<?xml version="1.0" encoding="UTF-8"?>
<document>
<title>THE EVOLUTION OF MAN</title>
<authors>ERNST HAECKEL</authors>
<publisher>WATTS CO.</publisher>
<publishedplace>LONDON</publishedplace>
<publishedyear>1911</publishedyear>
<edition>VOLUME 2</edition>
<category>Natural History</category>
<section id="1" fatherid="0" title="STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT" lv="CHAPTER 2.16" priority="0" level="1">
In turning from the embryology to the phylogeny of man--from the
development of the individual to that of the species--we must bear in
mind the direct causal connection that exists between these two main
branches of the science of human evolution. This important causal
nexus finds its simplest expression in "the fundamental law of organic
development," the content and purport of which we have fully
considered in the first chapter. According to this biogenetic law,
ontogeny is a brief and condensed recapitulation of phylogeny. If this
compendious reproduction were complete in all cases, it would be very
easy to construct the whole story of evolution on an embryonic basis.
When we wanted to know the ancestors of any higher organism, and,
therefore, of man--to know from what forms the race as a whole has
been evolved we should merely have to follow the series of forms in
the development of the individual from the ovum; we could then regard
each of the successive forms as the representative of an extinct
ancestral form. However, this direct application of ontogenetic facts
to phylogenetic ideas is possible, without limitations, only in a very
small section of the animal kingdom. There are, it is true, still a
number of lower invertebrates (for instance, some of the Zoophyta and
Vermalia) in which we are justified in recognising at once each
embryonic form as the historical reproduction, or silhouette, as it
were, of an extinct ancestor. But in the great majority of the
animals, and in the case of man, this is impossible, because the
embryonic forms themselves have been modified through the change of
the conditions of existence, and have lost their original character to
some extent. During the immeasurable course of organic history, the
many millions of years during which life was developing on our planet,
secondary changes of the embryonic forms have taken place in most
animals. The young of animals (not only detached larvae, but also the
embryos enclosed in the womb) may be modified by the influence of the
environment, just as well as the mature organisms are by adaptation to
the conditions of life; even species are altered during the embryonic
development. Moreover, it is an advantage for all higher organisms
(and the advantage is greater the more advanced they are) to curtail
and simplify the original course of development, and thus to
obliterate the traces of their ancestors. The higher the individual
organism is in the animal kingdom, the less completely does it
reproduce in its embryonic development the series of its ancestors,
for reasons that are as yet only partly known to us. The fact is
easily proved by comparing the different developments of higher and
lower animals in any single stem.

In order to appreciate this important feature, we have distributed the
embryological phenomena in two groups, palingenetic and cenogenetic.
Under palingenesis we count those facts of embryology that we can
directly regard as a faithful synopsis of the corresponding
stem-history. By cenogenesis we understand those embryonic processes
which we cannot directly correlate with corresponding evolutionary
processes, but must regard as modifications or falsifications of them.
With this careful discrimination between palingenetic and cenogenetic
phenomena, our biogenetic law assumes the following more precise
shape:--The rapid and brief development of the individual (ontogeny)
is a condensed synopsis of the long and slow history of the stem
(phylogeny): this synopsis is the more faithful and complete in
proportion as the original features have been preserved by heredity,
and modifications have not been introduced by adaptation.

In order to distinguish correctly between palingenetic and cenogenetic
phenomena in embryology, and deduce sound conclusions in connection
with stem-history, we must especially make a comparative study of the
former. In doing this it is best to employ the methods that have long
been used by geologists for the purpose of establishing the succession
of the sedimentary rocks in the crust of the earth. This solid crust,
which encloses the glowing central mass like a thin shell, is composed
of different kinds of rocks: there are, firstly, the volcanic rocks
which were formed directly by the cooling at the surface of the molten
mass of the earth; secondly, there are the sedimentary rocks, that
have been made out of the former by the action of water, and have been
laid in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course
of thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
 characteristic impressions of their forms in the soft slime, we
have especially the more solid parts of the animals and plants that
lived and died during the deposit of the slimy strata.

Hence each of the sedimentary strata has its characteristic fossils,
the remains of the animals and plants that lived during that
particular period of the earth's history. When we make a comparative
study of these strata, we can survey the whole series of such periods.
All geologists are now agreed that we can demonstrate a definite
historical succession in the strata, and that the lowest of them were
deposited in very remote, and the uppermost in comparatively recent,
times. However, there is no part of the earth where we find the series
of strata in its entirety, or even approximately complete. The
succession of strata and of corresponding historical periods generally
given in geology is an ideal construction, formed by piecing together
the various partial discoveries of the succession of strata that have
been made at different points of the earth's surface (cf. Chapter
2.18).

We must act in this way in constructing the phylogeny of man. We must
try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are
really in a position to form an approximate picture of the evolution
of man and the mammals by a proper comparison of the embryology of
very different animals--a picture that we could never have framed from
the ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes--those of disturbed and curtailed
heredity--whole series of lower stages have dropped out in the
embryonic development of man and the other mammals especially from the
earliest periods, or been falsified by modification. But we find these
lower stages in their original purity in the lower vertebrates and
their invertebrate ancestors. Especially in the lowest of all the
vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms
completely preserved in the embryonic development. We also find
important evidence in the fishes, which stand between the lower and
higher vertebrates, and throw further light on the course of evolution
in certain periods. Next to the fishes come the amphibia, from the
embryology of which we can also draw instructive conclusions. They
represent the transition to the higher vertebrates, in which the
middle and older stages of ancestral development have been either
distorted or curtailed, but in which we find the more recent stages of
the phylogenetic process well preserved in ontogeny. We are thus in a
position to form a fairly complete idea of the past development of
man's ancestors within the vertebrate stem by putting together and
comparing the embryological developments of the various groups of
vertebrates. And when we go below the lowest vertebrates and compare
their embryology with that of their invertebrate relatives, we can
follow the genealogical tree of our animal ancestors much farther,
down to the very lowest groups of animals.

In entering the obscure paths of this phylogenetic labyrinth, clinging
to the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we
have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that we can employ this method with the same
confidence and  as the geologist. No geologist has ever had
ocular proof that the vast rocks that compose our Carboniferous or
Jurassic or Cretaceous strata were really deposited in water. Yet no
one doubts the fact. Further, no geologist has ever learned by direct
observation that these various sedimentary formations were deposited
in a certain order; yet all are agreed as to this order. This is
because the nature and origin of these rocks cannot be rationally
understood unless we assume that they were so deposited. These
hypotheses are universally received as safe and indispensable
"geological theories," because they alone give a rational explanation
of the strata.

Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of "biological
theories" for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning
of the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in
certain quarters, will sooner or later be generally received. It is
true that, as will soon appear, our task is not so simple as that of
the geologist. It is just as much more difficult and complex as man's
organisation is more elaborate than the structure of the rocks.

When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower
animal-forms. One of these animals is the lancelet (Amphioxus), the
other the sea-squirt (Ascidia). Both of these animals are very
instructive. Both are at the border between the two chief divisions of
the animal kingdom--the vertebrates and invertebrates. The vertebrates
comprise the already mentioned classes, from the Amphioxus to man
(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and
mammals). Following the example of Lamarck, it is usual to put all the
other animals together under the head of invertebrates. But, as I have
often mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the
vertebrates. On the other hand, we are greatly concerned with a very
interesting group that has only recently been carefully studied, and
that has a most important relation to the ancestral tree of the
vertebrates. This is the stem of the Tunicates. One member of this
group, the sea-squirt, very closely approaches the lowest vertebrate,
the Amphioxus, in its essential internal structure and embryonic
development. Until 1866 no one had any idea of the close connection of
these apparently very different animals; it was a very fortunate
accident that the embryology of these related forms was discovered
just at the time when the question of the descent of the vertebrates
from the invertebrates came to the front. In order to understand it
properly, we must first consider these remarkable animals in their
fully-developed forms and compare their anatomy.

We begin with the lancelet--after man the most important and
interesting of all animals. Man is at the highest summit, the lancelet
at the lowest root, of the vertebrate stem.

It lives on the flat, sandy parts of the Mediterranean coast, partly
buried in the sand, and is apparently found in a number of seas.* (*
See the ample monograph by Arthur Willey, Amphioxus and the Ancestry
of the Vertebrates; Boston, 1894.) It has been found in the North Sea
(on the British and Scandinavian coasts and in Heligoland), and at
various places on the Mediterranean (for instance, at Nice, Naples,
and Messina). It is also found on the coast of Brazil and in the most
distant parts of the Pacific Ocean (the coast of Peru, Borneo, China,
Australia, etc.). Recently eight to ten species of the amphioxus have
been determined, distributed in two or three genera.

(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural
size,  view. The long axis is vertical; the mouth-end is above,
the tail-end below; a mouth, surrounded by threads of beard; b anus, c
gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g
small intestine, h branchial cavity, i chorda (axial rod), underneath
it the aorta; k aortic arches, l trunk of the branchial artery, m
swellings on its branches, n vena cava, o visceral vein.

FIGURE 2.211. Transverse section of the head of the Amphioxus. (From
Boveri.) Above the branchial gut (kd) is the chorda, above this the
neural tube (in which we can distinguish the inner grey and the outer
white matter); above again is the dorsal fin (fh). To the  and
 above (in the episoma) are the thick muscular plates (m); below
(in the hyposoma) the gonads (g). ao aorta (here double), c corium, ec
endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel,
ld partition between the coeloma (sc) and atrium (p), mt transverse
ventral muscle, n renal canals, of upper and uf lower canals in the
mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal
body-cavity), si principal (or subintestinal) vein, sk perichorda
(skeletal layer).)

Johannes Muller classed the lancelet with the fishes, although he
pointed out that the differences between this simple vertebrate and
the lowest fishes are much greater than between the fishes and the
amphibia. But this was far from expressing the real significance of
the animal. We may confidently lay down the following principle: The
Amphioxus differs more from the fishes than the fishes do from man and
the other vertebrates. As a matter of fact, it is so different from
all the other vertebrates in its whole organisation that the laws of
logical classification compel us to distinguish two divisions of this
stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the
Craniota (man and the other vertebrates). The first and lower division
comprises the vertebrates that have no vertebrae or skull (cranium).
Of these the only living representatives are the Amphioxus and
Paramphioxus, though there must have been a number of different
species at an early period of the earth's history.

Opposed to the Acrania is the second division of the vertebrates,
which comprises all the other members of the stem, from the fishes up
to man. All these vertebrates have a head quite distinct from the
trunk, with a skull (cranium) and brain; all have a centralised heart,
fully-formed kidneys, etc. Hence they are called the Craniota. These
Craniotes are, however, without a skull in their earlier period. As we
already know from embryology, even man, like every other mammal,
passes in the earlier course of his development through the important
stage which we call the chordula; at this lower stage the animal has
neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even
after the formation of the primitive vertebrae has begun, the
segmented foetus of the amniotes still has for a long time the simple
form of a lyre-shaped disk or a sandal, without limbs or extremities.
When we compare this embryonic condition, the sandal-shaped foetus,
with the developed lancelet, we may say that the amphioxus is, in a
certain sense, a permanent sandal-embryo, or a permanent embryonic
form of the Acrania; it never rises above a low grade of development
which we have long since passed.

The fully-developed lancelet (Figure 2.210) is about two inches long,
is colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is pointed at both ends, but much
compressed at the sides. There is no trace of limbs. The outer skin is
very thin and delicate, naked, transparent, and composed of two
different layers, a simple external stratum of cells, the epidermis,
and a thin underlying cutis-layer. Along the middle line of the back
runs a narrow fin-fringe which expands behind into an oval tail-fin,
and is continued below in a short anus-fin. The fin-fringe is
supported by a number of square elastic fin-plates.

In the middle of the body we find a thin string of cartilage, which
goes the whole length of the body from front to back, and is pointed
at both ends (Figure 2.210 i). This straight, cylindrical rod
(somewhat compressed for a time) is the axial rod or the chorda
dorsalis; in the lancelet this is the only trace of a vertebral
column. The chorda develops no further, but retains its original
simplicity throughout life. It is enclosed by a firm membrane, the
chorda-sheath or perichorda. The real features of this and of its
dependent formations are best seen in the transverse section of the
Amphioxus (Figure 2.211). The perichorda forms a cylindrical tube
immediately over the chorda, and the central nervous system, the
medullary tube, is enclosed in it. This important psychic organ also
remains in its simplest shape throughout life, as a cylindrical tube,
terminating with almost equal plainness at either end, and enclosing a
narrow canal in its thick wall. However, the fore end is a little
rounder, and contains a small, almost imperceptible bulbous swelling
of the canal. This must be regarded as the beginning of a rudimentary
brain. At the foremost end of it there is a small black pigment-spot,
a rudimentary eye; and a narrow canal leads to a superficial
sense-organ. In the vicinity of this optic spot we find at the 
side a small ciliated depression, the single olfactory organ. There is
no organ of hearing. This defective development of the higher
sense-organs is probably, in the main, not an original feature, but a
result of degeneration.

Underneath the axial rod or chorda runs a very simple alimentary
canal, a tube that opens on the ventral side of the animal by a mouth
in front and anus behind. The oval mouth is surrounded by a ring of
cartilage, on which there are twenty to thirty cartilaginous threads
(organs of touch, Figure 2.210 a). The alimentary canal divides into
sections of about equal length by a constriction in the middle. The
fore section, or head-gut, serves for respiration; the hind section,
or trunk-gut, for digestion. The limit of the two alimentary regions
is also the limit of the two parts of the body, the head and the
trunk. The head-gut or branchial gut forms a broad gill-crate, the
grilled wall of which is pierced by numbers of gill-cs (Figure
2.210 d). The fine bars of the gill-crate between the cs are
strengthened with firm parallel rods, and these are connected in pairs
by cross-rods. The water that enters the mouth of the Amphioxus passes
through these cs into the large surrounding branchial cavity or
atrium, and then pours out behind through a hole in it, the
respiratory pore (porus branchialis, Figure 2.210 c). Below, on the
ventral side of the gill-crate, there is in the middle line a ciliated
groove with a glandular wall (the hypobranchial groove), which is also
found in the Ascidia and the larvae of the Cyclostoma. It is
interesting because the thyroid gland in the larynx of the higher
vertebrates (underneath the "Adam's apple") has been developed from
it.

(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five
gill-cs, through the middle of the body.

FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis,
B medullary tube, C chorda, C1 inner chorda-sheath, D visceral
epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome),
3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal
and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel
(dorsal body-cavity), II splanchnocoel (ventral body-cavity).)

Behind the respiratory part of the gut we have the digestive section,
the trunk or liver (hepatic) gut. The small particles that the
Amphioxus takes in with the water--infusoria, diatoms, particles of
decomposed plants and animals, etc.--pass from the gill-crate into the
digestive part of the canal, and are used up as food. From a somewhat
enlarged portion, that corresponds to the stomach (Figure 2.210 e), a
long, pouch-like blind sac proceeds straight forward (f); it lies
underneath on the  side of the gill-crate, and ends blindly about
the middle of it. This is the liver of the Amphioxus, the simplest
kind of liver that we meet in any vertebrate. In man also the liver
develops, as we shall see, in the shape of a pouch-like blind sac,
that forms out of the alimentary canal behind the stomach.

The formation of the circulatory system in this animal is not less
interesting. All the other vertebrates have a compressed, thick,
pouch-shaped heart, which develops from the wall of the gut at the
throat, and from which the blood-vessels proceed; in the Amphioxus
there is no special centralised heart, driving the blood by its
pulsations. This movement is effected, as in the annelids, by the thin
blood-vessels themselves, which discharge the function of the heart,
contracting and pulsating in their whole length, and thus driving the
colourless blood through the entire body. On the under-side of the
gill-crate, in the middle line, there is the trunk of a large vessel
that corresponds to the heart of the other vertebrates and the trunk
of the branchial artery that proceeds from it; this drives the blood
into the gills (Figure 2.210 l). A number of small vascular arches
arise on each side from this branchial artery, and form little
heart-shaped swellings or bulbilla (m) at their points of departure;
they advance along the branchial arches, between the gill-cs and
the fore-gut, and unite, as branchial veins, above the gill-crate in a
large trunk blood-vessel that runs under the chorda dorsalis. This is
the principal artery or primitive aorta (Figure 2.214 D). The branches
which it gives off to all parts of the body unite again in a larger
venous vessel at the underside of the gut, called the subintestinal
vein (Figures 1.210 o and 2.212 E). This single main vessel of the
Amphioxus goes like a closed circular water-conduit along the
alimentary canal through the whole body, and pulsates in its whole
length above and below. When the upper tube contracts the lower one is
filled with blood, and vice versa. In the upper tube the blood flows
from front to rear, then back from rear to front in the lower vessel.
The whole of the long tube that runs along the ventral side of the
alimentary canal and contains venous blood may be called the
"principal vein," and may be compared to the ventral vessel in the
worms. On the other hand, the long straight vessel that runs along the
dorsal line of the gut above, between it and the chorda, and contains
arterial blood, is clearly identical with the aorta or principal
artery of the other vertebrates; and on the other side it may be
compared to the dorsal vessel in the worms.

(FIGURE 2.214. Transverse section of a young Amphioxus, immediately
after metamorphosis, through the hindermost third (between the
atrium-cavity and the anus).

FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B
medullary tube, C chorda, D aorta, E visceral epithelium, F
subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4
outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre
plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.)

The coeloma or body-cavity has some very important and distinctive
features in the Amphioxus. The embryology of it is most instructive in
connection with the stem-history of the body-cavity in man and the
other vertebrates. As we have already seen (Chapter 1.10), in these
the two coelom-pouches are divided at an early stage by transverse
constrictions into a double row of primitive segments (Figure 1.124),
and each of these subdivides, by a frontal or lateral constriction,
into an upper (dorsal) and lower (ventral) pouch.

These important structures are seen very clearly in the trunk of the
amphioxus (the latter third, Figures 2.212 to 2.215), but it is
otherwise in the head, the foremost third (Figure 2.216). Here we find
a number of complicated structures that cannot be understood until we
have studied them on the embryological side in the next chapter (cf.
Figure 1.81). The branchial gut lies free in a spacious cavity filled
with water, which was wrongly thought formerly to be the body-cavity
(Figure 2.216 A). As a matter of fact, this atrium (commonly called
the peribranchial cavity) is a secondary structure formed by the
development of a couple of lateral mantle-folds or gill-covers (M1,
U). The real body-cavity (Lh) is very narrow and entirely closed,
lined with epithelium. The peribranchial cavity (A) is full of water,
and its walls are lined with the skin-sense layer; it opens outwards
in the rear through the respiratory pore (Figure 2.210 c).

On the inner surface of these mantle-folds (M1), in the ventral half
of the wide mantle cavity (atrium), we find the sex-organs of the
Amphioxus. At each side of the branchial gut there are between twenty
and thirty roundish four-cornered sacs, which can clearly be seen from
without with the naked eye, as they shine through the thin transparent
body-wall. These sacs are the sexual glands they are the same size and
shape in both sexes, only differing in contents. In the female they
contain a quantity of simple ova (Figure 2.219 g); in the male a
number of much smaller cells that change into mobile ciliated cells
(sperm-cells). Both sacs lie on the inner wall of the atrium, and have
no special outlets. When the ova of the female and the sperm of the
male are ripe, they fall into the atrium, pass through the gill-cs
into the fore-gut, and are ejected through the mouth.

(FIGURE 2.216. Transverse section of the lancelet, in the fore half.
(From Ralph.) The outer covering is the simple cell-layer of the
epidermis (E). Under this is the thin corium, the subcutaneous tissue
of which is thickened; it sends connective-tissue partitions between
the muscles (M1) and to the chorda-sheath. N medullary tube, Ch
chorda, Lh body-cavity, A atrium, L upper wall of same, E1 inner wall,
E2 outer wall, Lh1 ventral remnant of same, Kst gill-reds, M ventral
muscles, R seam of the joining of the ventral folds (gill-covers), G
sexual glands.)

Above the sexual glands, at the dorsal angle of the atrium, we find
the kidneys. These important excretory organs could not be found in
the Amphioxus for a long time, on account of their remote position and
their smallness; they were discovered in 1890 by Theodor Boveri
(Figure 2.217 x). They are short segmented canals; corresponding to
the primitive kidneys of the other vertebrates (Figure 2.218 B). Their
internal aperture (Figure 2.217 B) opens into the body-cavity; their
outer aperture into the atrium (C). The prorenal canals lie in the
middle of the line of the head, outwards from the uppermost section of
the gill-arches, and have important relations to the branchial vessels
(H). For this reason, and in their whole arrangement, the primitive
kidneys of the Amphioxus show clearly that they are equivalent to the
prorenal canals of the Craniotes (Figure 2.218 B). The prorenal duct
of the latter (Figure 2.218 C) corresponds to the branchial cavity or
atrium of the former (Figure 2.217 C).

(FIGURE 2.217. Transverse section through the middle of the Amphioxus.
(From Boveri.) On the  a gill-rod has been struck, and on the
 a gill-c; consequently on the  we see the whole of a
prorenal canal (x), on the  only the section of its fore-leg. A
genital chamber (ventral section of the gonocoel), x pronephridium, B
its coelom-aperture, C atrium, D body-cavity, E visceral cavity, F
subintestinal vein, G aorta (the  branch connected by a branchial
vessel with the subintestinal vein), H renal vessel.

FIGURE 2.218. Transverse section of a primitive fish embryo
(Selachii-embryo, from Boveri.). To the  pronephridia (B), the
 primitive kidneys (A). The dotted lines on the  indicate
the later opening of the primitive kidney canals (A) into the prorenal
duct (C). D body-cavity, E visceral cavity, F subintestinal vein, G
aorta, H renal vessel.)

If we sum up the results of our anatomic study of the Amphioxus, and
compare them with the familiar organisation of man, we shall find an
immense distance between the two. As a fact, the highest summit of the
vertebrate organisation which man represents is in every respect so
far above the lowest stage, at which the lancelet remains, that one
would at first scarcely believe it possible to class both animals in
the same division of the animal kingdom. Nevertheless, this
classification is indisputably just. Man is only a more advanced stage
of the vertebral type that we find unmistakably in the Amphioxus in
its characteristic features. We need only recall the picture of the
ideal Primitive Vertebrate given in a former chapter, and compare it
with the lower stages of human embryonic development, to convince
ourselves of our close relationship to the lancelet. (Cf. Chapter
1.11.)

It is true that the Amphioxus is far below all other living
vertebrates. It is true that it has no separate head, no developed
brain or skull, the characteristic feature of the other vertebrates.
It is (probably as a result of degeneration) without the auscultory
organ and the centralised heart that all the others have; and it has
no fully-formed kidneys. Every single organ in it is simpler and less
advanced than in any of the others. Yet the characteristic connection
and arrangement of all the organs is just the same as in the other
vertebrates. All these, moreover, pass, during their embryonic
development, through a stage in which their whole organisation is no
higher than that of the Amphioxus, but is substantially identical with
it.

(FIGURE 2.219. Transverse section of the head of the Amphioxus (at the
limit of the first and second third of the body). (From Boveri) a
aorta (here double), b atrium, c chorda, co umlaut coeloma
(body-cavity), e endostyl (hypobranchial groove), g gonads (ovaries),
kb gill-arches, kd branchial gut, l liver-tube (on the ,
one-sided), m muscles, n renal canals, r spinal cord, sn spinal
nerves, sp gill-cs.)

In order to see this quite clearly, it is particularly useful to
compare the Amphioxus with the youthful forms of those vertebrates
that are classified next to it. This is the class of the Cyclostoma.
There are to-day only a few species of this once extensive class, and
these may be distributed in two groups. One group comprises the
hag-fishes or Myxinoides. The other group are the Petromyzontes, or
lampreys, which are a familiar delicacy in their marine form. These
Cyclostoma are usually classified with the fishes. But they are far
below the true fishes, and form a very interesting connecting-group
between them and the lancelet. One can see how closely they approach
the latter by comparing a young lamprey with the Amphioxus. The chorda
is of the same simple character in both; also the medullary tube, that
lies above the chorda, and the alimentary canal below it. However, in
the lamprey the spinal cord swells in front into a simple pear-shaped
cerebral vesicle, and at each side of it there are a very simple eye
and a rudimentary auditory vesicle. The nose is a single pit, as in
the Amphioxus. The two sections of the gut are also just the same and
very rudimentary in the lamprey. On the other hand, we see a great
advance in the structure of the heart, which is found underneath the
gills in the shape of a centralised muscular tube, and is divided into
an auricle and a ventricle. Later on the lamprey advances still
further, and gets a skull, five cerebral vesicles, a series of
independent gill-pouches, etc. This makes all the more interesting the
striking resemblance of its immature larva to the developed and
sexually mature Amphioxus.

While the Amphioxus is thus connected through the Cyclostoma with the
fishes, and so with the series of the higher vertebrates, it is, on
the other hand, very closely related to a lowly invertebrate marine
animal, from which it seems to be entirely remote at first glance.
This remarkable animal is the sea-squirt or Ascidia, which was
formerly thought to be closely related to the mussel, and so classed
in the molluscs. But since the remarkable embryology of these animals
was discovered in 1866, there can be no question that they have
nothing to do with the molluscs. To the great astonishment of
zoologists, they were found, in their whole individual development, to
be closely related to the vertebrates. When fully developed the
Ascidiae are shapeless lumps that would not, at first sight, be taken
for animals at all. The oval body, frequently studded with knobs or
uneven and lumpy, in which we can discover no special external organs,
is attached at one end to marine plants, rocks, or the floor of the
sea. Many species look like potatoes, others like melon-cacti, others
like prunes. Many of the Ascidiae form transparent crusts or deposits
on stones and marine plants. Some of the larger species are eaten like
oysters. Fishermen, who know them very well, think they are not
animals, but plants. They are sold in the fish markets of many of the
Italian coast-towns with other lower marine animals under the name of
"sea-fruit" (frutti di mare). There is nothing about them to show that
they are animals. When they are taken out of the water with the net
the most one can perceive is a slight contraction of the body that
causes water to spout out in two places. The bulk of the Ascidiae are
very small, at the most a few inches long. A few species are a foot or
more in length. There are many species of them, and they are found in
every sea. As in the case of the Acrania, we have no fossilised
remains of the class, because they have no hard and fossilisable
parts. However, they must be of great antiquity, and must go back to
the primordial epoch.

The name of "Tunicates" is given to the whole class to which the
Ascidiae belong, because the body is enclosed in a thick and stiff
covering like a mantle (tunica). This mantle--sometimes soft like
jelly, sometimes as tough as leather, and sometimes as stiff as
cartilage--has a number of peculiarities. The most remarkable of them
is that it consists of a woody matter, cellulose--the same vegetal
substance that forms the stiff envelopes of the plant-cells, the
substance of the wood. The tunicates are the only class of animals
that have a real cellulose or woody coat. Sometimes the cellulose
mantle is bly coloured, at other times colourless. Not
infrequently it is set with needles or hairs, like a cactus. Often we
find a mass of foreign bodies--stone, sand, fragments of
mussel-shells, etc.--worked into the mantle. This has earned for the
Ascidia the name of "the microcosm."

(FIGURE 2.220. Organisation of an Ascidia ( view); the dorsal side
is turned to the  and the ventral side to the , the mouth (o)
above; the ascidia is attached at the tail end. The branchial gut
(br), which is pierced by a number of cs, continues below in the
visceral gut. The rectum opens through the anus (a) into the atrium
(cl), from which the excrements are ejected with the respiratory water
through the mantle-hole or cloaca (a); m mantle. (From Gegenbaur.)

FIGURE 2.221. Organisation of an Ascidia (as in Figure 2.220, seen
from the ). sb branchial sac, v stomach, i small intestine, c
heart, t testicle, vd sperm-duct, o ovary, o apostrophe ripe ova in
the branchial cavity. The two small arrows indicate the entrance and
exit of the water through the openings of the mantle. (From
Milne-Edwards.))

The hind end, which corresponds to the tail of the Amphioxus, is
usually attached, often by means of regular roots. The dorsal and
ventral sides differ a good deal internally, but frequently cannot be
distinguished externally. If we open the thick tunic or mantle in
order to examine the internal organisation, we first find a spacious
cavity filled with water--the mantle-cavity or respiratory cavity
(Figure 2.220 cl). It is also called the branchial cavity and the
cloaca, because it receives the excrements and sexual products as well
as the respiratory water. The greater part of the respiratory cavity
is occupied by the large grated branchial sac (br). This is so like
the gill-crate of the Amphioxus in its whole arrangement that the
resemblance was pointed out by the English naturalist Goodsir, years
ago, before anything was known of the relationship of the two animals.
As a fact, even in the Ascidia the mouth (o) opens first into this
wide branchial sac. The respiratory water passes through the
lattice-work of the branchial sac into the branchial cavity, and is
ejected from this by the respiratory pore (a apostrophe). Along the
ventral side of the branchial sac runs a ciliated groove--the
hypobranchial groove which we have previously found at the same spot
in the Amphioxus. The food of the Ascidia also consists of tiny
organisms, infusoria, diatoms, parts of decomposed marine plants and
animals; etc. These pass with the water into the gill-crate and the
digestive part of the gut at the end of it, at first into an
enlargement of it that represents the stomach. The adjoining small
intestine usually forms a loop, bends forward, and opens by an anus
(Figure 2.220 a), not directly outwards, but first into the mantle
cavity; from this the excrements are ejected by a common outlet (a
apostrophe) together with the used-up water and the sexual products.
The outlet is sometimes called the branchial pore, and sometimes the
cloaca or ejection-aperture. In many of the Ascidiae a glandular mass
opens into the gut, and this represents the liver. In some there is
another gland besides the liver, and this is taken to represent the
kidneys. The body-cavity proper, or coeloma, which is filled with
blood and encloses the hepatic gut, is very narrow in the Ascidia, as
in the Amphioxus, and is here also usually confounded with the wide
atrium, or peribranchial cavity, full of water.

There is no trace in the fully-developed Ascidia of a chorda dorsalis,
or internal axial skeleton. It is the more interesting that the young
animal that emerges from the ovum HAS a chorda, and that there is a
rudimentary medullary tube above it. The latter is wholly atrophied in
the developed Ascidia, and looks like a small nerve-ganglion in front
above the gill-crate. It corresponds to the upper "gullet-ganglion" or
"primitive brain" in other vermalia. Special sense-organs are either
wanting altogether or are only found in a very rudimentary form, as
simple optic spots and touch-corpuscles or tentacles that surround the
mouth. The muscular system is very slightly and irregularly developed.
Immediately under the thin corium, and closely connected with it, we
find a thin muscle tube, as in the worms. On the other hand, the
Ascidia has a centralised heart, and in this respect it seems to be
more advanced than the Amphioxus. On the ventral side of the gut, some
distance behind the gill-crate, there is a spindle-shaped heart. It
retains permanently the simple tubular form that we find temporarily
as the first structure of the heart in the vertebrates. This simple
heart of the Ascidia has, however, a remarkable peculiarity. It
contracts in alternate directions. In all other animals the beat of
the heart is always in the same direction (generally from rear to
front); it changes in the Ascidia to the reverse direction. The heart
contracts first from the rear to the front, stands still for a minute,
and then begins to beat the opposite way, now driving the blood from
front to rear; the two large vessels that start from either end of the
heart act alternately as arteries and veins. This feature is found in
the Tunicates alone.

Of the other chief organs we have still to mention the sexual glands,
which lie  behind in the body-cavity. All the Ascidiae are
hermaphrodites. Each individual has a male and a female gland, and so
is able to fertilise itself. The ripe ova (Figure 2.221 o apostrophe)
fall directly from the ovary (o) into the mantle-cavity. The male
sperm is conducted into this cavity from the testicle (t) by a special
duct (vd). Fertilisation is accomplished here, and in many of the
Ascidiae developed embryos are found. These are then ejected with the
breathing-water through the cloaca (q), and so "born alive."

If we now glance at the entire structure of the simple Ascidia
(especially Phallusia, Cynthia, etc.) and compare it with that of the
Amphioxus, we shall find that the two have few points of contact. It
is true that the fully-developed Ascidia resembles the Amphioxus in
several important features of its internal structure, and especially
in the peculiar character of the gill-crate and gut. But in most other
features of organisation it is so far removed from it, and is so
unlike it in external appearance, that the really close relationship
of the two was not discovered until their embryology was studied. We
will now compare the embryonic development of the two animals, and
find to our great astonishment that the same embryonic form develops
from the ovum of the Amphioxus as from that of the Ascidia--a typical
chordula.

</section>
<section id="2" fatherid="0" title="EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT" lv="CHAPTER 2.17." priority="0" level="1">
The structural features that distinguish the vertebrates from the
invertebrates are so prominent that there was the greatest difficulty
in the earlier stages of classification in determining the affinity of
these two great groups. When scientists began to speak of the affinity
of the various animal groups in more than a figurative--in a
genealogical--sense, this question came at once to the front, and
seemed to constitute one of the chief obstacles to the carrying-out of
the evolutionary theory. Even earlier, when they had studied the
relations of the chief groups, without any idea of real genealogical
connection, they believed they had found here and there among the
invertebrates points of contact with the vertebrates: some of the
worms, especially, seemed to approach the vertebrates in structure,
such as the marine arrow-worm (Sagitta). But on closer study the
analogies proved untenable. When Darwin gave an impulse to the
construction of a real stem-history of the animal kingdom by his
reform of the theory of evolution, the solution of this problem was
found to be particularly difficult. When I made the first attempt in
my General Morphology (1866) to work out the theory and apply it to
classification, I found no problem of phylogeny that gave me so much
trouble as the linking of the vertebrates with the invertebrates.

But just at this time the true link was discovered, and at a point
where it was least expected. Towards the end of 1866 two works of the
Russian zoologist, Kowalevsky, who had lived for some time at Naples,
and studied the embryology of the lower animals, were issued in the
publications of the St. Petersburg Academy. A fortunate accident had
directed the attention of this able observer almost simultaneously to
the embryology of the lowest vertebrate, the Amphioxus, and that of an
invertebrate, the close affinity of which to the Amphioxus had been
least suspected, the Ascidia. To the extreme astonishment of all
zoologists who were interested in this important question, there
turned out to be the utmost resemblance in structure from the
commencement of development between these two very different
animals--the lowest vertebrate and the mis-shaped, sessile
invertebrate. With this undeniable identity of ontogenesis, which can
be demonstrated to an astounding extent, we had, in virtue of the
biogenetic law, discovered the long-sought genealogical link, and
definitely identified the invertebrate group that represents the
nearest blood-relatives of the vertebrates. The discovery was
confirmed by other zoologists, and there can no longer be any doubt
that of all the classes of invertebrates that of the Tunicates is most
closely related to the vertebrates, and of the Tunicates the nearest
are the Ascidiae. We cannot say that the vertebrates are descended
from the Ascidiae--and still less the reverse--but we can say that of
all the invertebrates it is the Tunicates, and, within this group, the
Ascidiae, that are the nearest blood-relatives of the ancient
stem-form of the vertebrates. We must assume as the common ancestral
group of both stems an extinct family of the extensive vermalia-stem,
the Prochordonia or Prochordata ("primitive chorda-animals").

In order to appreciate fully this remarkable fact, and especially to
secure the sound basis we seek for the genealogical tree of the
vertebrates, it is necessary to study thoroughly the embryology of
both these animals, and compare the individual development of the
Amphioxus step by step with that of the Ascidia. We begin with the
ontogeny of the Amphioxus.

From the concordant observations of Kowalevsky at Naples and Hatschek
at Messina, it follows, firstly, that the ovum-segmentation and
gastrulation of the Amphioxus are of the simplest character. They take
place in the same way as we find them in many of the lower animals of
different invertebrate stems, which we have already described as
original or primordial; the development of the Ascidia is of the same
type. Sexually mature specimens of the Amphioxus, which are found in
great quantities at Messina from April or May onwards, begin as a rule
to eject their sexual products in the evening; if you catch them about
the middle of a warm night and put them in a glass vessel with
seawater, they immediately eject through the mouth their accumulated
sexual products, in consequence of the disturbance. The males give out
masses of sperm, and the females discharge ova in such quantity that
many of them stick to the fibrils about their mouths. Both kinds of
cells pass first into the mantle-cavity after the opening of the
gonads, proceed through the gill-cs into the branchial gut, and
are discharged from this through the mouth.

The ova are simply round cells. They are only 1/250 of an inch in
diameter, and thus are only half the size of the mammal ova, and have
no distinctive features. The clear protoplasm of the mature ovum is
made so turbid by the numbers of dark granules of food-yelk or
deutoplasm scattered in it that it is difficult to follow the process
of fecundation and the behaviour of the two nuclei during it (Chapter
1.7). The active elements of the male sperm, the cone-shaped
spermatozoa, are similar to those of most other animals (cf. Figure
1.20). Fecundation takes place when these lively ciliated cells of the
sperm approach the ovum, and seek to penetrate into the yelk-matter or
the cellular substance of the ovum with their head-part--the thicker
part of the cell that encloses the nucleus. Only one spermatozoon can
bore its way into the yelk at one pole of the ovum-axis; its head or
nucleus coalesces with the female nucleus, which remains after the
extrusion of the directive bodies from the germinal vesicle. Thus is
formed the "stem-nucleus," or the nucleus of the "stem-cell" (cytula,
Figure 1.2). This now undergoes total segmentation, dividing into two,
four, eight, sixteen, thirty-two cells, and so on. In this way we get
the spherical, mulberry-shaped body, which we call the morula.

The segmentation of the Amphioxus is not entirely regular, as was
supposed after the first observations of Kowalevsky (1866). It is not
completely equal, but a little unequal. As Hatschek afterwards found
(1879), the segmentation-cells only remain equal up to the
morula-stage, the spherical body of which consists of thirty-two
cells. Then, as always happens in unequal segmentation, the more
sluggish vegetal cells are outstripped in the cleavage. At the lower
or vegetal pole of the ovum a crown of eight large entodermic cells
remains for a long time unchanged, while the other cells divide, owing
to the formation of a series of horizontal circles, into an increasing
number of crowns of sixteen cells each. Afterwards the
segmentation-cells get more or less irregularly displaced, while the
segmentation-cavity enlarges in the centre of the morula; in the end
the former all lie on the surface of the latter, so that the foetus
attains the familiar blastula shape and forms a hollow ball, the wall
of which consists of a single stratum of cells (Figure 1.38 A to C).
This layer is the blastoderm, the simple epithelium from the cells of
which all the tissues of the body proceed.

These important early embryonic processes take place so quickly in the
Amphioxus that four or five hours after fecundation, or about
midnight, the spherical blastula is completed. A pit-like depression
is then formed at the vegetal pole of it, and in consequence of this
the hollow sphere doubles on itself (Figure 1.38 D). This pit becomes
deeper and deeper (Figure 1.38 E and F); at last the invagination (or
doubling) is complete, and the inner or folded part of the
blastula-wall lies on the inside of the outer wall. We thus get a
hollow hemisphere, the thin wall of which is made up of two layers of
cells (Figure 1.38 E). From hemispherical the body soon becomes almost
spherical once more, and then oval, the internal cavity enlarging
considerably and its mouth growing narrower (Figure 2.213). The form
which the Amphioxus-embryo has thus reached is a real "cup-larva" or
gastrula, of the original simple type that we have previously
described as the "bell-gastrula" or archigastrula (Figures 1.29 to
1.35).

As in all the other animals that form an archigastrula, the whole body
is nothing but a simple gastric sac or stomach; its internal cavity is
the primitive gut (progaster or archenteron, Figure 1.38 g, 1.35 d),
and its aperture the primitive mouth (prostoma or blastoporus, o). The
wall is at once gut-wall and body-wall. It is composed of two simple
cell-layers, the familiar primary germinal layers. The inner layer or
the invaginated part of the blastoderm, which immediately encloses the
gut-cavity is the entoderm, the inner or vegetal germ-layer, from
which develop the wall of the alimentary canal and all its appendages,
the coelom-pouches, etc. (Figures 1.35 and 1.36 i). The outer stratum
of cells, or the non-invaginated part of the blastoderm, is the
ectoderm, the outer or animal germ-layer, which provides the outer
skin (epidermis) and the nervous system (e). The cells of the entoderm
are much larger, darker, and more fatty than those of the ectoderm,
which are clearer and less rich in fatty particles. Hence before and
during invagination there is an increasing differentiation of the
inner from the outer layer. The animal cells of the outer layer soon
develop vibratory hairs; the vegetal cells of the inner layer do so
much later. A thread-like process grows out of each cell, and effects
continuous vibratory movements. By the vibrations of these slender
hairs the gastrula of the Amphioxus swims about in the sea, when it
has pierced the thin ovolemma, like the gastrula of many other animals
(Figure 1.36). As in many other lower animals, the cells have only one
whip-like hair each, and so are called flagellate (whip) cells (in
contrast with the ciliated cells, which have a number of short lashes
or cilia).

In the further course of its rapid development the roundish
bell-gastrula becomes elongated, and begins to flatten on one side,
parallel to the long axis. The flattened side is the subsequent dorsal
side; the opposite or ventral side remains curved. The latter grows
more quickly than the former, with the result that the primitive mouth
is forced to the dorsal side (Figure 1.39). In the middle of the
dorsal surface a shallow longitudinal groove or furrow is formed
(Figure 1.79), and the edges of the body rise up on each side of this
groove in the shape of two parallel swellings. This groove is, of
course, the dorsal furrow, and the swellings are the dorsal or
medullary swellings; they form the first structure of the central
nervous system, the medullary tube. The medullary swellings now rise
higher; the groove between them becomes deeper and deeper. The edges
of the parallel swellings curve towards each other, and at last unite,
and the medullary tube is formed (Figures 1.83 m and 1.84 m). Hence
the formation of a medullary tube out of the outer skin takes place in
the naked dorsal surface of the free-swimming larva of the Amphioxus
in just the same way as we have found in the embryo of man and the
higher animals within the foetal membranes.

Simultaneously with the construction of the medullary tube we have in
the Amphioxus-embryo the formation of the chorda, the coelom-pouches,
and the mesoderm proceeding from their wall. These processes also take
place with characteristic simplicity and clearness, so that they are
very instructive to compare with the vermalia on the one hand and with
the higher vertebrates on the other. While the medullary groove is
sinking in the middle line of the flat dorsal side of the oval embryo,
and its parallel edges unite to form the ectodermic neural tube, the
single chorda is formed directly underneath them, and on each side of
this a parallel longitudinal fold, from the dorsal wall of the
primitive gut. These longitudinal folds of the entoderm proceed from
the primitive mouth, or from its lower and hinder edge. Here we see at
an early stage a couple of large entodermic cells, which are
distinguished from all the others by their great size, round form, and
fine-grained protoplasm; they are the two promesoblasts, or polar
cells of the mesoderm (Figure 1.83 p). They indicate the original
starting-point of the two coelom-pouches, which grow from this spot
between the inner and outer germinal layers, sever themselves from the
primitive gut, and provide the cellular material for the middle layer.

Immediately after their formation the two coelom-pouches of the
Amphioxus are divided into several parts by longitudinal and
transverse folds. Each of the primary pouches is divided into an upper
dorsal and a lower ventral section by a couple of lateral longitudinal
folds (Figure 1.82). But these are again divided by several parallel
transverse folds into a number of successive sacs, the primitive
segments or somites (formerly called by the unsuitable name of
"primitive vertebrae"). They have a different future above and below.
The upper or dorsal segments, the episomites, lose their cavity later
on, and form with their cells the muscular plates of the trunk. The
lower or ventral segments, the hyposomites, corresponding to the
lateral plates of the craniote-embryo, fuse together in the upper part
owing to the disappearance of their lateral walls, and thus form the
later body-cavity (metacoel); in the lower part they remain separate,
and afterwards form the segmental gonads.

In the middle, between the two lateral coelom-folds of the primitive
gut, a single central organ detaches from this at an early stage in
the middle line of its dorsal wall. This is the dorsal chorda (Figures
1.83 and 1.84 ch). This axial rod, which is the first foundation of
the later vertebral column in all the vertebrates, and is the only
representative of it in the Amphioxus, originates from the entoderm.

In consequence of these important folding-processes in the primitive
gut, the simple entodermic tube divides into four different
sections:--

1. underneath, at the ventral side, the permanent alimentary canal or
permanent gut;

2. above, at the dorsal side, the axial rod or chorda; and

3. the two coelom-sacs, which immediately sub-divide into two
structures:--

3A. above, on the dorsal side, the episomites, the double row of
primitive or muscular segments; and

3B. below, on each side of the gut, the hyposomites, the two lateral
plates that give rise to the sex-glands, and the cavities of which
partly unite to form the body-cavity. At the same time, the neural or
medullary tube is formed above the chorda, on the dorsal surface, by
the closing of the parallel medullary swellings.

All these processes, which outline the typical structure of the
vertebrate, take place with astonishing rapidity in the embryo of the
Amphioxus; in the afternoon of the first day, or twenty-four hours
after fertilisation, the young vertebrate, the typical embryo, is
formed; it then has, as a rule, six to eight somites.

The chief occurrence on the second day of development is the
construction of the two permanent openings of the gut--the mouth and
anus. In the earlier stages the alimentary tube is found to be
entirely closed, after the closing of the primitive mouth; it only
communicates behind by the neurenteric canal with the medullary tube.
The permanent mouth is a secondary formation, at the opposite end.
Here, at the end of the second day, we find a pit-like depression in
the outer skin, which penetrates inwards into the closed gut. The anus
is formed behind in the same way a few hours later (in the vicinity of
the additional gastrula-mouth). In man and the higher vertebrates also
the mouth and anus are formed, as we have seen, as flat pits in the
outer skin; they then penetrate inwards, gradually becoming connected
with the blind ends of the closed gut-tube. During the second day the
Amphioxus-embryo undergoes few other changes. The number of primitive
segments increases, and generally amounts to fourteen, some
forty-eight to fifty hours after impregnation.

Almost simultaneously with the formation of the mouth the first
gill-c breaks through in the fore section of the Amphioxus-embryo
(generally forty hours after the commencement of development). It now
begins to nourish itself independently, as the food material stored up
in the ovum is completely used up. The further development of the free
larvae takes place very slowly, and extends over several months. The
body becomes much longer, and is compressed at the sides, the head-end
being broadened in a sort of triangle. Two rudimentary sense-organs
are developed in it. Inside we find the first blood-vessels, an upper
or dorsal vessel, corresponding to the aorta, between the gut and the
dorsal cord, and a lower or ventral vessel, corresponding to the
subintestinal vein, at the lower border of the gut. Now, the gills or
respiratory organs also are formed at the fore-end of the alimentary
canal. The whole of the anterior or respiratory section of the gut is
converted into a gill-crate, which is pierced trellis-wise by numbers
of branchial-holes, as in the ascidia. This is done by the foremost
part of the gut-wall joining star-wise with the outer skin, and the
formation of cs at the point of connection, piercing the wall and
leading into the gut from without. At first there are very few of
these branchial cs; but there are soon a number of them--first in
one, then in two, rows. The foremost gill-c is the oldest. In the
end we have a sort of lattice work of fine gill-cs, supported on a
number of stiff branchial rods; these are connected in pairs by
transverse rods.

(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae
(diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222
there is free communication from without with the gut-cavity (D)
through the gill-cs (K). In Figure 2.223 the lateral folds of the
body-wall, or the gill-covers, which grow downwards, are formed. In
Figure 2.224 these lateral folds have united underneath and joined
their edges in the middle line of the ventral side (R seam). The
respiratory water now passes from the gut-cavity (D) into the
mantle-cavity (A). The letters have the same meaning throughout: N
medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part
of the body-cavity in which the sexual organs are subsequently formed.
D gut-cavity, clothed with the gut-gland layer (a). A mantle-cavity, K
gill-cs, b = E epidermis, E1 the same as visceral epithelium of
the mantle-cavity, E2 as parietal epithelium of the mantle-cavity.)

At an early stage of embryonic development the structure of the
Amphioxus-larva is substantially the same as the ideal picture we have
previously formed of the "Primitive Vertebrate" (Figures 1.98 to
1.102). But the body afterwards undergoes various modifications,
especially in the fore-part. These modifications do not concern us, as
they depend on special adaptations, and do not affect the hereditary
vertebrate type. When the free-swimming Amphioxus-larva is three
months old, it abandons its pelagic habits and changes into the young
animal that lives in the sand. In spite of its smallness (one-eighth
of an inch), it has substantially the same structure as the adult. As
regards the remaining organs of the Amphioxus, we need only mention
that the gonads or sexual glands are developed very late, immediately
out of the inner cell-layer of the body-cavity. Although we can find
afterwards no continuation of the body-cavity (Figure 2.216 U) in the
lateral walls of the mantle-cavity, in the gill-covers or mantle-folds
(Figure 2.224 U), there is one present in the beginning (Figure 2.224
Lh). The sexual cells are formed below, at the bottom of this
continuation (Figure 2.224 S). For the rest, the subsequent
development into the adult Amphioxus of the larva we have followed is
so simple that we need not go further into it here.

We may now turn to the embryology of the Ascidia, an animal that seems
to stand so much lower and to be so much more simply organised,
remaining for the greater part of its life attached to the bottom of
the sea like a shapeless lump. It was a fortunate accident that
Kowalevsky first examined just those larger specimens of the Ascidiae
that show most clearly the relationship of the vertebrates to the
invertebrates, and the larvae of which behave exactly like those of
the Amphioxus in the first stages of development. This resemblance is
so close in the main features that we have only to repeat what we have
already said of the ontogenesis of the Amphioxus.

The ovum of the larger Ascidia (Phallusia, Cynthia, etc.) is a simple
round cell of 1/250 to 1/125 of an inch in diameter. In the thick
fine-grained yelk we find a clear round germinal vesicle of about
1/750 of an inch in diameter, and this encloses a small embryonic spot
or nucleolus. Inside the membrane that surrounds the ovum, the
stem-cell of the Ascidia, after fecundation, passes through just the
same metamorphoses as the stem-cell of the Amphioxus. It undergoes
total segmentation; it divides into two, four, eight, sixteen,
thirty-two cells, and so on. By continued total cleavage the morula,
or mulberry-shaped cluster of cells, is formed. Fluid gathers inside
it, and thus we get once more a globular vesicle (the blastula); the
wall of this is a single stratum of cells, the blastoderm. A real
gastrula (a simple bell-gastrula) is formed from the blastula by
invagination, in the same way as in the amphioxus.

Up to this there is no definite ground in the embryology of the
Ascidiae for bringing them into close relationship with the
Vertebrates; the same gastrula is formed in the same way in many other
animals of different stems. But we now find an embryonic process that
is peculiar to the Vertebrates, and that proves irrefragably the
affinity of the Ascidiae to the Vertebrates. From the epidermis of the
gastrula a medullary tube is formed on the dorsal side, and, between
this and the primitive gut, a chorda; these are the organs that are
otherwise only found in Vertebrates. The formation of these very
important organs takes place in the Ascidia-gastrula in precisely the
same way as in that of the Amphioxus. In the Ascidia (as in the other
case) the oval gastrula is first flattened on one side--the subsequent
dorsal side. A groove or furrow (the medullary groove) is sunk in the
middle line of the flat surface, and two parallel longitudinal
swellings arise on either side from the skin layer. These medullary
swellings join together over the furrow, and form a tube; in this
case, again, the neural or medullary tube is at first open in front,
and connected with the primitive gut behind by the neurenteric canal.
Further, in the Ascidia-larva also the two permanent apertures of the
alimentary canal only appear later, as independent and new formations.
The permanent mouth does not develop from the primitive mouth of the
gastrula; this primitive mouth closes up, and the later anus is formed
near it by invagination from without, on the hinder end of the body,
opposite to the aperture of the medullary tube.

During these important processes, that take place in just the same way
in the Amphioxus, a tail-like projection grows out of the posterior
end of the larva-body, and the larva folds itself up within the round
ovolemma in such a way that the dorsal side is curved and the tail is
forced on to the ventral side. In this tail is developed--starting
from the primitive gut--a cylindrical string of cells, the fore end of
which pushes into the body of the larva, between the alimentary canal
and the neural canal, and is no other than the chorda dorsalis. This
important organ had hitherto been found only in the Vertebrates, not a
single trace of it being discoverable in the Invertebrates. At first
the chorda only consists of a single row of large entodermic cells. It
is afterwards composed of several rows of cells. In the Ascidia-larva,
also, the chorda develops from the dorsal middle part of the primitive
gut, while the two coelom-pouches detach themselves from it on both
sides. The simple body-cavity is formed by the coalescence of the two.

When the Ascidia-larva has attained this stage of development it
begins to move about in the ovolemma. This causes the membrane to
burst. The larva emerges from it, and swims about in the sea by means
of its oar-like tail. These free-swimming larvae of the Ascidia have
been known for a long time. They were first observed by Darwin during
his voyage round the world in 1833. They resemble tadpoles in outward
appearance, and use their tails as oars, as the tadpoles do. However,
this lively and highly-developed condition does not last long. At
first there is a progressive development; the foremost part of the
medullary tube enlarges into a brain, and inside this two single
sense-organs are developed, a dorsal auditory vesicle and a ventral
eye. Then a heart is formed on the ventral side of the animal, or the
lower wall of the gut, in the same simple form and at the same spot at
which the heart is developed in man and all the other vertebrates. In
the lower muscular wall of the gut we find a weal-like thickening, a
solid, spindle-shaped string of cells, which becomes hollow in the
centre; it begins to contract in different directions, now forward and
now backward, as is the case with the adult Ascidia. In this way the
sanguineous fluid accumulated in the hollow muscular tube is driven in
alternate directions into the blood-vessels, which develop at both
ends of the cardiac tube. One principal vessel runs along the dorsal
side of the gut, another along its ventral side. The former
corresponds to the aorta and the dorsal vessel in the worms. The other
corresponds to the subintestinal vein and the ventral vessel of the
worms.

With the formation of these organs the progressive development of the
Ascidia comes to an end, and degeneration sets in. The free-swimming
larva sinks to the floor of the sea, abandons its locomotive habits,
and attaches itself to stones, marine plants, mussel-shells, corals,
and other objects; this is done with the part of the body that was
foremost in movement. The attachment is effected by a number of
out-growths, usually three, which can be seen even in the
free-swimming larva. The tail is lost, as there is no further use for
it. It undergoes a fatty degeneration, and disappears with the chorda
dorsalis. The tailless body changes into an unshapely tube, and, by
the atrophy of some parts and the modification of others, gradually
assumes the appearance we have already described.

(FIGURE 2.225. An Appendicaria (Copelata), seen from the . m
mouth, k branchial gut, o gullet, v stomach, a anus, n brain (ganglion
above the gullet), g auditory vesicle, f ciliated groove under the
gills, h heart, t testicles, e ovary, c chorda, s tail.)

Among the living Tunicates there is a very interesting group of small
animals that remain throughout life at the stage of development of the
tailed, free Ascidia-larva, and swim about briskly in the sea by means
of their broad oar-tail. These are the remarkable Copelata
(Appendicaria and Vexillaria, Figure 2.225). They are the only living
Vertebrates that have throughout life a chorda dorsalis and a neural
string above it; the latter must be regarded as the prolongation of
the cerebral ganglion and the equivalent of the medullary tube. Their
branchial gut also opens directly outwards by a pair of branchial
cs. These instructive Copelata, comparable to permanent
Ascidia-larvae, come next to the extinct Prochordonia, those ancient
worms which we must regard as the common ancestors of the Tunicates
and Vertebrates. The chorda of the Appendicaria is a long, cylindrical
string (Figure 2.225 c), and serves as an attachment for the muscles
that work the flat oar-tail.

Among the various modifications which the Ascidia-larva undergoes
after its establishment at the sea-floor, the most interesting (after
the loss of the axial rod) is the atrophy of one of its chief organs,
the medullary tube. In the Amphioxus the spinal marrow continues to
develop, but in the Ascidia the tube soon shrinks into a small and
insignificant nervous ganglion that lies above the mouth and the
gill-crate, and is in accord with the extremely slight mental power of
the animal. This insignificant relic of the medullary tube seems to be
quite beyond comparison with the nervous centre of the vertebrate, yet
it started from the same structure as the spinal cord of the
Amphioxus. The sense-organs that had been developed in the fore part
of the neural tube are also lost; no trace of which can be found in
the adult Ascidia. On the other hand, the alimentary canal becomes a
most extensive organ. It divides presently into two sections--a wide
fore or branchial gut that serves for respiration, and a narrower hind
or hepatic gut that accomplishes digestion. The branchial or head-gut
of the Ascidia is small at first, and opens directly outwards only by
a couple of lateral ducts or gill-cs--a permanent arrangement in
the Copelata. The gill-cs are developed in the same way as in the
Amphioxus. As their number greatly increases we get a large
gill-crate, pierced like lattice work. In the middle line of its
ventral side we find the hypobranchial groove. The mantle or
cloaca-cavity (the atrium) that surrounds the gill-crate is also
formed in the same way in the Ascidia as in the Amphioxus. The
ejection-opening of this peribranchial cavity corresponds to the
branchial pore of the Amphioxus. In the adult Ascidia the branchial
gut and the heart on its ventral side are almost the only organs that
recall the original affinity with the vertebrates.

The further development of the Ascidia in detail has no particular
interest for us, and we will not go into it. The chief result that we
obtain from its embryology is the complete agreement with that of the
Amphioxus in the earliest and most important embryonic stages. They do
not begin to diverge until after the medullary tube and alimentary
canal, and the axial rod with the muscles between the two, have been
formed. The Amphioxus continues to advance, and resembles the
embryonic forms of the higher vertebrates; the Ascidia degenerates
more and more, and at last, in its adult condition, has the appearance
of a very imperfect invertebrate.

If we now look back on all the remarkable features we have encountered
in the structure and the embryonic development of the Amphioxus and
the Ascidia, and compare them with the features of man's embryonic
development which we have previously studied, it will be clear that I
have not exaggerated the importance of these very interesting animals.
It is evident that the Amphioxus from the vertebrate side and the
Ascidia from the invertebrate form the bridge by which we can span the
deep gulf that separates the two great divisions of the animal
kingdom. The radical agreement of the lancelet and the sea-squirt in
the first and most important stages of development shows something
more than their close anatomic affinity and their proximity in
classification; it shows also their real blood-relationship and their
common origin from one and the same stem-form. In this way, it throws
considerable light on the oldest roots of man's genealogical tree.

</section>
<section id="3" fatherid="0" title="DURATION OF THE HISTORY OF OUR STEM" lv="CHAPTER 2.18." priority="0" level="1">
Our comparative investigation of the anatomy and ontogeny of the
Amphioxus and Ascidia has given us invaluable assistance. We have, in
the first place, bridged the wide gulf that has existed up to the
present between the Vertebrates and Invertebrates; and, in the second
place, we have discovered in the embryology of the Amphioxus a number
of ancient evolutionary stages that have long since disappeared from
human embryology, and have been lost, in virtue of the law of
curtailed heredity. The chief of these stages are the spherical
blastula (in its simplest primary form), and the succeeding
archigastrula, the pure, original form of the gastrula which the
Amphioxus has preserved to this day, and which we find in the same
form in a number of Invertebrates of various classes. Not less
important are the later embryonic forms of the coelomula, the
chordula, etc.

Thus the embryology of the Amphioxus and the Ascidia has so much
increased our knowledge of man's stem-history that, although our
empirical information is still very incomplete, there is now no defect
of any great consequence in it. We may now, therefore, approach our
proper task, and reconstruct the phylogeny of man in its chief lines
with the aid of this evidence of comparative anatomy and ontogeny. In
this the reader will soon see the immense importance of the direct
application of the biogenetic law. But before we enter upon the work
it will be useful to make a few general observations that are
necessary to understand the processes a.

We must say a few words with regard to the period in which the human
race was evolved from the animal kingdom. The first thought that
occurs to one in this connection is the vast difference between the
duration of man's ontogeny and phylogeny. The individual man needs
only nine months for his complete development, from the fecundation of
the ovum to the moment when he leaves the maternal womb. The human
embryo runs its whole course in the brief space of forty weeks (as a
rule, 280 days). In many other mammals the time of the embryonic
development is much the same as in man--for instance, in the cow. In
the horse and ass it takes a little longer, forty-three to forty-five
weeks; in the camel, thirteen months. In the largest mammals, the
embryo needs a much longer period for its development in the womb--a
year and a half in the rhinoceros, and ninety weeks in the elephant.
In these cases pregnancy lasts twice as long as in the case of man, or
one and three-quarter years. In the smaller mammals the embryonic
period is much shorter. The smallest mammals, the dwarf-mice, develop
in three weeks; hares in four weeks, rats and marmots in five weeks,
the dog in nine, the pig in seventeen, the sheep in twenty-one and the
goat in thirty-six. Birds develop still more quickly. The chick only
needs, in normal circumstances, three weeks for its full development.
The duck needs twenty-five days, the turkey twenty-seven, the peacock
thirty-one, the swan forty-two, and the cassowary sixty-five. The
smallest bird, the humming-bird, leaves the egg after twelve days.
Hence the duration of individual development within the foetal
membranes is, in the mammals and birds, clearly related to the
absolute size of the body of the animal in question. But this is not
the only determining feature. There are a number of other
circumstances that have an influence on the period of embryonic
development. In the Amphioxus the earliest and most important
embryonic processes take place so rapidly that the blastula is formed
in four hours, the gastrula in six, and the typical vertebrate form in
twenty-four.

In every case the duration of ontogeny shrinks into insignificance
when we compare it with the enormous period that has been necessary
for phylogeny, or the gradual development of the ancestral series.
This period is not measured by years or centuries, but by thousands
and millions of years. Many millions of years had to pass before the
most advanced vertebrate, man, was evolved, step by step, from his
ancient unicellular ancestors. The opponents of evolution, who declare
that this gradual development of the human form from lower animal
forms, and ultimately from a unicellular organism, is an incredible
miracle, forget that the same miracle takes place within the space of
mine months in the embryonic development of every human being. Each of
us has, in the forty weeks--properly speaking, in the first four
weeks--of his development in the womb, passed through the same series
of transformations that our animal ancestors underwent in the course
of millions of years.

It is impossible to determine even approximately, in hundreds or even
thousands of years, the real and absolute duration of the phylogenetic
period. But for some time now we have, through the research of
geologists, been in a position to assign the relative length of the
various sections of the organic history of the earth. The immediate
data for determining this relative length of the geological periods
are found in the thickness of the sedimentary strata--the strata that
have been formed at the bottom of the sea or in fresh water from the
mud or slime deposited there. These successive layers of limestone,
sandstone, slate, marl, etc., which make up the greater part of the
rocks, and are often several thousand feet thick, give us a standard
for computing the relative length of the various periods.

To make the point quite clear, I must say a word about the evolution
of the earth in general, and point out briefly the chief features of
the story. In the first place, we encounter the principle that on our
planet organic life began to exist at a definite period. That
statement is no longer disputed by any competent geologist or
biologist. The organic history of the earth could not commence until
it was possible for water to settle on our planet in fluid condition.
Every organism, without exception, needs fluid water as a condition of
existence, and contains a considerable quantity of it. Our own body,
when fully formed, contains sixty to seventy per cent of water in its
tissues, and only thirty to forty per cent of solid matter. There is
even more water in the body of the child, and still more in the
embryo. In the earlier stages of development the human foetus contains
more than ninety per cent of water, and not ten per cent of solids. In
the lower marine animals, especially certain medusae, the body
consists to the extent of more than ninety-nine per cent of sea-water,
and has not one per cent of solid matter. No organism can exist or
discharge its functions without water. No water, no life!

But fluid water, on which the existence of life primarily depends,
could not exist on our planet until the temperature of the surface of
the incandescent sphere had sunk to a certain point. Up to that time
it remained in the form of steam. But as soon as the first fluid water
could be condensed from the envelope of steam, it began its geological
action, and has continued down to the present day to modify the solid
crust of the earth. The final outcome of this incessant action of the
water--wearing down and dissolving the rocks in the form of rain,
hail, snow, and ice, as running stream or boiling surge--is the
formation of mud. As Huxley says in his admirable Lectures on the
Causes of Phenomena in Organic Nature, the chief document as to the
past history of our earth is mud; the question of the history of past
ages resolves itself into a question about the formation of mud.

As I have said, it is possible to form an approximate idea of the
relative age of the various strata by comparing them at different
parts of the earth's surface. Geologists have long been agreed that
there is a definite historical succession of the different strata. The
various superimposed layers correspond to successive periods in the
organic history of the earth, in which they were deposited in the form
of mud at the bottom of the sea. The mud was gradually converted into
stone. This was lifted out of the water owing to variations in the
earth's surface, and formed the mountains. As a rule, four or five
great divisions are distinguished in the organic history of the earth,
corresponding to the larger and smaller groups of the sedimentary
strata. The larger periods are then sub-divided into a series of
smaller ones, which usually number from twelve to fifteen. The
comparative thickness of the groups of strata enables us to make an
approximate calculation of the relative length of these various
periods of time. We cannot say, it is true, "In a century a stratum of
a certain thickness (about two feet) is formed on the average;
therefore, a layer 1000 feet thick must be 500,000 years old."
Different strata of the same thickness may need very different periods
for their formation. But from the thickness or size of the stratum we
can draw some conclusion as to the RELATIVE length of the period.

The first and oldest of the four or five chief divisions of the
organic history of the earth is called the primordial, archaic, or
archeozoic period. If we compute the total average thickness of the
sedimentary strata at about 130,000 feet, this first period comprises
70,000 feet, or the greater part of the whole. For this and other
reasons we may at once conclude that the corresponding primordial or
archeolithic period must have been in itself much longer than the
whole of the remaining periods together, from its close to the present
day. It was probably much longer than the figures I have quoted (7 :
6) indicate--possibly 9 : 6. Of late years the thickness of the
archaic rocks has been put at 90,000 feet.

SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS, OR THE FOSSILIFEROUS
STRATA OF THE CRUST.

COLUMN 1 : Groups (V. down to I.).

COLUMN 2 : Systems (XIV. down to I.).

COLUMN 3 : Formations (38 down to 1).

COLUMN 4 : Synonyms of Formations.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata
: XIV. Recent (alluvium) : 38. Present : Upper alluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata
: XIV. Recent (alluvium) : 37. Recent : Lower alluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata
: XIII. Pleistocene (diluvium) : 36. Post-glacial : Upper diluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata
: XIII. Pleistocene (diluvium) : 35. Glacial : Lower diluvial.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII.
Pliocene (neo-tertiary) : 34. Arverne : Upper pliocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII.
Pliocene (neo-tertiary) : 33. Subapennine : Lower pliocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI.
Miocene (middle tertiary) : 32. Falun : Upper miocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI.
Miocene (middle tertiary) : 31. Limbourg : Lower miocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb.
Oligocene (old tertiary) : 30. Aquitaine : Upper oligocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb.
Oligocene (old tertiary) : 29. Ligurium : Lower oligocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa.
Eocene (primitive tertiary) : 28. Gypsum : Upper eocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa.
Eocene (primitive tertiary) : 27. Coarse chalk : Middle eocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa.
Eocene (primitive tertiary) : 26. London clay : Lower eocene.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX.
Chalk (cretaceous) : 25. White chalk. : Upper cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX.
Chalk (cretaceous) : 24. Green Sand : Middle cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX.
Chalk (cretaceous) : 23. Neocomian : Lower cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX.
Chalk (cretaceous) : 22. Wealden : Weald-formation.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII.
Jurassic : 21. Portland : Upper oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII.
Jurassic : 20. Oxford : Middle oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII.
Jurassic : 19. Bath : Lower oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII.
Jurassic : 18. Lias : Liassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII.
Triassic : 17. Keuper : Upper triassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII.
Triassic : 16. Muschelkalk : Middle triassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII.
Triassic : 15. Bunter : Lower triassic.

II. Paleolithic group, or paleozoic (primary) group of strata : VIb.
Permian : 14. Zechstein : Upper permian.

II. Paleolithic group, or paleozoic (primary) group of strata : VIb.
Permian : 13. Neurot sand : Lower permian.

II. Paleolithic group, or paleozoic (primary) group of strata : VIa.
Carboniferous (coal-measures) : 12. Carboniferous sandstone : Upper
carboniferous.

II. Paleolithic group, or paleozoic (primary) group of strata : VIa.
Carboniferous (coal-measures) : 11. Carboniferous limestone : Lower
carboniferous.

II. Paleolithic group, or paleozoic (primary) group of strata : V.
Devonian : 10. Pilton : Upper devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : V.
Devonian : 9. Ilfracombe : Middle devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : V.
Devonian : 8. Linton : Lower devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV.
Silurian : 7. Ludlow : Upper silurian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV.
Silurian : 6. Wenlock : Middle silurian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV.
Silurian : 5. Llandeilo : Lower silurian.

I. Archeolithic group, or archeozoic (primordial) group of strata :
III. Cambrian : 4. Potsdam : Upper cambrian.

I. Archeolithic group, or archeozoic (primordial) group of strata :
III. Cambrian : 3. Longmynd : Lower cambrian.

I. Archeolithic group, or archeozoic (primordial) group of strata :
II. Huronian : 2. Labrador : Upper laurentian.

I. Archeolithic group, or archeozoic (primordial) group of strata : I.
Laurentian : 1. Ottawa : Lower laurentian.

The primordial period falls into three subordinate sections--the
Laurentian, Huronian, and Cambrian, corresponding to the three chief
groups of rocks that comprise the archaic formation. The immense
period during which these rocks were forming in the primitive ocean
probably comprises more than 50,000,000 years. At the commencement of
it the oldest and simplest organisms were formed by spontaneous
generation--the Monera, with which the history of life on our planet
opened. From these were first developed unicellular organisms of the
simplest character, the Protophyta and Protozoa (paulotomea, amoebae,
rhizopods, infusoria, and other Protists). During this period the
whole of the invertebrate ancestors of the human race were evolved
from the unicellular organisms. We can deduce this from the fact that
we already find remains of fossilised fishes (Selachii and Ganoids)
towards the close of the following Silurian period. These are much
more advanced and much younger than the lowest vertebrate, the
Amphioxus, and the numerous skull-less vertebrates, related to the
Amphioxus, that must have lived at that time. The whole of the
invertebrate ancestors of the human race must have preceded these.

The primordial age is followed by a much shorter division, the
paleozoic or Primary age. It is divided into four long periods, the
Silurian, Devonian, Carboniferous, and Permian. The Silurian strata
are particularly interesting because they contain the first fossil
traces of vertebrates--teeth and scales of Selachii (Palaeodus) in the
lower, and Ganoids (Pteraspis) in the upper Silurian. During the
Devonian period the "old red sandstone" was formed; during the
Carboniferous period were deposited the vast coal-measures that yield
us our chief combustive material; in the Permian (or the Dyas), in
fine, the new red sandstone, the Zechstein (magnesian limestone), and
the Kupferschiefer (marl-slate) were formed. The collective depth of
these strata is put at 40,000 to 45,000 feet. In any case, the
paleozoic age, taken as a whole, was much shorter than the preceding
and much longer than the subsequent periods. The strata that were
deposited during this primary epoch contain a large number of fossils;
besides the invertebrate species there are a good many vertebrates,
and the fishes preponderate. There were so many fishes, especially
primitive fishes (of the shark type) and plated fishes, during the
Devonian, and also during the Carboniferous and Permian periods, that
we may describe the whole paleozoic period as "the age of fishes."
Among the paleozoic plated fishes or Ganoids the Crossopterygii and
the Ctenodipterina (dipneusts) are of great importance.

During this period some of the fishes began to adapt themselves to
living on land, and so gave rise to the class of the amphibia. We find
in the Carboniferous period fossilised remains of five-toed amphibia,
the oldest terrestrial, air-breathing vertebrates. These amphibia
increase in variety in the Permian epoch. Towards the close of it we
find the first Amniotes, the ancestors of the three higher classes of
Vertebrates. These are lizard-like animals; the first to be discovered
was the Proterosaurus, from the marl at Eisenach. The rise of the
earliest Amniotes, among which must have been the common ancestor of
the reptiles, birds, and mammals, is put back towards the close of the
paleozoic age by the discovery of these reptile remains. The ancestors
of our race during this period were at first represented by true
fishes, then by dipneusts and amphibia, and finally by the earliest
Amniotes, or the Protamniotes.

The third chief section of the organic history of the earth is the
Mesozoic or Secondary period. This again is subdivided into three
divisions Triassic, Jurassic, and Cretaceous. The thickness of the
strata that were deposited in this period, from the beginning of the
Triassic to the end of the Cretaceous period, is altogether about
15,000 feet, or not half as much as the paleozoic deposits. During
this period there was a very brisk and manifold development in all
branches of the animal kingdom. There were especially a number of new
and interesting forms evolved in the vertebrate stem. Bony fishes
(Teleostei) make their first appearance. Reptiles are found in
extraordinary variety and number; the extinct giant-serpents
(dinosauria), the sea-serpents (halisauria), and the flying lizards
(pterosauria) are the most remarkable and best known of these. On
account of this predominance of the reptile-class, the period is
called "the age of reptiles." But the bird-class was also evolved
during this period; they certainly originated from some division of
the lizard-like reptiles. This is proved by the embryological identity
of the birds and reptiles and their comparative anatomy, and, among
other features, from the circumstance that in this period there were
birds with teeth in their jaws and with tails like lizards
(Archeopteryx, Odontornis).

Finally, the most advanced and (for us) the most important class of
the vertebrates, the mammals, made their appearance during the
mesozoic period. The earliest fossil remains of them were found in the
latest Triassic strata--lower jaws of small ungulates and marsupials.
More numerous remains are found a little later in the Jurassic, and
some in the Cretaceous. All the mammal remains that we have from this
section belong to the lower promammals and marsupials; among these
were most certainly the ancestors of the human race. On the other
hand, we have not found a single indisputable fossil of any higher
mammal (a placental) in the whole of this period. This division of the
mammals, which includes man, was not developed until later, towards
the close of this or in the following period.

The fourth section of the organic history of the earth, the Tertiary
or Cenozoic age, was much shorter than the preceding. The strata that
were deposited during this period have a collective thickness of only
about 3,000 feet. It is subdivided into four sections--the Eocene,
Oligocene, Miocene, and Pliocene. During these periods there was a
very varied development of higher plant and animal forms; the fauna
and flora of our planet approached nearer and nearer to the character
that they bear to-day. In particular, the most advanced class, the
mammals, began to preponderate. Hence the Tertiary period may be
called "the age of mammals." The highest section of this class, the
placentals, now made their appearance; to this group the human race
belongs. The first appearance of man, or, to be more precise, the
development of man from some closely-related group of apes, probably
falls in either the miocene or the pliocene period, the middle or the
last section of the Tertiary period. Others believe that man properly
so-called--man endowed with speech--was not evolved from the
non-speaking ape-man (Pithecanthropus) until the following, the
anthropozoic, age.

In this fifth and last section of the organic history of the earth we
have the full development and dispersion of the various races of men,
and so it is called the Anthropozoic as well as the Quaternary period.
In the imperfect condition of paleontological and ethnographical
science we cannot as yet give a confident answer to the question
whether the evolution of the human race from some extinct ape or lemur
took place at the beginning of this or towards the middle or the end
of the Tertiary period. However, this much is certain: the development
of civilisation falls in the anthropozoic age, and this is merely an
insignificant fraction of the vast period of the whole history of
life. When we remember this, it seems ridiculous to restrict the word
"history" to the civilised period. If we divide into a hundred equal
parts the whole period of the history of life, from the spontaneous
generation of the first Monera to the present day, and if we then
represent the relative duration of the five chief sections or ages, as
calculated from the average thickness of the strata they contain, as
percentages of this, we get something like the following relation:--

I. Archeolithic or archeozoic (primordial) age : 53 : 6.

II. Paleolithic or paleozoic (primary) age : 32 : 1.

III. Mesolithic or mesozoic (secondary) age : 11 : 5.

IV. Cenolithic or cenozoic (tertiary) age : 2 : 3

V. Anthropolithic or anthropozoic (quaternary) age : 0 : 5.

Total : 100 : 0.

In any case, the "historical period" is an insignificant quantity
compared with the vast length of the preceding ages, in which there
was no question of human existence on our planet. Even the important
Cenozoic or Tertiary period, in which the first placentals or higher
mammals appear, probably amounts to little over two per cent of the
whole organic age.

Before we approach our proper task, and, with the aid of our
ontogenetic acquirements and the biogenetic law, follow step by step
the paleontological development of our animal ancestors, let us glance
for a moment at another, and apparently quite remote, branch of
science, a general consideration of which will help us in the solving
of a difficult problem. I mean the science of comparative philology.
Since Darwin gave new life to biology by his theory of selection, and
raised the question of evolution on all sides, it has often been
pointed out that there is a remarkable analogy between the development
of languages and the evolution of species. The comparison is perfectly
just and very instructive. We could hardly find a better analogy when
we are dealing with some of the difficult and obscure features of the
evolution of species. In both cases we find the action of the same
natural laws.

All philologists of any competence in their science now agree that all
human languages have been gradually evolved from very rudimentary
beginnings. The idea that speech is a gift of the gods--an idea held
by distinguished authorities only fifty years ago--is now generally
abandoned, and only supported by theologians and others who admit no
natural development whatever. Speech has been developed simultaneously
with its organs, the larynx and tongue, and with the functions of the
brain. Hence it will be quite natural to find in the evolution and
classification of languages the same features as in the evolution and
classification of organic species. The various groups of languages
that are distinguished in philology as primitive, fundamental, parent,
and daughter languages, dialects, etc., correspond entirely in their
development to the different categories which we classify in zoology
and botany as stems, classes, orders, families, genera, species, and
varieties. The relation of these groups, partly co-ordinate and partly
subordinate, in the general scheme is just the same in both cases; and
the evolution follows the same lines in both.

When, with the assistance of this tree, we follow the formation of the
various languages that have been developed from the common root of the
ancient Indo-Germanic tongue, we get a very clear idea of their
phylogeny. We shall see at the same time how analogous this is to the
development of the various groups of vertebrates that have arisen from
the common stem-form of the primitive vertebrate. The ancient
Indo-Germanic root-language divided first into two principal
stems--the Slavo-Germanic and the Aryo-Romanic. The Slavo-Germanic
stem then branches into the ancient Germanic and the ancient
Slavo-Lettic tongues; the Aryo-Romanic into the ancient Aryan and the
ancient Greco-Roman. If we still follow the genealogical tree of these
four Indo-Germanic tongues, we find that the ancient Germanic divides
into three branches--the Scandinavian, the Gothic, and the German.
From the ancient German came the High German and Low German; to the
latter belong the Frisian, Saxon, and modern Low-German dialects. The
ancient Slavo-Lettic divided first into a Baltic and a Slav language.
The Baltic gave rise to the Lett, Lithuanian, and old-Prussian
varieties; the Slav to the Russian and South-Slav in the south-east,
and to the Polish and Czech in the west.

We find an equally prolific branching of its two chief stems when we
turn to the other division of the Indo-Germanic languages. The
Greco-Roman divided into the Thracian (Albano-Greek) and the
Italo-Celtic. From the latter came the divergent branches of the
Italic (Roman and Latin) in the south, and the Celtic in the north:
from the latter have been developed all the British (ancient British,
ancient Scotch, and Irish) and Gallic varieties. The ancient Aryan
gave rise to the numerous Iranian and Indian languages.

This "comparative anatomy" and evolution of languages admirably
illustrates the phylogeny of species. It is clear that in structure
and development the primitive languages, mother and daughter
languages, and varieties, correspond exactly to the classes, orders,
genera, and species of the animal world. In both cases the "natural"
system is phylogenetic. As we have been convinced from comparative
anatomy and ontogeny, and from paleontology, that all past and living
vertebrates descend from a common ancestor, so the comparative study
of dead and living Indo-Germanic tongues proves beyond question that
they are all modifications of one primitive language. This view of
their origin is now accepted by all the chief philologists who have
worked in this branch and are unprejudiced.

But the point to which I desire particularly to draw the reader's
attention in this comparison of the Indo-Germanic languages with the
branches of the vertebrate stem is, that one must never confuse direct
descendants with collateral branches, nor extinct forms with living.
This confusion is very common, and our opponents often make use of the
erroneous ideas it gives rise to for the purpose of attacking
evolution generally. When, for instance, we say that man descends from
the ape, this from the lemur, and the lemur from the marsupial, many
people imagine that we are speaking of the living species of these
orders of mammals that they find stuffed in our museums. Our opponents
then foist this idea on us, and say, with more astuteness than
intelligence, that it is quite impossible; or they ask us, by way of
physiological experiment, to turn a kangaroo into a lemur, a lemur
into a gorilla, and a gorilla into a man! The demand is childish, and
the idea it rests on erroneous. All these living forms have diverged
more or less from the ancestral form; none of them could engender the
same posterity that the stem-form really produced thousands of years
ago.

It is certain that man has descended from some extinct mammal; and we
should just as certainly class this in the order of apes if we had it
before us. It is equally certain that this primitive ape descended in
turn from an unknown lemur, and this from an extinct marsupial. But it
is just as clear that all these extinct ancestral forms can only be
claimed as belonging to the living order of mammals in virtue of their
essential internal structure and their resemblance in the decisive
anatomic characteristics of each ORDER. In external appearance, in the
characteristics of the GENUS or SPECIES, they would differ more or
less, perhaps very considerably, from all living representatives of
those orders. It is a universal and natural procedure in phylogenetic
development that the stem-forms themselves, with their specific
peculiarities, have been extinct for some time. The forms that
approach nearest to them among the living species are more or
less--perhaps very substantially--different from them. Hence in our
phylogenetic inquiry and in the comparative study of the living,
divergent descendants, there can only be a question of determining the
greater or less remoteness of the latter from the ancestral form. Not
a single one of the older stem-forms has continued unchanged down to
our time.

We find just the same thing in comparing the various dead and living
languages that have developed from a common primitive tongue. If we
examine our genealogical tree of the Indo-Germanic languages in this
light, we see at once that all the older or parent tongues, of which
we regard the living varieties of the stem as divergent daughter or
grand-daughter languages, have been extinct for some time. The
Aryo-Romanic and the Slavo-Germanic tongues have completely
disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and
the ancient Germanic. Even their daughters and grand-daughters have
been lost; all the living Indo-Germanic languages are only related in
the sense that they are divergent descendants of common stem-forms.
Some forms have diverged more, and some less, from the original
stem-form.

This easily demonstrable fact illustrates very well the analogous case
of the origin of the vertebrate species. Phylogenetic comparative
philology here yields a strong support to phylogenetic comparative
zoology. But the one can adduce more direct evidence than the other,
as the paleontological material of philology--the old monuments of the
extinct tongue--have been preserved much better than the
paleontological material of zoology, the fossilised bones and imprints
of vertebrates.

We may, however, trace man's genealogical tree not only as far as the
lower mammals, but much further--to the amphibia, to the shark-like
primitive fishes, and, in fine, to the skull-less vertebrates that
closely resembled the Amphioxus. But this must not be understood in
the sense that the existing Amphioxus, or the sharks or amphibia of
to-day, can give us any idea of the external appearance of these
remote stem-forms. Still less must it be thought that the Amphioxus or
any actual shark, or any living species of amphibia, is a real
ancestral form of the higher vertebrates and man. The statement can
only rationally mean that the living forms I have referred to are
COLLATERAL LINES that are much more closely related to the extinct
stem-forms, and have retained the resemblance much better, than any
other animals we know. They are still so like them in regard to their
distinctive internal structure that we should put them in the same
class with the extinct forms if we had these before us. But no direct
descendants of these earlier forms have remained unchanged. Hence we
must entirely abandon the idea of finding direct ancestors of the
human race in their characteristic EXTERNAL FORM among the living
species of animals. The essential and distinctive features that still
connect living forms more or less closely with the extinct common
stem-forms lie in the internal structure, not the external appearance.
The latter has been much modified by adaptation. The former has been
more or less preserved by heredity.

Comparative anatomy and ontogeny prove beyond question that man is a
true vertebrate, and, therefore, man's special genealogical tree must
be connected with that of the other Vertebrates, which spring from a
common root with him. But we have also many important grounds in
comparative anatomy and ontogeny for assuming a common origin for all
the Vertebrates. If the general theory of evolution is correct, all
the Vertebrates, including man, come from a single common ancestor, a
long-extinct "Primitive Vertebrate." Hence the genealogical tree of
the Vertebrates is at the same time that of the human race.

Our task, therefore, of constructing man's genealogy becomes the
larger aim of discovering the genealogy of the entire vertebrate stem.
As we now know from the comparative anatomy and ontogeny of the
Amphioxus and the Ascidia, this is in turn connected with the
genealogical tree of the Invertebrates (directly with that of the
Vermalia), but has no direct connection with the independent stems of
the Articulates, Molluscs, and Echinoderms. If we do thus follow our
ancestral tree through various stages down to the lowest worms, we
come inevitably to the Gastraea, that most instructive form that gives
the clearest possible picture of an animal with two germinal layers.
The Gastraea itself has originated from the simple multicellular
vesicle, the Blastaea, and this in turn must have been evolved from
the lowest circle of unicellular animals, to which we give the name of
Protozoa. We have already considered the most important primitive type
of these, the unicellular Amoeba, which is extremely instructive when
compared with the human ovum. With this we reach the lowest of the
solid data to which we are to apply our biogenetic law, and by which
we may deduce the extinct ancestor from the embryonic form. The
amoeboid nature of the young ovum and the unicellular condition in
which (as stem-cell or cytula) every human being begins its existence
justify us in affirming that the earliest ancestors of the human race
were simple amoeboid coils.

But the further question now arises: "Whence came these first amoebae
with which the history of life began at the commencement of the
Laurentian epoch?" There is only one answer to this. The earliest
unicellular organisms can only have been evolved from the simplest
organisms we know, the Monera. These are the simplest living things
that we can conceive. Their whole body is nothing but a particle of
plasm, a granule of living albuminous matter, discharging of itself
all the essential vital functions that form the material basis of
life. Thus we come to the last, or, if you prefer, the first, question
in connection with evolution--the question of the origin of the
Monera. This is the real question of the origin of life, or of
spontaneous generation.

We have neither space nor occasion to go further in this Chapter into
the question of spontaneous generation. For this I must refer the
reader to the fifteenth chapter of the History of Creation, and
especially to the second book of the General Morphology, or to the
essay on "The Monera and Spontaneous Generation" in my Studies of the
Monera and other Protists.* (* The English reader will find a luminous
and up-to-date chapter on the subject in Haeckel's recently written
and translated Wonders of Life.--Translator.) I have given there fully
my own view of this important question. The famous botanist Nageli
afterwards (1884) developed the same ideas. I will only say a few
words here about this obscure question of the origin of life, in so
far as our main subject, organic evolution in general, is affected by
it. Spontaneous generation, in the definite and restricted sense in
which I maintain it, and claim that it is a necessary hypothesis in
explaining the origin of life, refers solely to the evolution of the
Monera from inorganic carbon-compounds. When living things made their
first appearance on our planet, the very complex nitrogenous compound
of carbon that we call plasson, which is the earliest material
embodiment of vital action, must have been formed in a purely chemical
way from inorganic carbon-compounds. The first Monera were formed in
the sea by spontaneous generation, as crystals are formed in the
mother-water. Our demand for a knowledge of causes compels us to
assume this. If we believe that the whole inorganic history of the
earth has proceeded on mechanical principles without any intervention
of a Creator, and that the history of life also has been determined by
the same mechanical laws; if we see that there is no need to admit
creative action to explain the origin of the various groups of
organisms; it is utterly irrational to assume such creative action in
dealing with the first appearance of organic life on the earth.

This much-disputed question of "spontaneous generation" seems so
obscure, because people have associated with the term a mass of very
different, and often very absurd, ideas, and have attempted to solve
the difficulty by the crudest experiments. The real doctrine of the
spontaneous generation of life cannot possibly be refuted by
experiments. Every experiment that has a negative result only proves
that no organism has been formed out of inorganic matter in the
conditions--highly artificial conditions--we have established. On the
other hand, it would be exceedingly difficult to prove the theory by
way of experiment; and even if Monera were still formed daily by
spontaneous generation (which is quite possible), it would be very
difficult, if not impossible, to find a solid proof of it. Those who
will not admit the spontaneous generation of the first living things
in our sense must have recourse to a supernatural miracle; and this
is, as a matter of fact, the desperate resource to which our "exact"
scientists are driven, to the complete abdication of reason.

A famous English physicist, Lord Kelvin (then Sir W. Thomson),
attempted to dispense with the hypothesis of spontaneous generation by
assuming that the organic inhabitants of the earth were developed from
germs that came from the inhabitants of other planets, and that
chanced to fall on our planet on fragments of their original home, or
meteorites. This hypothesis found many supporters, among others the
distinguished German physicist, Helmholtz. However, it was refuted in
1872 by the able physicist, Friedrich Zollner, of Leipzig, in his
work, On the Nature of Comets. He showed clearly how unscientific this
hypothesis is; firstly in point of logic, and secondly in point of
scientific content. At the same time he pointed out that our
hypothesis of spontaneous generation is "a necessary condition for
understanding nature according to the law of causality."

I repeat that we must call in the aid of the hypothesis only as
regards the Monera, the structureless "organisms without organs."
Every complex organism must have been evolved from some lower
organism. We must not assume the spontaneous generation of even the
simplest cell, for this itself consists of at least two parts--the
internal, firm nuclear substance, and the external, softer cellular
substance or the protoplasm of the cell-body. These two parts must
have been formed by differentiation from the indifferent plasson of a
moneron, or a cytode. For this reason the natural history of the
Monera is of great interest; here alone can we find the means to
overcome the chief difficulties of the problem of spontaneous
generation. The actual living Monera are specimens of such organless
or structureless organisms, as they must have boon formed by
spontaneous generation at the commencement of the history of life.

</section>
<section id="4" fatherid="0" title="OUR PROTIST ANCESTORS" lv="CHAPTER 2.19." priority="0" level="1">
Under the guidance of the biogenetic law, and on the basis of the
evidence we have obtained, we now turn to the interesting task of
determining the series of man's animal ancestors. Phylogeny us a whole
is an inductive science. From the totality of the biological processes
in the life of plants, animals, and man we have gathered a confident
inductive idea that the whole organic population of our planet has
been moulded on a harmonious law of evolution. All the interesting
phenomena that we meet in ontogeny and paleontology, comparative
anatomy and dysteleology, the distribution and habits of
organisms--all the important general laws that we abstract from the
phenomena of these sciences, and combine in harmonious unity--are the
broad bases of our great biological induction.

But when we come to the application of this law, and seek to determine
with its aid the origin of the various species of organisms, we are
compelled to frame hypotheses that have essentially a DEDUCTIVE
character, and are inferences from the general law to particular
cases. But these special deductions are just as much justified and
necessitated by the rigorous laws of logic as the inductive
conclusions on which the whole theory of evolution is built. The
doctrine of the animal ancestry of the human race is a special
deduction of this kind, and follows with logical necessity from the
general inductive law of evolution.

I must point out at once, however, that the certainty of these
evolutionary hypotheses, which rest on clear special deductions, is
not always equally strong. Some of these inferences are now beyond
question; in the case of others it depends on the knowledge and the
competence of the inquirer what degree of certainty he attributes to
them. In any case, we must distinguish between the ABSOLUTE certainty
of the general (inductive) theory of descent and the RELATIVE
certainty of special (deductive) evolutionary hypotheses. We can never
determine the whole ancestral series of an organism with the same
confidence with which we hold the general theory of evolution as the
sole scientific explanation of organic modifications. The special
indication of stem-forms in detail will always be more or less
incomplete and hypothetical. This is quite natural. The evidence on
which we build is imperfect, and always will be imperfect; just as in
comparative philology.

The first of our documents, paleontology, is exceedingly incomplete.
We know that all the fossils yet discovered are only an insignificant
fraction of the plants and animals that have lived on our planet. For
every single species that has been preserved for us in the rocks there
are probably hundreds, perhaps thousands, of extinct species that have
 no trace behind them. This extreme and very unfortunate
incompleteness of the paleontological evidence, which cannot be
pointed out too often, is easily explained. It is absolutely
inevitable in the circumstances of the fossilisation of organisms. It
is also due in part to the incompleteness of our knowledge in this
branch. It must be borne in mind that the great majority of the
stratified rocks that compose the crust of the earth have not yet been
opened. We have only a few specimens of the innumerable fossils that
are buried in the vast mountain ranges of Asia and Africa. Only a part
of Europe and North America has been investigated carefully. The whole
of the fossils known to us certainly do not amount to a hundredth part
of the remains that are really buried in the crust of the earth. We
may, therefore, look forward to a rich harvest in the future as
regards this science. However, our paleontological evidence will (for
reasons that I have fully explained in the sixteenth chapter of the
History of Creation) always be defective.

The second chief source of evidence, ontogeny, is not less incomplete.
It is the most important source of all for special phylogeny; but it
has great defects, and often fails us. We must, above all, clearly
distinguish between palingenetic and cenogenetic phenomena. We must
never forget that the laws of curtailed and disturbed heredity often
make the original course of development almost unrecognisable. The
recapitulation of phylogeny by ontogeny is only fairly complete in a
few cases, and is never wholly complete. As a rule, it is precisely
the earliest and most important embryonic stages that suffer most from
alteration and condensation. The earlier embryonic forms have had to
adapt themselves to new circumstances, and so have been modified. The
struggle for existence has had just as profound an influence on the
freely moving and still immature young forms as on the adult forms.
Hence in the embryology of the higher animals, especially,
palingenesis is much restricted by cenogenesis; it is to-day, as a
rule, only a faded and much altered picture of the original evolution
of the animal's ancestors. We can only draw conclusions from the
embryonic forms to the stem-history with the greatest caution and
discrimination. Moreover, the embryonic development itself has only
been fully studied in a few species.

Finally, the third and most valuable source of evidence, comparative
anatomy, is also, unfortunately, very imperfect; for the simple reason
that the whole of the living species of animals are a mere fraction of
the vast population that has dwelt on our planet since the beginning
of life. We may confidently put the total number of these at more than
a million species. The number of animals whose organisation has been
studied up to the present in comparative anatomy is proportionately
very small. Here, again, future research will yield incalculable
treasures. But, for the present, in view of this patent incompleteness
of our chief sources of evidence, we must naturally be careful not to
lay too much stress in human phylogeny on the particular animals we
have studied, or regard all the various stages of development with
equal confidence as stem-forms.

In my first efforts to construct the series of man's ancestors I drew
up a list of, at first ten, afterwards twenty to thirty, forms that
may be regarded more or less certainly as animal ancestors of the
human race, or as stages that in a sense mark off the chief sections
in the long story of evolution from the unicellular organism to man.
Of these twenty to thirty stages, ten to twelve belong to the older
group of the Invertebrates and eighteen to twenty to the younger
division of the Vertebrates.

In approaching, now, the difficult task of establishing the
evolutionary succession of these thirty ancestors of humanity since
the beginning of life, and in venturing to lift the veil that covers
the earliest secrets of the earth's history, we must undoubtedly look
for the first living things among the wonderful organisms that we call
the Monera; they are the simplest organisms known to us--in fact, the
simplest we can conceive. Their whole body consists merely of a simple
particle or globule of structureless plasm or plasson. The discoveries
of the last four decades have led us to believe with increasing
certainty that wherever a natural body exhibits the vital processes of
nutrition, reproduction, voluntary movement, and sensation, we have
the action of a nitrogenous carbon-compound of the chemical group of
the albuminoids; this plasm (or protoplasm) is the material basis of
all vital functions. Whether we regarded the function, in the monistic
sense, as the direct action of the material substratum, or whether we
take matter and force to be distinct things in the dualistic sense, it
is certain that we have not as yet found any living organism in which
the exercise of the vital functions is not inseparably bound up with
plasm.

The soft slimy plasson of the body of the moneron is generally called
"protoplasm," and identified with the cellular matter of the ordinary
plant and animal cells. But we must, to be accurate, distinguish
between the plasson of the cytodes and the protoplasm of the cells.
This distinction is of the utmost importance for the purposes of
evolution. As I have often said, we must recognise two different
stages of development in these "elementary organisms," or plastids
("builders"), that represent the ultimate units of organic
individuality. The earlier and lower stage are the unnucleated
cytodes, the body of which consists of only one kind of albuminous
matter--the homogeneous plasson or "formative matter." The later and
higher stage are the nucleated cells, in which we find a
differentiation of the original plasson into two different formative
substances--the caryoplasm of the nucleus and the cytoplasm of the
body of the cell (cf. Chapter 1.6.)

(FIGURE 2.226. Chroococcus minor (Nageli), magnified 1500 times. A
phytomoneron, the globular plastids of which secrete a gelatinous
structureless membrane. The unnucleated globule of plasm (bluish-green
in colour) increases by simple cleavage (a to d).

The Monera are permanent cytodes. Their whole body consists of soft,
structureless plasson. However carefully we examine it with our finest
chemical reagents and most powerful microscopes, we can find no
definite parts or no anatomic structure in it. Hence, the Monera are
literally organisms without organs; in fact, from the philosophic
point of view they are not organisms at all, since they have no
organs. They can only be called organisms in the sense that they are
capable of the vital functions of nutrition, reproduction, sensation,
and movement. If we were to try to imagine the simplest possible
organism, we should frame something like the moneron.

The Monera that we find to-day in various forms fall into two groups
according to the nature of their nutrition--the Phytomonera and the
Zoomonera; from the physiological point of view, the former are the
simplest specimens of the plant (phyton) kingdom, and the latter of
the animal (zoon) world. The Phytomonera, especially in their simplest
form, the Chromacea (Phycochromacea or Cyanophycea), are the most
primitive and the oldest of living organisms. The typical genus
Chroococcus (Figure 2.226) is represented by several fresh-water
species, and often forms a very delicate bluish-green deposit on
stones and wood in ponds and ditches. It consists of round, light
green particles, from 1/7000 to 1/2500 of an inch in diameter.

(FIGURE 2.227. Aphanocapsa primordialis (Nageli), magnified 1000
times. A phytomoneron, the round plastids of which (bluish-green in
colour) secrete a shapeless gelatinous mass; in this the unnucleated
cytodes increase continually by simple cleavage.)

The whole life of these homogeneous globules of plasm consists of
simple growth and reproduction by cleavage. When the tiny particle has
reached a certain size by the continuous assimilation of inorganic
matter, it divides into two equal halves, by a constriction in the
middle. The two daughter-monera that are thus formed immediately begin
a similar vital process. It is the same with the brown Procytella
primordialis (formerly called the Protococcus marinus); it forms large
masses of floating matter in the arctic seas. The tiny plasma-globules
of this species are of a greenish-brown colour, and have a diameter of
1/10,000 to 1/5000 of an inch. There is no membrane discoverable in
the simplest Chroococcacea, but we find one in other members of the
same family; in Aphanocapsa (Figure 2.227) the enveloping membranes of
the social plastids combine; in Gloecapsa they are retained through
several generations, so that the little plasma-globules are enfolded
in many layers of membrane.

Next to the Chromacea come the Bacteria, which have been evolved from
them by the remarkable change in nutrition which gives us the simple
explanation of the differentiation of plant and animal in the protist
kingdom. The Chromacea build up their plasm directly from inorganic
matter; the Bacteria feed on organic matter. Hence, if we logically
divide the protist kingdom into plasma-forming Protophyta and
plasma-consuming Protozoa, we must class the Bacteria with the latter;
it is quite illogical to describe them--as is still often done--as
Schizomycetes, and class them with the true fungi. The Bacteria, like
the Chromacea, have no nucleus. As is well-known, they play an
important part in modern biology as the causes of fermentation and
putrefaction, and of tuberculosis, typhus, cholera, and other
infectious diseases, and as parasites, etc. But we cannot linger now
to deal with these very interesting features; the Bacteria have no
relation to man's genealogical tree.

We may now turn to consider the remarkable Protamoeba, or unnucleated
Amoeba. I have, in the first volume, pointed out the great importance
of the ordinary Amoeba in connection with several weighty questions of
general biology. The tiny Protamoebae, which are found both in fresh
and salt water, have the same unshapely form and irregular movements
of their simple naked body as the real Amoebae; but they differ from
them very materially in having no nucleus in their cell-body. The
short, blunt, finger-like processes that are thrust out at the surface
of the creeping Protamoeba serve for getting food as well as for
locomotion. They multiply by simple cleavage (Figure 2.228).

(FIGURE 2.228. A moneron (Protamoeba) in the act of reproduction. A
The whole moneron, moving like an ordinary amoeba by thrusting out
changeable processes. B It divides into two halves by a constriction
in the middle. C The two halves separate, and each becomes an
independent individual. (Highly magnified.))

The next stage to the simple cytode-forms of the Monera in the
genealogy of mankind (and all other animals) is the simple cell, or
the most rudimentary form of the cell which we find living
independently to-day as the Amoeba. The earliest process of inorganic
differentiation in the structureless body of the Monera led to its
division into two different substances--the caryoplasm and the
cytoplasm. The caryoplasm is the inner and firmer part of the cell,
the substance of the nucleus. The cytoplasm is the outer and softer
part, the substance of the body of the cell. By this important
differentiation of the plasson into nucleus and cell-body, the
organised cell was evolved from the structureless cytode, the
nucleated from the unnucleated plastid. That the first cells to appear
on the earth were formed from the Monera by such a differentiation
seems to us the only possible view in the present condition of
science. We have a direct instance of this earliest process of
differentiation to-day in the ontogeny of many of the lower Protists
(such as the Gregarinae).

The unicellular form that we have in the ovum has already been
described as the reproduction of a corresponding unicellular
stem-form, and to this we have ascribed the organisation of an Amoeba
(cf. Chapter 1.6). The irregular-shaped Amoeba, which we find living
independently to-day in our fresh and salt water, is the least
definite and the most primitive of all the unicellular Protozoa
(Figure 1.16). As the unripe ova (the protova that we find in the
ovaries of animals) cannot be distinguished from the common Amoebae,
we must regard the Amoeba as the primitive form that is reproduced in
the embryonic stage of the amoeboid ovum to-day, in accordance with
the biogenetic law. I have already pointed out, in proof of the
striking resemblance of the two cells, that the ova of many of the
sponges were formerly regarded as parasitic Amoebae (Figure 1.18).
Large unicellular organisms like the Amoebae were found creeping about
inside the body of the sponge, and were thought to be parasites. It
was afterwards discovered that they were really the ova of the sponge
from which the embryos were developed. As a matter of fact, these
sponge-ova are so much like many of the Amoebae in size, shape, the
character of their nucleus, and movement of the pseudopodia, that it
is impossible to distinguish them without knowing their subsequent
development.

Our phylogenetic interpretation of the ovum, and the reduction of it
to some ancient amoeboid ancestral form, supply the answer to the old
problem: "Which was first, the egg or the chick?" We can now give a
very plain answer to this riddle, with which our opponents have often
tried to drive us into a corner. The egg came a long time before the
chick. We do not mean, of course, that the egg existed from the first
as a bird's egg, but as an indifferent amoeboid cell of the simplest
character. The egg lived for thousands of years as an independent
unicellular organism, the Amoeba. The egg, in the modern physiological
sense of the word, did not make its appearance until the descendants
of the unicellular Protozoon had developed into multicellular animals,
and these had undergone sexual differentiation. Even then the egg was
first a gastraea-egg, then a platode-egg, then a vermalia-egg, and
chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg,
reptile-egg, and finally bird's egg. The bird's egg we have experience
of daily is a highly complicated historical product, the result of
countless hereditary processes that have taken place in the course of
millions of years.

The earliest ancestors of our race were simple Protophyta, and from
these our protozoic ancestors were developed afterwards. From the
morphological point of view both the vegetal and the animal Protists
were simple organisms, individualities of the first order, or
plastids. All our later ancestors are complex organisms, or
individualities of a higher order--social aggregations of a plurality
of cells. The earliest of these, the Moraeada, which represent the
third stage in our genealogy, are very simple associations of
homogeneous, indifferent cells--undifferentiated colonies of social
Amoebae or Infusoria. To understand the nature and origin of these
protozoa-colonies we need only follow step by step the first embryonic
products of the stem-cell. In all the Metazoa the first embryonic
process is the repeated cleavage of the stem-cell, or first
segmentation-cell (Figure 2.229). We have already fully considered
this process, and found that all the different forms of it may be
reduced to one type, the original equal or primordial segmentation
(cf. Chapter 1.8). In the genealogical tree of the Vertebrates this
palingenetic form of segmentation has been preserved in the Amphioxus
alone, all the other Vertebrates having cenogenetically modified forms
of cleavage. In any case, the latter were developed from the former,
and so the segmentation of the ovum in the Amphioxus has a great
interest for us (cf. Figure 1.38). The outcome of this repeated
cleavage is the formation of a round cluster of cells, composed of
homogeneous, indifferent cells of the simplest character (Figure
2.230). This is called the morula (= mulberry-embryo) on account of
its resemblance to a mulberry or blackberry.

(FIGURE 2.229. Original or primordial ovum-cleavage. The stem-cell or
cytula, formed by fecundation of the ovum, divides by repeated regular
cleavage first into two (A), then four (B), then eight (C), and
finally a large number of segmentation-cells (D).

FIGURE 2.230. Morula, or mulberry-shaped embryo.)

It is clear that this morula reproduces for us to-day the simple
structure of the multicellular animal that succeeded the unicellular
amoeboid form in the early Laurentian period. In accordance with the
biogenetic law, the morula recalls the ancestral form of the Moraea,
or simple colony of Protozoa. The first cell-communities to be formed,
which laid the early foundation of the higher multicellular body, must
have consisted of homogeneous and simple amoeboid cells. The oldest
Amoebae lived isolated lives, and even the amoeboid cells that were
formed by the segmentation of these unicellular organisms must have
continued to live independently for a long time. But gradually small
communities of Amoebae arose by the side of these eremitical Protozoa,
the sister-cells produced by cleavage remaining joined together. The
advantages in the struggle for life which these communities had over
the isolated cells favoured their formation and their further
development. We find plenty of these cell-colonies or communities
to-day in both fresh and salt water. They belong to various groups
both of the Protophyta and Protozoa.

To have some idea of those ancestors of our race that succeeded
phylogenetically to the Moraeada, we have only to follow the further
embryonic development of the morula. We then see that the social cells
of the round cluster secrete a sort of jelly or a watery fluid inside
their globular body, and they themselves rise to the surface of it
(Figure 1.29 F, G). In this way the solid mulberry-embryo becomes a
hollow sphere, the wall of which is composed of a single layer of
cells. We call this layer the blastoderm, and the sphere itself the
blastula, or embryonic vesicle.

This interesting blastula is very important. The conversion of the
morula into a hollow ball proceeds on the same lines originally in the
most diverse stems--as, for instance, in many of the zoophytes and
worms, the ascidia, many of the echinoderms and molluscs, and in the
amphioxus. Moreover, in the animals in which we do not find a real
palingenetic blastula the defect is clearly due to cenogenetic causes,
such as the formation of food-yelk and other embryonic adaptations. We
may, therefore, conclude that the ontogenetic blastula is the
reproduction of a very early phylogenetic ancestral form, and that all
the Metazoa are descended from a common stem-form, which was in the
main constructed like the blastula. In many of the lower animals the
blastula is not developed within the foetal membranes, but in the open
water. In those cases each blastodermic cell begins at an early stage
to thrust out one or more mobile hair-like processes; the body swims
about by the vibratory movement of these lashes or whips (Figure 1.29
F).

We still find, both in the sea and in fresh water, various kinds of
primitive multicellular organisms that substantially resemble the
blastula in structure, and may be regarded in a sense as permanent
blastula-forms--hollow vesicles or gelatinous balls, with a wall
composed of a single layer of ciliated homogeneous cells. There are
"blastaeads" of this kind even among the Protophyta--the familiar
Volvocina, formerly classed with the infusoria. The common Volvox
globator is found in the ponds in the spring--a small, green,
gelatinous globule, swimming about by means of the stroke of its
lashes, which rise in pairs from the cells on its surface. In the
similar Halosphaera viridis also, which we find in the marine plancton
(floating matter), a number of green cells form a simple layer at the
surface of the gelatinous ball; but in this case there are no cilia.

Some of the infusoria of the flagellata-class (Signura, Magosphaera,
etc.) are similar in structure to these vegetal clusters, but differ
in their animal nutrition; they form the special group of the
Catallacta. In September, 1869, I studied the development of one of
these graceful animals on the island of Gis-Oe, off the coast of
Norway (Magosphaera planula), Figures 2.231 and 2.232). The
fully-formed body is a gelatinous ball, with its wall composed of
thirty-two to sixty-four ciliated cells; it swims about freely in the
sea. After reaching maturity the community is dissolved. Each cell
then lives independently for some time, grows, and changes into a
creeping amoeba. This afterwards contracts, and clothes itself with a
structureless membrane. The cell then looks just like an ordinary
animal ovum. When it has been in this condition for some time the cell
divides into two, four, eight, sixteen, thirty-two, and sixty-four
cells. These arrange themselves in a round vesicle, thrust out
vibratory lashes, burst the capsule, and swim about in the same
magosphaera-form with which we started. This completes the life-circle
of the remarkable and instructive animal.

If we compare these permanent blastulae with the free-swimming
ciliated larvae or blastulae, with similar construction, of many of
the lower animals, we can confidently deduce from them that there was
a very early and long-extinct common stem-form of substantially the
same structure as the blastula. We may call it the Blastaea. Its body
consisted, when fully formed, of a simple hollow ball, filled with
fluid or structureless jelly, with a wall composed of a single stratum
of ciliated cells. There were probably many genera and species of
these blastaeads in the Laurentian period, forming a special class of
marine protists.

It is an interesting fact that in the plant kingdom also the simple
hollow sphere is found to be an elementary form of the multicellular
organism. At the surface and below the surface (down to a depth of
2000 yards) of the sea there are green globules swimming about, with a
wall composed of a single layer of chlorophyll-bearing cells. The
botanist Schmitz gave them the name of Halosphaera viridis in 1879.

The next stage to the Blastaea, and the sixth in our genealogical
tree, is the Gastraea that is developed from it. As we have already
seen, this ancestral form is particularly important. That it once
existed is proved with certainty by the gastrula, which we find
temporarily in the ontogenesis of all the Metazoa (Figure 1.29 J, K).
As we saw, the original, palingenetic form of the gastrula is a round
or oval uni-axial body, the simple cavity of which (the primitive gut)
has an aperture at one pole of its axis (the primitive mouth). The
wall of the gut consists of two strata of cells, and these are the
primary germinal layers, the animal skin-layer (ectoderm) and vegetal
gut-layer (entoderm).

The actual ontogenetic development of the gastrula from the blastula
furnishes sound evidence as to the phylogenetic origin of the Gastraea
from the Blastaea. A pit-shaped depression appears at one side of the
spherical blastula (Figure 1.29 H). In the end this invagination goes
so far that the outer or invaginated part of the blastoderm lies close
on the inner or non-invaginated part (Figure 1.29 J). In explaining
the phylogenetic origin of the gastraea in the light of this
ontogenetic process, we may assume that the one-layered cell-community
of the blastaea began to take in food more largely at one particular
part of its surface. Natural selection would gradually lead to the
formation of a depression or pit at this alimentary spot on the
surface of the ball. The depression would grow deeper and deeper. In
time the vegetal function of taking in and digesting food would be
confined to the cells that lined this hole; the other cells would see
to the animal functions of locomotion, sensation, and protection. This
was the first division of labour among the originally homogeneous
cells of the blastaea.

(FIGURE 2.231. The Norwegian Magosphaera planula, swimming about by
means of the lashes or cilia at its surface.

FIGURE 2.232. Section of Magosphaera planula, showing how the
pear-shaped cells in the centre of the gelatinous ball are connected
by a fibrous process. Each cell has a contractile vacuole as well as a
nucleus.)

The effect, then, of this earliest histological differentiation was to
produce two different kinds of cells--nutritive cells in the
depression and locomotive cells on the surface outside. But this
involved the severance of the two primary germinal layers--a most
important process. When we remember that even man's body, with all its
various parts, and the body of all the other higher animals, are built
up originally out of these two simple layers, we cannot lay too much
stress on the phylogenetic significance of this gastrulation. In the
simple primitive gut or gastric cavity of the gastrula and its
rudimentary mouth we have the first real organ of the animal frame in
the morphological sense; all the other organs were developed
afterwards from these. In reality, the whole body of the gastrula is
merely a "primitive gut." I have shown already (Chapters 1.8 and 1.9)
that the two-layered embryos of all the Metazoa can be reduced to this
typical gastrula. This important fact justifies us in concluding, in
accordance with the biogenetic law, that their ancestors also were
phylogenetically developed from a similar stem-form. This ancient
stem-form is the gastraea.

The gastraea probably lived in the sea during the Laurentian period,
swimming about in the water by means of its ciliary coat much as free
ciliated gastrulae do to-day. Probably it differed from the existing
gastrula only in one essential point, though extinct millions of years
ago. We have reason, from comparative anatomy and ontogeny, to believe
that it multiplied by sexual generation, not merely asexually (by
cleavage, gemmation, and spores), as was no doubt the case with the
earlier ancestors. Some of the cells of the primary germ-layers
probably became ova and others fertilising sperm. We base these
hypotheses on the fact that we do to-day find the simplest form of
sexual reproduction in some of the living gastraeads and other lower
animals, especially the sponges.

The fact that there are still in existence various kinds of
gastraeads, or lower Metazoa with an organisation little higher than
that of the hypothetical gastraea, is a strong point in favour of our
theory. There are not very many species of these living gastraeads;
but their morphological and phylogenetic interest is so great, and
their intermediate position between the Protozoa and Metazoa so
instructive, that I proposed long ago (1876) to make a special class
of them. I distinguished three orders in this class--the Gastremaria,
Physemaria, and Cyemaria (or Dicyemida). But we might also regard
these three orders as so many independent classes in a primitive
gastraead stem.

The Gastremaria and Cyemaria, the chief of these living gastraeads,
are small Metazoa that live parasitically inside other Metazoa, and
are, as a rule, 1/50 to 1/25 of an inch long, often much less (Figure
2.233, 1 to 15). Their soft body, devoid of skeleton, consists of two
simple strata of cells, the primary germinal layers; the outer of
these is thickly clothed with long hair-like lashes, by which the
parasites swim about in the various cavities of their host. The inner
germinal layer furnishes the sexual products. The pure type of the
original gastrula (or archigastrula, Figure 1.29 I) is seen in the
Pemmatodiscus gastrulaceus, which Monticelli discovered in the
umbrella of a large medusa (Pilema pulmo) in 1895; the convex surface
of this gelatinous umbrella was covered with numbers of clear
vesicles, of 1/25 to 1/8 inch in diameter, in the fluid contents of
which the little parasites were swimming. The cup-shaped body of the
Pemmatodiscus (Figure 2.233, 1) is sometimes rather flat, and shaped
like a hat or cone, at other times almost curved into a semi-circle.
The simple hollow of the cup, the primitive gut (g), has a narrow
opening (o). The skin layer (e) consists of long slender cylindrical
cells, which bear long vibratory hairs; it is separated by a thin
structureless, gelatinous plate (f) from the visceral or gut layer
(i), the prismatic cells of which are much smaller and have no cilia.
Pemmatodiscus propagates asexually, by simple longitudinal cleavage;
on this account it has recently been regarded as the representative of
a special order of gastraeads (Mesogastria).

Probably a near relative of the Pemmatodiscus is the Kunstleria
Gruveli (Figure 2.233, 2). It lives in the body-cavity of Vermalia
(Sipunculida), and differs from the former in having no lashes either
on the large ectodermic cells (e) or the small entodermic (i); the
germinal layers are separated by a thick, cup-shaped, gelatinous mass,
which has been called the "clear vesicle" (f). The primitive mouth is
surrounded by a dark ring that bears very strong and long vibratory
lashes, and effects the swimming movements.

Pemmatodiscus and Kunstleria may be included in the family of the
Gastremaria. To these gastraeads with open gut are closely related the
Orthonectida (Rhopalura, Figure 2.233, 3 to 5). They live
parasitically in the body-cavity of echinoderms (Ophiura) and
vermalia; they are distinguished by the fact that their primitive
gut-cavity is not empty, but filled with entodermic cells, from which
the sexual cells are developed. These gastraeads are of both sexes,
the male (Figure 1.3) being smaller and of a somewhat different shape
from the oval female (Figure 1.4).

The somewhat similar Dicyemida (Figure 1.6) are distinguished from the
preceding by the fact that their primitive gut-cavity is occupied by a
single large entodermic cell instead of a crowded group of sexual
cells. This cell does not yield sexual products, but afterwards
divides into a number of cells (spores), each of which, without being
impregnated, grows into a small embryo. The Dicyemida live
parasitically in the body-cavity, especially the renal cavities, of
the cuttle-fishes. They fall in several genera, some of which are
characterised by the possession of special polar cells; the body is
sometimes roundish, oval, or club-shaped, at other times long and
cylindrical. The genus Conocyema (Figures 1.7 to 1.15) differs from
the ordinary Dicyema in having four polar pimples in the form of a
cross, which may be incipient tentacles.

The classification of the Cyemaria is much disputed; sometimes they
are held to be parasitic infusoria (like the Opalina), sometimes
platodes or vermalia, related to the suctorial worms or rotifers, but
having degenerated through parasitism. I adhere to the
phylogenetically important theory that I advanced in 1876, that we
have here real gastraeads, primitive survivors of the common
stem-group of all the Metazoa. In the struggle for life they have
found shelter in the body-cavity of other animals.

(FIGURE 2.233. Modern gastraeads.
Figure 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal
section.
Figure 2. Kunstleria gruveli (Delage), in longitudinal section. (From
Kunstler and Gruvel.)
Figures 3 to 5. Rhopalura Giardi (Julin): Figure 3 male, Figure 4
female, Figure 5 planula.
Figure 6. Dicyema macrocephala (Van Beneden).
Figures 7 to 15. Conocyema polymorpha (Van Beneden): Figure 7 the
mature gastraead, Figures 8 to 15 its gastrulation. d primitive gut, o
primitive mouth, e ectoderm, i entoderm, f gelatinous plate between e
and i (supporting plate, blastocoel).)

The small Coelenteria attached to the floor of the sea that I have
called the Physemaria (Haliphysema and Gastrophysema) probably form a
third order (or class) of the living gastraeads. The genus Haliphysema
(Figures 2.234 and 2.235) is externally very similar to a large
rhizopod (described by the same name in 1862) of the family of the
Rhabdamminida, which was at first taken for a sponge. In order to
avoid confusion with these, I afterwards gave them the name of
Prophysema. The whole mature body of the Prophysema is a simple
cylindrical or oval tube, with a two-layered wall. The hollow of the
tube is the gastric cavity, and the upper opening of it the mouth
(Figure 2.235 m). The two strata of cells that form the wall of the
tube are the primary germinal layers. These rudimentary zoophytes
differ from the swimming gastraeads chiefly in being attached at one
end (the end opposite to the mouth) to the floor of the sea.

In Prophysema the primitive gut is a simple oval cavity, but in the
closely related Gastrophysema it is divided into two chambers by a
transverse constriction; the hind and smaller chamber above furnishes
the sexual products, the anterior one being for digestion.

The simplest sponges (Olynthus, Figure 2.238) have the same
organisation as the Physemaria. The only material difference between
them is that in the sponge the thin two-layered body-wall is pierced
by numbers of pores. When these are closed they resemble the
Physemaria. Possibly the gastraeads that we call Physemaria are only
olynthi with the pores closed. The Ammoconida, or the simple tubular
sand-sponges of the deep-sea (Ammolynthus, etc.), do not differ from
the gastraeads in any important point when the pores are closed. In my
Monograph on the Sponges (with sixty plates) I endeavoured to prove
analytically that all the species of this class can be traced
phylogenetically to a common stem-form (Calcolynthus).

(FIGURES 2.234 AND 2.235. Prophysema primordiale, a living gastraead.

FIGURE 2.234. The whole of the spindle-shaped animal (attached below
to the floor of the sea).

FIGURE 2.235. The same in longitudinal section. The primitive gut (d)
opens above at the primitive mouth (m). Between the ciliated cells (g)
are the amoeboid ova (e). The skin-layer (h) is encrusted with grains
of sand below and sponge-spicules above.

FIGURES 2.236 TO 2.237. Ascula of gastrophysema, attached to the floor
of the sea. Figure 2.236 external view, 2.237 longitudinal section. g
primitive gut, o primitive mouth, i visceral layer, e cutaneous layer.
(Diagram.)

FIGURE 2.238. Olynthus, a very rudimentary sponge. A piece cut away in
front.)

The lowest form of the Cnidaria is also not far removed from the
gastraeads. In the interesting common fresh-water polyp (Hydra) the
whole body is simply an oval tube with a double wall; only in this
case the mouth has a crown of tentacles. Before these develop the
hydra resembles an ascula (Figures 2.236 and 2.237). Afterwards there
are slight histological differentiations in its ectoderm, though the
entoderm remains a single stratum of cells. We find the first
differentiation of epithelial and stinging cells, or of muscular and
neural cells, in the thick ectoderm of the hydra.

In all these rudimentary living coelenteria the sexual cells of both
kinds--ova and sperm cells--are formed by the same individual; it is
possible that the oldest gastraeads were hermaphroditic. It is clear
from comparative anatomy that hermaphrodism--the combination of both
kinds of sexual cells in one individual--is the earliest form of
sexual differentiation; the separation of the sexes (gonochorism) was
a much later phenomenon. The sexual cells originally proceeded from
the edge of the primitive mouth of the gastraead.

</section>
<section id="5" fatherid="0" title="OUR WORM-LIKE ANCESTORS" lv="CHAPTER 2.20." priority="0" level="1">
The gastraea theory has now convinced us that all the Metazoa or
multicellular animals can be traced to a common stem-form, the
Gastraea. In accordance with the biogenetic law, we find solid proof
of this in the fact that the two-layered embryos of all the Metazoa
can be reduced to a primitive common type, the gastrula. Just as the
countless species of the Metazoa do actually develop in the individual
from the simple embryonic form of the gastrula, so they have all
descended in past time from the common stem-form of the Gastraea. In
this fact, and the fact we have already established that the Gastraea
has been evolved from the hollow vesicle of the one-layered Blastaea,
and this again from the original unicellular stem-form, we have
obtained a solid basis for our study of evolution. The clear path from
the stem-cell to the gastrula represents the first section of our
human stem-history (Chapters 1.8, 1.9, and 2.19).

The second section, that leads from the Gastraea to the Prochordonia,
is much more difficult and obscure. By the Prochordonia we mean the
ancient and long-extinct animals which the important embryonic form of
the chordula proves to have once existed (cf. Figures 1.83 to 1.86).
The nearest of living animals to this embryonic structure are the
lowest Tunicates, the Copelata (Appendicaria) and the larvae of the
Ascidia. As both the Tunicates and the Vertebrates develop from the
same chordula, we may infer that there was a corresponding common
ancestor of both stems. We may call this the Chordaea, and the
corresponding stem-group the Prochordonia or Prochordata.

From this important stem-group of the unarticulated Prochordonia (or
"primitive chorda-animals") the stems of the Tunicates and Vertebrates
have been divergently evolved. We shall see presently how this
conclusion is justified in the present condition of morphological
science.

We have first to answer the difficult and much-discussed question of
the development of the Chordaea from the Gastraea; in other words,
"How and by what transformations were the characteristic animals,
resembling the embryonic chordula, which we regard as the common
stem-forms of all the Chordonia, both Tunicates and Vertebrates,
evolved from the simplest two-layered Metazoa?"

The descent of the Vertebrates from the Articulates has been
maintained by a number of zoologists during the last thirty years with
more zeal than discernment; and, as a vast amount has been written on
the subject, we must deal with it to some extent. All three classes of
Articulates in succession have been awarded the honour of being
considered the "real ancestors" of the Vertebrates: first, the
Annelids (earth-worms, leeches, and the like), then the Crustacea
(crabs, etc.), and, finally, the Tracheata (spiders, insects, etc.).
The most popular of these hypotheses was the annelid theory, which
derived the Vertebrates from the Worms. It was almost simultaneously
(1875) formulated by Carl Semper, of Wurtzburg, and Anton Dohrn, of
Naples. The latter advanced this theory originally in favour of the
failing degeneration theory, with which I dealt in my work, Aims and
Methods of Modern Embryology.

This interesting degeneration theory--much discussed at that time, but
almost forgotten now--was formed in 1875 with the aim of harmonising
the results of evolution and ever-advancing Darwinism with religious
belief. The spirited struggle that Darwin had occasioned by the
reformation of the theory of descent in 1859, and that lasted for a
decade with varying fortunes in every branch of biology, was drawing
to a close in 1870-1872, and soon ended in the complete victory of
transformism. To most of the disputants the chief point was not the
general question of evolution, but the particular one of "man's place
in nature"--"the question of questions," as Huxley ly called it.
It was soon evident to every clear-headed thinker that this question
could only be answered in the sense of our anthropogeny, by admitting
that man had descended from a long series of Vertebrates by gradual
modification and improvement.

In this way the real affinity of man and the Vertebrates came to be
admitted on all hands. Comparative anatomy and ontogeny spoke too
clearly for their testimony to be ignored any longer. But in order
still to save man's unique position, and especially the dogma of
personal immortality, a number of natural philosophers and theologians
discovered an admirable way of escape in the "theory of degeneration."
Granting the affinity, they turned the whole evolutionary theory
upside down, and boldly contended that "man is not the most highly
developed animal, but the animals are degenerate men." It is true that
man is closely related to the ape, and belongs to the vertebrate stem;
but the chain of his ancestry goes upward instead of downward. In the
beginning "God created man in his own image," as the prototype of the
perfect vertebrate; but, in consequence of original sin, the human
race sank so low that the apes branched off from it, and afterwards
the lower Vertebrates. When this theory of degeneration was
consistently developed, its supporters were bound to hold that the
entire animal kingdom was descended from the debased children of men.

This theory was most strenuously defended by the Catholic priest and
natural philosopher, Michelis, in his Haeckelogony: An Academic
Protest against Haeckel's Anthropogeny (1875). In still more
"academic" and somewhat mystic form the theory was advanced by a
natural philosopher of the older Jena school--the mathematician and
physicist, Carl Snell. But it received its chief support on the
zoological side from Anton Dohrn, who maintained the anthropocentric
ideas of Snell with particular ability. The Amphioxus, which modern
science now almost unanimously regards as the real Primitive
Vertebrate, the ancient model of the original vertebrate structure,
is, according to Dohrn, a late, degenerate descendant of the stem, the
"prodigal son" of the vertebrate family. It has descended from the
Cyclostoma by a profound degeneration, and these in turn from the
fishes; even the Ascidia and the whole of the Tunicates are merely
degenerate fishes! Following out this curious theory, Dohrn came to
contest the general belief that the Coelenterata and Worms are "lower
animals"; he even declared that the unicellular Protozoa were
degenerate Coelenterata. In his opinion "degeneration is the great
principle that explains the existence of all the lower forms."

If this Michelis-Dohrn theory were true, and all animals were really
degenerate descendants of an originally perfect humanity, man would
assuredly be the true centre and goal of all terrestrial life; his
anthropocentric position and his immortality would be saved.
Unfortunately, this trustful theory is in such flagrant contradiction
to all the known facts of paleontology and embryology that it is no
longer worth serious scientific consideration.

But the case is no better for the much-discussed descent of the
Vertebrates from the Annelids, which Dohrn afterwards maintained with
great zeal. Of late years this hypothesis, which raised so much dust
and controversy, has been entirely abandoned by most competent
zoologists, even those who once supported it. Its chief supporter,
Dohrn, admitted in 1890 that it is "dead and buried," and made a
blushing retraction at the end of his Studies of the Early History of
the Vertebrate.

Now that the annelid-hypothesis is "dead and buried," and other
attempts to derive the Vertebrates from Medusae, Echinoderms, or
Molluscs, have been equally unsuccessful, there is only one hypothesis
 to answer the question of the origin of the Vertebrates--the
hypothesis that I advanced thirty-six years ago and called the
"chordonia-hypothesis." In view of its sound establishment and its
profound significance, it may very well claim to be a THEORY, and so
should be described as the chordonia or chordaea theory.

I first advanced this theory in a series of university lectures in
1867, from which the History of Creation was composed. In the first
edition of this work (1868) I endeavoured to prove, on the strength of
Kowalevsky's epoch-making discoveries, that "of all the animals known
to us the Tunicates are undoubtedly the nearest blood-relatives of the
Vertebrates; they are the most closely related to the Vermalia, from
which the Vertebrates have been evolved. Naturally, I do not mean that
the Vertebrates have descended from the Tunicates, but that the two
groups have sprung from a common root. It is clear that the real
Vertebrates (primarily the Acrania) were evolved in very early times
from a group of Worms, from which the degenerate Tunicates also
descended in another and retrogressive direction." This common extinct
stem-group are the Prochordonia; we still have a silhouette of them in
the chordula-embryo of the Vertebrates and Tunicates; and they still
exist independently, in very modified form, in the class of the
Copelata (Appendicaria, Figure 2.225).

The chordaea-theory received the most valuable and competent support
from Carl Gegenbaur. This able comparative morphologist defended it in
1870, in the second edition of his Elements of Comparative Anatomy; at
the same time he drew attention to the important relations of the
Tunicates to a curious worm, Balanoglossus: he ly regards this as
the representative of a special class of worms, which he called
"gut-breathers" (Enteropneusta). Gegenbaur referred on many other
occasions to the close blood-relationship of the Tunicates and
Vertebrates, and luminously explained the reasons that justify us in
framing the hypothesis of the descent of the two stems from a common
ancestor, an unsegmented worm-like animal with an axial chorda between
the dorsal nerve-tube and the ventral gut-tube.

The theory afterwards received a good deal of support from the
research made by a number of distinguished zoologists and anatomists,
especially C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and
Julin. Since Hatschek's Studies of the Development of the Amphioxus
gave us full information as to the embryology of this lowest
vertebrate, it has become so important for our purpose that we must
consider it a document of the first rank for answering the question we
are dealing with.

The ontogenetic facts that we gather from this sole survivor of the
Acrania are the more valuable for phylogenetic purposes, as
paleontology, unfortunately, throws no light whatever on the origin of
the Vertebrates. Their invertebrate ancestors were soft organisms
without skeleton, and thus incapable of fossilisation, as is still the
case with the lowest vertebrates--the Acrania and Cyclostoma. The same
applies to the greater part of the Vermalia or worm-like animals, the
various classes and orders of which differ so much in structure. The
isolated groups of this rich stem are living branches of a huge tree,
the greater part of which has long been dead, and we have no fossil
evidence as to its earlier form. Nevertheless, some of the surviving
groups are very instructive, and give us clear indications of the way
in which the Chordonia were developed from the Vermalia, and these
from the Coelenteria.

While we seek the most important of these palingenetic forms among the
groups of Coelenteria and Vermalia, it is understood that not a single
one of them must be regarded as an unchanged, or even little changed,
copy of the extinct stem-form. One group has retained one feature,
another a different feature, of the original organisation, and other
organs have been further developed and characteristically modified.
Hence here, more than in any other part of our genealogical tree, we
have to keep before our mind the FULL PICTURE of development, and
separate the unessential secondary phenomena from the essential and
primary. It will be useful first to point out the chief advances in
organisation by which the simple Gastraea gradually became the more
developed Chordaea.

We find our first solid datum in the gastrula of the Amphioxus (Figure
1.38). Its bilateral and tri-axial type indicates that the
Gastraeads--the common ancestors of all the Metazoa--divided at an
early stage into two divergent groups. The uni-axial Gastraea became
sessile, and gave rise to two stems, the Sponges and the Cnidaria (the
latter all reducible to simple polyps like the hydra). But the
tri-axial Gastraea assumed a certain pose or direction of the body on
account of its swimming or creeping movement, and in order to sustain
this it was a great advantage to share the burden equally between the
two halves of the body ( and ). Thus arose the typical
bilateral form, which has three axes. The same bilateral type is found
in all our artificial means of locomotion--carts, ships, etc.; it is
by far the best for the movement of the body in a certain direction
and steady position. Hence natural selection early developed this
bilateral type in a section of the Gastraeads, and thus produced the
stem-forms of all the bilateral animals.

The Gastraea bilateralis, of which we may conceive the bilateral
gastrula of the amphioxus to be a palingenetic reproduction,
represented the two-sided organism of the earliest Metazoa in its
simplest form. The vegetal entoderm that lined their simple gut-cavity
served for nutrition; the ciliated ectoderm that formed the external
skin attended to locomotion and sensation; finally, the two primitive
mesodermic cells, that lay to the  and  at the ventral border
of the primitive mouth, were sexual cells, and effected reproduction.
In order to understand the further development of the gastraea, we
must pay particular attention to: (1) the careful study of the
embryonic stages of the amphioxus that lie between the gastrula and
the chordula; (2) the morphological study of the simplest Platodes
(Platodaria and Turbellaria) and several groups of unarticulated
Vermalia (Gastrotricha, Nemertina, Enteropneusta).

We have to consider the Platodes first, because they are on the border
between the two principal groups of the Metazoa, the Coelenteria and
the Coelomaria. With the former they share the lack of body-cavity,
anus, and vascular system; with the latter they have in common the
bilateral type, the possession of a pair of nephridia or renal canals,
and the formation of a vertical brain or cerebral ganglion. It is now
usual to distinguish four classes of Platodes: the two free-living
classes of the primitive worms (Platodaria) and the coiled-worms
(Turbellaria), and the two parasitic classes of the suctorial worms
(Trematoda) and the tape-worms (Cestoda). We have only to consider the
first two of these classes; the other two are parasites, and have
descended from the former by adaptation to parasitic habits and
consequent degeneration.

(FIGURE 2.239. Aphanostomum Langii (Haeckel), a primitive worm of the
platodaria class, of the order of Cryptocoela or Acoela. This new
species of the genus Aphanostomum, named after Professor Arnold Lang
of Zurich, was found in September, 1899, at Ajaccio in Corsica
(creeping between fucoidea). It is one-twelfth of an inch long,
one-twenty-fifth of an inch broad, and violet in colour. a mouth, g
auditory vesicle, e ectoderm, i entoderm, o ovaries, a spermaries, f
female aperture, m male aperture.)

The primitive worms (Platodaria) are very small flat worms of simple
construction, but of great morphological and phylogenetic interest.
They have been hitherto, as a rule, regarded as a special order of the
Turbellaria, and associated with the Rhabdocoela; but they differ
considerably from these and all the other Platodes (flat worms) in the
absence of renal canals and a special central nervous system; the
structure of their tissue is also simpler than in the other Platodes.
Most of the Platodes of this group (Aphanostomum, Amphichoerus,
Convoluta, Schizoprora, etc.) are very soft and delicate animals,
swimming about in the sea by means of a ciliary coat, and very small
(1/10 to 1/20 inch long). Their oval body, without appendages, is
sometimes spindle-shaped or cylindrical, sometimes flat and
leaf-shaped. Their skin is merely a layer of ciliated ectodermic
cells. Under this is a soft medullary substance, which consists of
entodermic cells with vacuoles. The food passes through the mouth
directly into this digestive medullary substance, in which we do not
generally see any permanent gut-cavity (it may have entirely
collapsed); hence these primitive Platodes have been called Acoela
(without gut-cavity or coelom), or, more correctly, Cryptocoela, or
Pseudocoela. The sexual organs of these hermaphroditic Platodaria are
very simple--two pairs of strings of cells, the inner of which (the
ovaries, Figure 2.239 o) produce ova, and the outer (the spermaria, s)
sperm-cells. These gonads are not yet independent sexual glands, but
sexually differentiated cell-groups in the medullary substance, or, in
other words, parts of the gut-wall. Their products, the sex-cells, are
conveyed out behind by two pairs of short canals; the male opening (m)
lies just behind the female (f). Most of the Platodaria have not the
muscular pharynx, which is very advanced in the Turbellaria and
Trematoda. On the other hand, they have, as a rule, before or behind
the mouth, a bulbous sense-organ (auditory vesicle or organ of
equilibrium, g), and many of them have also a couple of simple optic
spots. The cell-pit of the ectoderm that lies underneath is rather
thick, and represents the first rudiment of a neural ganglion
(vertical brain or acroganglion).

The Turbellaria, with which the similar Platodaria were formerly
classed, differ materially from them in the more advanced structure of
their organs, and especially in having a central nervous system
(vertical brain) and excretory renal canals (nephridia); both
originate from the ectoderm. But between the two germinal layers a
mesoderm is developed, a soft mass of connective tissue, in which the
organs are embedded. The Turbellaria are still represented by a number
of different forms, in both fresh and sea-water. The oldest of these
are the very rudimentary and tiny forms that are known as Rhabdocoela
on account of the simple construction of their gut; they are, as a
rule, less than a quarter of an inch long and of a simple oval or
lancet shape (Figure 2.240). The surface is covered with ciliated
epithelium, a stratum of ectodermic cells. The digestive gut is still
the simple primitive gut of the gastraea (d), with a single aperture
that is both mouth and anus (m). There is, however, an invagination of
the ectoderm at the mouth, which has given rise to a muscular pharynx
(sd). It is noteworthy that the mouth of the Turbellaria (like the
primitive mouth of the Gastraea) may, in this class, change its
position considerably in the middle line of the ventral surface;
sometimes it lies behind (Opisthostomum), sometimes in the middle
(Mesostomum), sometimes in front (Prosostomum). This displacement of
the mouth from front to rear is very interesting, because it
corresponds to a phylogenetic displacement of the mouth. This probably
occurred in the Platode ancestors of most (or all?) of the Coelomaria;
in these the permanent mouth (metastoma) lies at the fore end (oral
pole), whereas the primitive mouth (prostoma) lay at the hind end of
the bilateral body.

In most of the Turbellaria there is a narrow cavity, containing a
number of secondary organs, between the two primary germinal layers,
the outer or animal layer of which forms the epidermis and the inner
vegetal layer the visceral epithelium. The earliest of these organs
are the sexual organs; they are very variously constructed in the
Platode-class; in the simplest case there are merely two pairs of
gonads or sexual glands--a pair of testicles (Figure 2.241 h) and a
pair of ovaries (e). They open externally, sometimes by a common
aperture (Monogonopora), sometimes by separate ones, the female behind
the male (Digonopora, Figure 2.241). The sexual glands develop
originally from the two promesoblasts or primitive mesodermic cells
(Figure 1.83 p). As these earliest mesodermic structures extended, and
became spacious sexual pouches in the later descendants of the
Platodes, probably the two coelom-pouches were formed from them, the
first trace of the real body-cavity of the higher Metazoa
(Enterocoela).

The gonads are among the oldest organs, the few other organs that we
find in the Platodes between the gut-wall and body-wall being later
evolutionary products. One of the oldest and most important of these
are the kidneys or nephridia, which remove unusable matter from the
body (Figure 2.240 nc). These urinary or excretory organs were
originally enlarged skin-glands--a couple of canals that run the
length of the body, and have a separate or common external aperture
(nm). They often have a number of branches. These special excretory
organs are not found in the other Coelenteria (Gastraeads, Sponges,
Cnidaria) or the Cryptocoela. They are first met in the Turbellaria,
and have been transmitted direct from these to the Vermalia, and from
these to the higher stems.

Finally, there is a very important new organ in the Turbellaria, which
we do not find in the Cryptocoela (Figure 2.239) and their gastraead
ancestors--the rudimentary nervous system. It consists of a couple of
simple cerebral ganglia (Figure 2.241 g) and fine nervous fibres that
radiate from them; these are partly voluntary nerves (or motor fibres)
that go to the thin muscular layer developing under the skin; and
partly sensory nerves that proceed to the sense-cells of the ciliated
epiderm (f). Many of the Turbellaria have also special sense-organs; a
couple of ciliated smell pits (na), rudimentary eyes (au), and, less
frequently, auditory vesicles.

On these principles I assume that the oldest and simplest Turbellaria
arose from Platodaria, and these directly from bilateral Gastraeads.
The chief advances were the formation of gonads and nephridia, and of
the rudimentary brain. On this hypothesis, which I advanced in 1872 in
the first sketch of the gastraea-theory (Monograph on the Sponges),
there is no direct affinity between the Platodes and the Cnidaria.

(FIGURE 2.240. A simple turbellarian (Rhabdocoelum). m mouth, sd
gullet epithelium, sm gullet muscles, d gastric gut, nc renal canals,
nm renal aperture, au eye, na olfactory pit. (Diagram.)

FIGURE 2.241. The same, showing the other organs. g brain, au eye, na
olfactory pit, n nerves, h testicles, male symbol male aperture,
female symbol female aperture, e ovary, f ciliated epiderm. (Diagram.)

(FIGURES 242 AND 243. Chaetonotus, a rudimentary vermalian, of the
group of Gastrotricha. m mouth, s gullet, d gut, a anus, g brain, n
nerves, ss sensory hairs, au eye, ms muscular cells, h skin, f
ciliated bands of the ventral surface, nc nephridia, nm their
aperture, e ovaries.))

Next to the ancient stem-group of the Turbellaria come a number of
more recent chordonia ancestors, which we class with the Vermalia or
Helminthes, the unarticulated worms. These true worms (Vermes, lately
also called Scolecida) are the difficulty or the lumber-room of the
zoological classifier, because the various classes have very
complicated relations to the lower Platodes on the one hand and the
more advanced animals on the other. But if we exclude the Platodes and
the Annelids from this stem, we find a fairly satisfactory unity of
organisation in the remaining classes. Among these worms we find some
important forms that show considerable advance in organisation from
the platode to the chordonia stage. Three of these phenomena are
particularly instructive: (1) The formation of a true (secondary)
body-cavity (coeloma); (2) the formation of a second aperture of the
gut, the anus; and (3) the formation of a vascular system. The great
majority of the Vermalia have these three features, and they are all
wanting in the Platodes; in the rest of the worms at least one or two
of them are developed.

Next and very close to the Platodes we have the Ichthydina
(Gastrotricha), little marine and fresh-water worms, about 1/250 to
1/1000 inch long. Zoologists differ as to their position in
classification. In my opinion, they approach very close to the
Rhabdocoela (Figures 2.240 and 2.241), and differ from them chiefly in
the possession of an anus at the posterior end (Figure 2.242 a).
Further, the cilia that cover the whole surface of the Turbellaria are
confined in the Gastrotricha to two ciliated bands (f) on the ventral
surface of the oval body, the dorsal surface having bristles.
Otherwise the organisation of the two classes is the same. In both the
gut consists of a muscular gullet (s) and a glandular primitive gut
(d). Over the gullet is a double brain (acroganglion, g). At the side
of the gut are two serpentine prorenal canals (water-vessels or
pronephridia, nc), which open on the ventral side (nm). Behind are a
pair of simple sexual glands or gonads (Figure 2.243 e).

While the Ichthydina are thus closely related to the Platodes, we have
to go farther away for the two classes of Vermalia which we unite in
the group of the "snout-worms" (Frontonia). These are the Nemertina
and the Enteropneusta. Both classes have a complete ciliary coat on
the epidermis, a heritage from the Turbellaria and the Gastraeads;
also, both have two openings of the gut, the mouth and anus, like the
Gastrotricha. But we find also an important organ that is wanting in
the preceding forms--the vascular system. In their more advanced
mesoderm we find a few contractile longitudinal canals which force the
blood through the body by their contractions; these are the first
blood-vessels.

(FIGURE 2.244. A simple Nemertine. m mouth, d gut, a anus, g brain, n
nerves, h ciliary coat, ss sensory pits (head-cs), au eyes, r
dorsal vessel, l lateral vessels. (Diagram.)

FIGURE 2.245. A young Enteropneust (Balanaglossus). (From Alexander
Agassiz.) r acorn-shaped snout, h neck, k gill-cs and gill-arches
of the fore-gut, in long rows on each side, d digestive hind-gut,
filling the greater part of the body-cavity, v intestinal vein or
ventral vessel, lying between the parallel folds of the skin, a anus.

Figure 2.246. Transverse section of the branchial gut. A of
Balanoglossus, B of Ascidia. r branchial gut, n pharyngeal groove,
asterisk ventral folds between the two. Diagrammatic illustration from
Gegenbaur, to show the relation of the dorsal branchial-gut cavity (r)
to the pharyngeal or hypobranchial groove (n).)

The Nemertina were formerly classed with the much less advanced
Turbellaria. But they differ essentially from them in having an anus
and blood-vessels, and several other marks of higher organisation.
They have generally long and narrow bodies, like a more or less
flattened cord; there are, besides several small species, giant-forms
with a width of 1/5 to 2/5 inch and a length of several yards (even
ten to fifteen). Most of them live in the sea, but some in fresh water
and moist earth. In their internal structure they approach the
Turbellaria on the one hand and the higher Vermalia (especially the
Enteropneusta) on the other. They have a good deal of interest as the
lowest and oldest of all animals with blood. In them we find
blood-vessels for the first time, distributing real blood through the
body. The blood is red, and the red colouring-matter is haemoglobin,
connected with elliptic discoid blood-cells, as in the Vertebrates.
Most of them have two or three parallel blood-canals, which run the
whole length of the body, and are connected in front and behind by
loops, and often by a number of ring-shaped pieces. The chief of these
primitive blood-vessels is the one that lies above the gut in the
middle line of the back (Figure 2.244 r); it may be compared to either
the dorsal vessel of the Articulates or the aorta of the Vertebrates.
To the  and  are the two serpentine lateral vessels (Figure
2.244 l).

After the Nemertina, I take (as distant relatives) the Enteropneusta;
they may be classed together with them as Frontonia or Rhyncocoela
(snout-worms). There is now only one genus of this class, with several
species (Balanoglossus); but it is very remarkable, and may be
regarded as the last survivor of an ancient and long-extinct class of
Vermalia. They are related, on the one hand, to the Nemertina and
their immediate ancestors, the Platodes, and to the lowest and oldest
forms of the Chordonia on the other.

The Enteropneusta (Figure 2.245) live in the sea sand, and are long
worms of very simple shape, like the Nemertina. From the latter they
have inherited: (1) The bilateral type, with incomplete segmentation;
(2) the ciliary coat of the soft epidermis; (3) the double rows of
gastric pouches, alternating with a single or double row of gonads;
(4) separation of the sexes (the Platode ancestors were
hermaphroditic); (5) the ventral mouth, underneath a protruding snout;
(6) the anus terminating the simple gut-tube; and (7) several parallel
blood-canals, running the length of the body, a dorsal and a ventral
principal stem.

On the other hand, the Enteropneusta differ from their Nemertine
ancestors in several features, some of which are important, that we
may attribute to adaptation. The chief of these is the branchial gut
(Figure 2.245 k). The anterior section of the gut is converted into a
respiratory organ, and pierced by two rows of gill-cs; between
these there is a branchial (gill) skeleton, formed of rods and plates
of chitine. The water that enters at the mouth makes its exit by these
cs. They lie in the dorsal half of the fore-gut, and this is
completely separated from the ventral half by two longitudinal folds
(Figure 2.246 A*). This ventral half, the glandular walls of which are
clothed with ciliary epithelium and secrete mucus, corresponds to the
pharyngeal or hypo-branchial groove of the Chordonia (Bn), the
important organ from which the later thyroid gland is developed in the
Craniota (cf. Chapter 2.16). The agreement in the structure of the
branchial gut of the Enteropneusts, Tunicates, and Vertebrates was
first recognised by Gegenbaur (1878); it is the more significant as at
first we find only a couple of gill-cs in the young animals of all
three groups; the number gradually increases. We can infer from this
the common descent of the three groups with all the more confidence
when we find the Balanoglossus approaching the Chordonia in other
respects. Thus, for instance, the chief part of the central nervous
system is a long dorsal neural string that runs above the gut and
corresponds to the medullary tube of the Chordonia. Bateson believes
he has detected a rudimentary chorda between the two.

Of all extant invertebrate animals the Enteropneusts come nearest to
the Chordonia in virtue of these peculiar characters; hence we may
regard them as the survivors of the ancient gut-breathing Vermalia
from which the Chordonia also have descended. Again, of all the
chorda-animals the Copelata (Figure 2.225) and the tailed larvae of
the ascidia approach nearest to the young Balanoglossus. Both are, on
the other hand, very closely related to the Amphioxus, the Primitive
Vertebrate of which we have considered the importance (Chapters 2.16
and 2.17). As we saw there, the unarticulated Tunicates and the
articulated Vertebrates must be regarded as two independent stems,
that have developed in divergent directions. But the common root of
the two stems, the extinct group of the Prochordonia, must be sought
in the vermalia stem; and of all the living Vermalia those we have
considered give us the safest clue to their origin. It is true that
the actual representatives of the important groups of the Copelata,
Balanoglossi, Nemertina, Icthydina, etc., have more or less departed
from the primitive model owing to adaptation to special environment.
But we may just as confidently affirm that the main features of their
organisation have been preserved by heredity.

We must grant, however, that in the whole stem-history of the
Vertebrates the long stretch from the Gastraeads and Platodes up to
the oldest Chordonia remains by far the most obscure section. We might
frame another hypothesis to raise the difficulty--namely, that there
was a long series of very different and totally extinct forms between
the Gastraea and the Chordaea. Even in this modified chordaea-theory
the six fundamental organs of the chordula would retain their great
value. The medullary tube would be originally a chemical sensory
organ, a dorsal olfactory tube, taking in respiratory-water and food
by the neuroporus in front and conveying them by the neurenteric canal
into the primitive gut. This olfactory tube would afterwards become
the nervous centre, while the expanding gonads (lying to  and
 of the primitive mouth) would form the coeloma. The chorda may
have been originally a digestive glandular groove in the dorsal middle
line of the primitive gut. The two secondary gut-openings, mouth and
anus, may have arisen in various ways by change of functions. In any
case, we should ascribe the same high value to the chordula as we did
before to the gastrula.

In order to explain more fully the chief stages in the advance of our
race, I add the hypothetical sketch of man's ancestry that I published
in my Last Link a translation by Dr. Gadow of the paper read at the
International Zoological Congress at Cambridge in 1898:--

   A. MAN'S GENEALOGICAL TREE, FIRST HALF: EARLIER SERIES OF ANCESTORS,
      WITHOUT FOSSIL EVIDENCE.

COLUMN 1 : CHIEF STAGES.
COLUMN 2 : ANCESTRAL STEM-GROUPS.
COLUMN 3 : LIVING RELATIVES OF ANCESTORS.


STAGES 1 TO 5. PROTIST ANCESTORS. UNICELLULAR ORGANISMS.

1 to 2. Protophytes. : 1. Monera. Without nucleus. : Chromacea.
(Chroococcus.) Phycochromacea.

1 to 2. Protophytes. : 2. Algaria. Unicellular algae. : 2. Paulotomea.
Palmellacea. Eremosphaera.

3 to 5. Protozoa. : 3. Lobosa. Unicellular (amoebina) rhizopods. : 3.
Amoebina. Amoeba Leucocyta.

3 to 5. Protozoa. : 4. Infusoria. Unicellular. : 4. Flagellata.
Euflagellata. Zoomonades.

3 to 5. Protozoa. : 5. Blastaeades. Multicellular hollow spheres. : 5.
Catallacta. Magosphaera, Volvocina, Blastula.

STAGES 6 TO 11. INVERTEBRATE METAZOA ANCESTORS.

6 to 8. Coelenteria, without anus and body-cavity. : 6. Gastraeades.
With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria.

6 to 8. Coelenteria, without anus and body-cavity. : 7. Platodes I.
Platodaria (without nephridia). : 7. Cryptocoela. Convoluta, Proporus.

6 to 8. Coelenteria, without anus and body-cavity. : 8. Platodes II.
Platodinia (with nephridia). : 8. Rhabdocoela. Vortex, Monotus.

9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia.
(Primitive Worms.) Rotatoria. : 9. Gastrotricha. Trochozoa,
Trochophora.

9 to 11. Vermalia, with anus and body-cavity. : 10. Frontonia.
(Rhynchelminthes.) Snout-worms. : 10. Enteropneusta. Balanoglossus,
Cephalodiscus.

9 to 11. Vermalia, with anus and body-cavity. : 11. Prochordonia.
Chorda-worms. : 11. Copelata. Appendicaria. Chordula-larvae.

STAGES 12 TO 15. MONORHINA ANCESTORS.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 12.
Acrania I. (Prospondylia.) : 12. Amphioxus larva.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 13.
Acrania II. More recent. : 13. Leptocardia. Amphioxus.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 14.
Cyclostoma I. (Archicrania.) : 14. Petromyzonta larvae.

Oldest vertebrates without jaws or pairs of limbs, single nose. : 15.
Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta.

B. MAN'S GENEALOGICAL TREE, SECOND HALF: LATER ANCESTORS, WITH FOSSIL
EVIDENCE.

COLUMN 1 : GEOLOGICAL PERIODS.
COLUMN 2 : ANCESTRAL STEM-GROUPS.
COLUMN 3 : LIVING RELATIVES OF ANCESTORS.

Silurian. :  16. Selachii. Primitive fishes. Proselachii. : 16.
Natidanides. Chlamydoselachius. Heptanchus.

Silurian. 17. Ganoides. Plated-fishes. Proganoides. : 17.
Accipenserides. (Sturgeons.) Polypterus.

Devonian. : 18. Dipneusta. Paladipneusta. : 18. Neodipneusta.
Ceratodus. Protopterus.

Carboniferous. : 19. Amphibia. Stegocephala. : 19. Phanerobranchia.
Salamandrina. (Proteus, triton.)

Permian. : 20. Reptilia. Proreptilia. : 20. Rhynchocephalia. Primitive
lizards. Hatteria.

Triassic. : 21. Monotrema. Promammalia. : 21. Ornithodelphia. Echidna.
Ornithorhyncus.

Jurassic. : 22. Marsupalia. Prodidelphia. : 22. Didelphia. Didelphys.
Perameles.

Cretaceous. : 23. Mallotheria. Prochoriata. : 23. Insectivora.
Erinaceida. (Ictopsida +.)

Older Eocene. : 24. Lemuravida. Older lemurs. Dentition. 3. 1. 4. 3. :
24. Pachylemures. (Hyopsodus +), (Adapis +).

Neo-Eocene. : 25. Lemurogona. Later lemurs. Dentition. 2. 1. 4. 3. :
25. Autolemures. Eulemur. Stenops.

Oligocene. : 26. Dysmopitheca. Western apes. Dentition. 2. 1. 3. 3. :
26. Platyrrhinae. (Anthropops +), (Homunculus +).

Older Miocene. : 27. Cynopitheca. Dog-faced apes (tailed). : 27.
Papiomorpha. Cynocephalus.

Neo-Miocene. : 28. Anthropoides. Man-like apes (tail-less). : 28.
Hylobatida. Hylobates. Satyrus.

Pliocene. : 29. Pithecanthropi. Ape-men (alali, speechless). : 29.
Anthropitheca. Chimpanzee. Gorilla.

Pleistocene. : 30. Homines. Men, with speech. : 30. Weddahs.
Australian negroes.

</section>
<section id="6" fatherid="0" title="OUR FISH-LIKE ANCESTORS" lv="CHAPTER 2.21." priority="0" level="1">
Our task of detecting the extinct ancestors of our race among the vast
numbers of animals known to us encounters very different difficulties
in the various sections of man's stem-history. These were very great
in the series of our invertebrate ancestors; they are much slighter in
the subsequent series of our vertebrate ancestors. Within the
vertebrate stem there is, as we have already seen, so complete an
agreement in structure and embryology that it is impossible to doubt
their phylogenetic unity. In this case the evidence is much clearer
and more abundant.

The characteristics that distinguish the Vertebrates as a whole from
the Invertebrates have already been discussed in our description of
the hypothetical Primitive Vertebrate (Chapter 1.11, Figure 1.98 to
1.102). The chief of these are: (1) The evolution of the primitive
brain into a dorsal medullary tube; (2) the formation of the chorda
between the medullary tube and the gut; (3) the division of the gut
into branchial (gill) and hepatic (liver) gut; and (4) the internal
articulation or metamerism. The first three features are shared by the
Vertebrates with the ascidia-larvae and the Prochordonia; the fourth
is peculiar to them. Thus the chief advantage in organisation by which
the earliest Vertebrates took precedence of the unsegmented Chordonia
consisted in the development of internal segmentation.

The whole vertebrate stem divides first into the two chief sections of
Acrania and Craniota. The Amphioxus is the only surviving
representative of the older and lower section, the Acrania
("skull-less"). All the other vertebrates belong to the second
division, the Craniota ("skull-animals"). The Craniota descend
directly from the Acrania, and these from the primitive Chordonia. The
exhaustive study that we made of the comparative anatomy and ontogeny
of the Ascidia and the Amphioxus has proved these relations for us.
(See Chapters 2.16 and 2.17.) The Amphioxus, the lowest Vertebrate,
and the Ascidia, the nearest related Invertebrate, descend from a
common extinct stem-form, the Chordaea; and this must have had,
substantially, the organisation of the chordula.

However, the Amphioxus is important not merely because it fills the
deep gulf between the Invertebrates and Vertebrates, but also because
it shows us to-day the typical vertebrate in all its simplicity. We
owe to it the most important data that we proceed on in reconstructing
the gradual historical development of the whole stem. All the Craniota
descend from a common stem-form, and this was substantially identical
in structure with the Amphioxus. This stem-form, the Primitive
Vertebrate (Prospondylus, Figures 1.98 to 1.102), had the
characteristics of the vertebrate as such, but not the important
features that distinguish the Craniota from the Acrania. Though the
Amphioxus has many peculiarities of structure and has much
degenerated, and though it cannot be regarded as an unchanged
descendant of the Primitive Vertebrate, it must have inherited from it
the specific characters we enumerated above. We may not say that
"Amphioxus is the ancestor of the Vertebrates"; but we can say:
"Amphioxus is the nearest relation to the ancestor of all the animals
we know." Both belong to the same small family, or lowest class of the
Vertebrates, that we call the Acrania. In our genealogical tree this
group forms the twelfth stage, or the first stage among the vertebrate
ancestors (Chapter 2.20). From this group of Acrania both the
Amphioxus and the Craniota were evolved.

The vast division of the Craniota embraces all the Vertebrates known
to us, with the exception of the Amphioxus. All of them have a head
clearly differentiated from the trunk, and a skull enclosing a brain.
The head has also three pairs of higher sense-organs (nose, eyes, and
ears). The brain is very rudimentary at first, a mere bulbous
enlargement of the fore end of the medullary tube. But it is soon
divided by a number of transverse constrictions into, first three,
then five successive cerebral vesicles. In this formation of the head,
skull, and brain, with further development of the higher sense-organs,
we have the advance that the Craniota made beyond their skull-less
ancestors. Other organs also attained a higher development; they
acquired a compact centralised heart with valves and a more advanced
liver and kidneys, and made progress in other important respects.

We may divide the Craniota generally into Cyclostoma ("round-mouthed")
and Gnathostoma ("jaw-mouthed"). There are only a few groups of the
former in existence now, but they are very interesting, because in
their whole structure they stand midway between the Acrania and the
Gnathostoma. They are much more advanced than the Acrania, much less
so than the fishes, and thus form a very welcome connecting-link
between the two groups. We may therefore consider them a special
intermediate group, the fourteenth and fifteenth stages in the series
of our ancestors.

(FIGURE 2.247. The large marine lamprey (Petromyzon marinus), much
reduced. Behind the eye there is a row of seven gill-cs visible on
the , in front the round suctorial mouth.)

The few surviving species of the Cyclostoma are divided into two
orders--the Myxinoides and the Petromyzontes. The former, the
hag-fishes, have a long, cylindrical, worm-like body. They were
classed by Linne with the worms, and by later zoologists, with the
fishes, or the amphibia, or the molluscs. They live in the sea,
usually as parasites of fishes, into the skin of which they bore with
their round suctorial mouths and their tongues, armed with horny
teeth. They are sometimes found alive in the body cavity of fishes
(such as the torsk or sturgeon); in these cases they have passed
through the skin into the interior. The second order consists of the
Petromyzontes or lampreys; the small river lamprey (Petromyzon
fluviatilis) and the large marine lamprey (Petromyzon marinus, Figure
2.247). They also have a round suctorial mouth, with horny teeth
inside it; by means of this they attach themselves by sucking to
fishes, stones, and other objects (hence the name Petromyzon =
stone-sucker). It seems that this habit was very widespread among the
earlier Vertebrates; the larvae of many of the Ganoids and frogs have
suctorial disks near the mouth.

The class that is formed of the Myxinoides and Petromyzontes is called
the Cyclostoma (round-mouthed), because their mouth has a circular or
semi-circular aperture. The jaws (upper and lower) that we find in all
the higher Vertebrates are completely wanting in the Cyclostoma, as in
the Amphioxus. Hence the other Vertebrates are collectively opposed to
them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called
Monorhina (single-nosed), because they have only a single nasal
passage, while all the Gnathostoma have two nostrils (Amphirhina =
double-nosed). But apart from these peculiarities the Cyclostoma
differ more widely from the fishes in other special features of their
structure than the fishes do from man. Hence they are obviously the
last survivors of a very ancient class of Vertebrates, that was far
from attaining the advanced organisation of the true fish. To mention
only the chief points, the Cyclostoma show no trace of pairs of limbs.
Their mucous skin is quite naked and smooth and devoid of scales.
There is no bony skeleton. A very rudimentary skull is developed at
the foremost end of their chorda. At this point a soft membranous
(partly turning into cartilage), small skull-capsule is formed, and
encloses the brain.

The brain of the Cyclostoma is merely a very small and comparatively
insignificant swelling of the spinal marrow, a simple vesicle at
first. It afterwards divides into five successive cerebral vesicles,
like the brain of the Gnathostoma. These five primitive cerebral
vesicles, that are found in the embryos of all the higher vertebrates
from the fishes to man, and grow into very complex structures, remain
at a very rudimentary stage in the Cyclostoma. The histological
structure of the nerves is also less advanced than in the rest of the
vertebrates. In these the auscultory organ always contains three
circular canals, but in the lampreys there are only two, and in the
hag-fishes only one. In most other respects the organisation of the
Cyclostoma is much simpler--for instance, in the structure of the
heart, circulation, and kidneys. We must especially note the absence
of a very important organ that we find in the fishes, the
floating-bladder, from which the lungs of the higher Vertebrates have
been developed.

When we consider all these peculiarities in the structure of the
Cyclostoma, we may formulate the following thesis: Two divergent lines
proceeded from the earliest Craniota, or the primitive Craniota
(Archicrania). One of these lines is preserved in a greatly modified
condition: these are the Cyclostoma, a very backward and partly
degenerate side-line. The other, the chief line of the Vertebrate
stem, advanced straight to the fishes, and by fresh adaptations
acquired a number of important improvements.

(FIGURE 2.248. Fossil Permian primitive fish (Pleuracanthus Dechenii),
from the red sandstone of Saarbrucken. (From Doderlein.) I Skull and
branchial skeleton: o eye-region, pq palatoquadratum, nd lower jaw, hm
hyomandibular, hy tongue-bone, k gill-radii, kb gill-arches, z
jaw-teeth, sz gullet-teeth, st neck-spine. II Vertebral column: ob
upper arches, ub lower arches, hc intercentra, r ribs. III Single
fins: d dorsal fin, c tail-fin (tail-end wanting), an anus-fin, ft
supporter of fin-rays. IV Breast-fin: sg shoulder-zone, ax fin-axis,
ss double lines of fin-rays, bs additional rays, sch plates. V Ventral
fin: p pelvis, ax fin-axis, ss single row of fin-rays, bs additional
rays, sch scales, cop penis.

FIGURE 2.249. Embryo of a shark (Scymnus lichia), seen from the
ventral side, v breast-fins (in front five pairs of gill-cs), h
belly-fins, a anus, s tail-fin, k external gill-tuft, d yelk-sac
(removed for most part), g eye, n nose, m mouth-c.)

The Cyclostoma are almost always classified by zoologists among the
fishes; but the incorrectness of this may be judged from the fact that
in all the chief and distinctive features of organisation they are
further removed from the fishes than the fishes are from the Mammals,
and even man. With the fishes we enter upon the vast division of the
jaw-mouthed or double-nosed Vertebrates (Gnathostoma or Amphirhina).
We have to consider the fishes carefully as the class which, on the
evidence of palaeontology, comparative anatomy, and ontogeny, may be
regarded with absolute certainty as the stem-class of all the higher
Vertebrates or Gnathostomes. Naturally, none of the actual fishes can
be considered the direct ancestor of the higher Vertebrates. But it is
certain that all the Vertebrates or Gnathostomes, from the fishes to
man, descend from a common, extinct, fish-like ancestor. If we had
this ancient stem-form before us, we would undoubtedly class it as a
true fish. Fortunately the comparative anatomy and classification of
the fishes are now so far advanced that we can get a very clear idea
of these interesting and instructive features.

In order to understand properly the genealogical tree of our race
within the vertebrate stem, it is important to bear in mind the
characteristics that separate the whole of the Gnathostomes from the
Cyclostomes and Craniota. In these respects the fishes agree entirely
with all the other Gnathostomes up to man, and it is on this that we
base our claim of relationship to the fishes. The following
characteristics of the Gnathostomes are anatomic features of this
kind: (1) The internal gill-arch apparatus with the jaw arches; (2)
the pair of nostrils; (3) the floating bladder or lungs; and (4) the
two pairs of limbs.

The peculiar formation of the frame work of the branchial (gill)
arches and the connected maxillary (jaw) apparatus is of importance in
the whole group of the Gnathostomes. It is inherited in rudimentary
form by all of them, from the earliest fishes to man. It is true that
the primitive transformation (which we find even in the Ascidia) of
the fore gut into the branchial gut can be traced in all the
Vertebrates to the same simple type; in this respect the gill-cs,
which pierce the walls of the branchial gut in all the Vertebrates and
in the Ascidia, are very characteristic. But the EXTERNAL, superficial
branchial skeleton that supports the gill-crate in the Cyclostoma is
replaced in the Gnathostomes by an INTERNAL branchial skeleton. It
consists of a number of successive cartilaginous arches, which lie in
the wall of the gullet between the gill-cs, and run round the
gullet from both sides. The foremost pair of gill-arches become the
maxillary arches, from which we get our upper and lower jaws.

The olfactory organs are at first found in the same form in all the
Gnathostomes, as a pair of depressions in the fore part of the skin of
the head, above the mouth; hence, they are also called the Amphirhina
("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their
nose is a single passage in the middle of the frontal surface. But as
the olfactory nerve is double in both cases, it is possible that the
peculiar form of the nose in the actual Cyclostomes is a secondary
acquisition (by adaptation to suctorial habits).

A third essential character of the Gnathostomes, that distinguishes
them very conspicuously from the lower vertebrates we have dealt with,
is the formation of a blind sac by invagination from the fore part of
the gut, which becomes in the fishes the air-filled floating-bladder.
This organ acts as a hydrostatic apparatus, increasing or reducing the
specific gravity of the fish by compressing or altering the quantity
of air in it. The fish can rise or sink in the water by means of it.
This is the organ from which the lungs of the higher vertebrates are
developed.

(FIGURE 2.250. Fully developed man-eating shark (Carcharias
melanopterus),  view. r1 first, r2 second dorsal fin, s tail-fin,
a anus-fin, v breast-fins, h belly-fins.)

Finally, the fourth character of the Gnathostomes in their simple
embryonic form is the two pairs of extremities or limbs--a pair of
fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind
legs (ventral fins in the fish, Figure 2.250 h). The comparative
anatomy of these fins is very interesting, because they contain the
rudiments of all the skeletal parts that form the framework of the
fore and hind legs in all the higher vertebrates  up to man.
There is no trace of these pairs of limbs in the Acrania and
Cyclostomes.

Turning, now, to a closer inspection of the fish class, we may first
divide it into three groups or sub-classes, the genealogy of which is
well known to us. The first and oldest group is the sub-class of the
Selachii or primitive fishes; the best-known representatives of which
to-day are the orders of the sharks and rays (Figures 2.248 to 2.252).
Next to this is the more advanced sub-class of the plated fishes or
Ganoids (Figures 2.253 to 2.255). It has been long extinct for the
most part, and has very few living representatives, such as the
sturgeon and the bony pike; but we can form some idea of the earlier
extent of this interesting group from the large numbers of fossils.
From these plated fishes the sub-class of the bony fishes or Teleostei
was developed, to which the great majority of living fishes belong
(especially nearly all our river fishes). Comparative anatomy and
ontogeny show clearly that the Ganoids descended from the Selachii,
and the Teleostei from the Ganoids. On the other hand, a collateral
line, or rather the advancing chief line of the vertebrate stem, was
developed from the earlier Ganoids, and this leads us through the
group of the Dipneusta to the important division of the Amphibia.

(FIGURE 2.251. Fossil angel-shark (Squatina alifera), from the upper
Jurassic at Eichstatt. (From Zittel.) The cartilaginous skull is
clearly seen in the broad head, and the gill-arches behind. The wide
breast-fin and the narrower belly-fin have a number of radii; between
these and the vertebral column are a number of ribs.)

The earliest fossil remains of Vertebrates that we know were found in
the Upper Silurian (Chapter 2.18), and belong to two groups--the
Selachii and the Ganoids. The most primitive of all known
representatives of the earliest fishes are probably the remarkable
Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus,
etc. (Figure 2.248). These ancient cartilaginous fishes agree in most
points of structure with the real sharks (Figures 2.249 and 2.250);
but in other respects they seem to be so much simpler in organisation
that many palaeontologists separate them altogether, and regard them
as Proselachii; they are probably closely related to the extinct
ancestors of the Gnathostomes. We find well-preserved remains of them
in the Permian period. Well-preserved impressions of other sharks are
found in the Jurassic schist, such as of the angel-fish (Squatina,
Figure 2.251). Among the extinct earlier sharks of the Tertiary period
there were some twice as large as the biggest living fishes;
Carcharodon was more than 100 feet long. The sole surviving species of
this genus (C. Rondeleti) is eleven yards long, and has teeth two
inches long; but among the fossil species we find teeth six inches
long (Figure 2.252).

From the primitive fishes or Selachii, the earliest Gnathostomes, was
developed the legion of the Ganoids. There are very few genera now of
this interesting and varied group--the ancient sturgeons (Accipenser),
the eggs of which are eaten as caviare, and the stratified pikes
(Polypterus, Figure 2.255) in African rivers, and bony pikes
(Lepidosteus) in the rivers of North America. On the other hand, we
have a great variety of specimens of this group in the fossil state,
from the Upper Silurian onward. Some of these fossil Ganoids approach
closely to the Selachii; others are nearer to the Dipneusts; others
again represent a transition to the Teleostei. For our genealogical
purposes the most interesting are the intermediate forms between the
Selachii and the Dipneusts. Huxley, to whom we owe particularly
important works on the fossil Ganoids, classed them in the order of
the Crossopterygii. Many genera and species of this order are found in
the Devonian and Carboniferous strata (Figure 2.253); a single,
greatly modified survivor of the group is still found in the large
rivers of Africa (Polypterus, Figure 2.255, and the closely related
Calamichthys). In many impressions of the Crossopterygii the floating
bladder seems to be ossified, and therefore well preserved--for
instance, in the Undina (Figure 2.254, immediately behind the head).

Part of these Crossopterygii approach very closely in their chief
anatomic features to the Dipneusts, and thus represent
phylogenetically the transition from the Devonian Ganoids to the
earliest air-breathing vertebrates. This important advance was made in
the Devonian period. The numerous fossils that we have from the first
two geological sections, the Laurentian and Cambrian periods, belong
exclusively to aquatic plants and animals. From this paleontological
fact, in conjunction with important geological and biological
indications, we may infer with some confidence that there were no
terrestrial animals at that time. During the whole of the vast
archeozoic period--many millions of years--the living population of
our planet consisted almost exclusively of aquatic organisms; this is
a very remarkable fact, when we remember that this period embraces the
larger half of the whole history of life. The lower animal-stems are
wholly (or with very few exceptions) aquatic. But the higher stems
also remained in the water during the primordial epoch. It was only
towards its close that some of them came to live on land. We find
isolated fossil remains of terrestrial animals first in the Upper
Silurian, and in larger numbers in the Devonian strata, which were
deposited at the beginning of the second chief section of geology (the
paleozoic age). The number increases considerably in the Carboniferous
and Permian deposits. We find many species both of the articulate and
the vertebrate stem that lived on land and breathed the atmosphere;
their aquatic ancestors of the Silurian period only breathed water.
This important change in respiration is the chief modification that
the animal organism underwent in passing from the water to the solid
land. The first consequence was the formation of lungs for breathing
air; up to that time the gills alone had served for respiration. But
there was at the same time a great change in the circulation and its
organs; these are always very closely correlated to the respiratory
organs. Moreover, the limbs and other organs were also more or less
modified, either in consequence of remote correlation to the preceding
or owing to new adaptations.

(FIGURE 2.252. Tooth of a gigantic shark (Carcharodon megalodon), from
the Pliocene at Malta. Half natural size. (From Zittel.))

In the vertebrate stem it was unquestionably a branch of the
fishes--in fact, of the Ganoids--that made the first fortunate
experiment during the Devonian period of adapting themselves to
terrestrial life and breathing the atmosphere. This led to a
modification of the heart and the nose. The true fishes have merely a
pair of blind olfactory pits on the surface of the head; but a
connection of these with the cavity of the mouth was now formed. A
canal made its appearance on each side, and led directly from the
nasal depression into the mouth-cavity, thus conveying atmospheric air
to the lungs even when the mouth was closed. Further, in all true
fishes the heart has only two sections--an atrium that receives the
venous blood from the veins, and a ventricle that propels it through a
conical artery to the gills; the atrium was now divided into two
halves, or  and  auricles, by an incomplete partition. The
 auricle alone now received the venous blood from the body, while
the  auricle received the venous blood that flowed from the lungs
and gills to the heart. Thus the double circulation of the higher
vertebrates was evolved from the simple circulation of the true
fishes, and, in accordance with the laws of correlation, this advance
led to others in the structure of other organs.

(FIGURE 2.253. A Devonian Crossopterygius (Holoptychius nobilissimus,
from the Scotch old red sandstone. (From Huxley.)

FIGURE 2.254. A Jurassic Crossopterygius (Undina penicillata), from
the upper Jurassic at Eichstatt. (From Zittel.) j jugular plates, b
three ribbed scales.

FIGURE 2.255. A living Crossopterygius, from the Upper Nile
(Polypterus bichir).

FIGURE 2.256. Fossil Dipneust (Dipterus Valenciennesi), from the old
red sandstone (Devon). (From Pander.)

FIGURE 2.257. The Australian Dipneust (Ceratodus Forsteri). B view
from the , A lower side of the skull, C lower jaw. (From
Gunther.) Qu quadrate bone, Psph parasphenoid, PtP pterygopalatinum,
Vo vomer, d teeth, na nostrils, Br branchial cavity, C first rib. D
lower-jaw teeth of the fossil Ceratodus Kaupi (from the Triassic).)

The vertebrate class, that thus adapted itself to breathing the
atmosphere, and was developed from a branch of the Ganoids, takes the
name of the Dipneusts or Dipnoa ("double-breathers"), because they
retained the earlier gill-respiration along with the new pulmonary
(lung) respiration, like the lowest amphibia. This class was
represented during the paleozoic age (or the Devonian, Carboniferous,
and Permian periods) by a number of different genera. There are only
three genera of the class living to-day: Protopterus annectens in the
rivers of tropical Africa (the White Nile, the Niger, Quelliman,
etc.), Lepidosiren paradoxa in tropical South America (in the
tributaries of the Amazon), and Ceratodus Forsteri in the rivers of
East Australia. This wide distribution of the three isolated survivors
proves that they represent a group that was formerly very large. In
their whole structure they form a transition from the fishes to the
amphibia. The transitional formation between the two classes is so
pronounced in the whole organisation of these remarkable animals that
zoologists had a lively controversy over the question whether they
were really fishes or amphibia. Several distinguished zoologists
classed them with the amphibia, though most now associate them with
the fishes. As a matter of fact, the characters of the two classes are
so far united in the Dipneusts that the answer to the question depends
entirely on the definition we give of "fish" and "amphibian." In
habits they are true amphibia. During the tropical winter, in the
rainy season, they swim in the water like the fishes, and
breathe water by gills. During the dry season they bury themselves in
the dry mud, and breathe the atmosphere through lungs, like the
amphibia and the higher vertebrates. In this double respiration they
resemble the lower amphibia, and have the same characteristic
formation of the heart; in this they are much superior to the fishes.
But in most other features they approach nearer to the fishes, and are
inferior to the amphibia. Externally they are entirely fish-like.

(FIGURE 2.258. Young ceratodus, shortly after issuing from the egg,
magnified ten times. k gill-cover, l liver. (From Richard Semon.)

FIGURE 2.259. Young ceratodus six weeks after issuing from the egg. s
spiral fold of gut, b rudimentary belly-fin. (From Richard Semon.))

In the Dipneusts the head is not marked off from the trunk. The skin
is covered with large scales. The skeleton is soft, cartilaginous, and
at a low stage of development, as in the lower Selachii and the
earliest Ganoids. The chorda is completely retained, and surrounded by
an unsegmented sheath. The two pairs of limbs are very simple fins of
a primitive type, like those of the lowest Selachii. The formation of
the brain, the gut, and the sexual organs is also the same as in the
Selachii. Thus the Dipneusts have preserved by heredity many of the
less advanced features of our primitive fish-like ancestors, and at
the same time have made a great step forward in adaptation to
air-breathing by means of lungs and the correlative improvement of the
heart.

Ceratodus is particularly interesting on account of the primitive
build of its skeleton; the cartilaginous skeleton of its two pairs of
fins, for instance, has still the original form of a bi-serial or
feathered leaf, and was on that account described by Gegenbaur as a
"primitive fin-skeleton." On the other hand, the skeleton of the pairs
of fins is greatly reduced in the African dipneust (Protopterus) and
the American (Lepidosiren). Further, the lungs are double in these
modern dipneusts, as in all the other air-breathing vertebrates; they
have on that account been called "double-lunged" (Dipneumones) in
contrast to the Ceratodus; the latter has only a single lung
(Monopneumones). At the same time the gills also are developed as
water-breathing organs in all these lung-fishes. Protopterus has
external as well as internal gills.

The paleozoic Dipneusts that are in the direct line of our ancestry,
and form the connecting-bridge between the Ganoids and the Amphibia,
differ in many respects from their living descendants, but agree with
them in the above essential features. This is confirmed by a number of
interesting facts that have lately come to our knowledge in connection
with the embryonic development of the Ceratodus and Lepidosiren; they
give us important information as to the stem-history of the lower
Vertebrates, and therefore of our early ancestors of the paleozoic
age.

</section>
<section id="7" fatherid="0" title="OUR FIVE-TOED ANCESTORS" lv="CHAPTER 2.22." priority="0" level="1">
With the phylogenetic study of the four higher classes of Vertebrates,
which must now engage our attention, we reach much firmer ground and
more light in the construction of our genealogy than we have, perhaps,
enjoyed up to the present. In the first place, we owe a number of very
valuable data to the very interesting class of Vertebrates that come
next to the Dipneusts and have been developed from them--the Amphibia.
To this group belong the salamander, the frog, and the toad. In
earlier days all the reptiles were, on the example of Linne, classed
with the Amphibia (lizards, serpents, crocodiles, and tortoises). But
the reptiles are much more advanced than the Amphibia, and are nearer
to the birds in the chief points of their structure. The true Amphibia
are nearer to the Dipneusta and the fishes; they are also much older
than the reptiles. There were plenty of highly-developed (and
sometimes large) Amphibia during the Carboniferous period; but the
earliest reptiles are only found in the Permian period. It is probable
that the Amphibia were evolved even earlier--during the Devonian
period--from the Dipneusta. The extinct Amphibia of which we have
fossil remains from that remote period (very numerous especially in
the Triassic strata) were distinguished for a graceful scaly coat or a
powerful bony armour on the skin (like the crocodile), whereas the
living amphibia have usually a smooth and slippery skin.

The earliest of these armoured Amphibia (Phractamphibia) form the
order of Stegocephala ("roof-headed") (Figure 2.260). It is among
these, and not among the actual Amphibia, that we must look for the
forms that are directly related to the genealogy of our race, and are
the ancestors of the three higher classes of Vertebrates. But even the
existing Amphibia have such important relations to us in their
anatomic structure, and especially their embryonic development, that
we may say: Between the Dipneusts and the Amniotes there was a series
of extinct intermediate forms which we should certainly class with the
Amphibia if we had them before us. In their whole organisation even
the actual Amphibia seem to be an instructive transitional group. In
the important respects of respiration and circulation they approach
very closely to the Dipneusta, though in other respects they are far
superior to them.

This is particularly true of the development of their limbs or
extremities. In them we find these for the first time as five-toed
feet. The thorough investigations of Gegenbaur have shown that the
fish's fins, of which very erroneous opinions were formerly held, are
many-toed feet. The various cartilaginous or bony radii that are found
in large numbers in each fin correspond to the fingers or toes of the
higher Vertebrates. The several joints of each fin-radius correspond
to the various parts of the toe. Even in the Dipneusta the fin is of
the same construction as in the fishes; it was afterwards gradually
evolved into the five-toed form, which we first encounter in the
Amphibia. This reduction of the number of the toes to six, and then to
five, probably took place in the second half of the Devonian
period--at the latest, in the subsequent Carboniferous period--in
those Dipneusta which we regard as the ancestors of the Amphibia. We
have several fossil remains of five-toed Amphibia from this period.
There are numbers of fossil impressions of them in the Triassic of
Thuringia (Chirotherium).

(FIGURE 2.260. Fossil amphibian from the Permian, found in the Plauen
terrain near Dresden (Branchiosaurus amblystomus). (From Credner.) A
skeleton of a young larva. B larva, restored, with gills. C the adult
form, natural size.)

The fact that the toes number five is of great importance, because
they have clearly been transmitted from the Amphibia to all the higher
Vertebrates. Man entirely resembles his amphibian ancestors in this
respect, and indeed in the whole structure of the bony skeleton of his
five-toed extremities. A careful comparison of the skeleton of the
frog with our own is enough to show this. It is well known that this
hereditary number of the toes has assumed a very great practical
importance from remote times; on it our whole system of enumeration
(the decimal system applied to measurement of time, mass, weight,
etc.) is based. There is absolutely no reason why there should be five
toes in the fore and hind feet in the lowest Amphibia, the reptiles,
and the higher Vertebrates, unless we ascribe it to inheritance from a
common stem-form. Heredity alone can explain it. It is true that we
find less than five toes in many of the Amphibia and of the higher
Vertebrates. But in all these cases we can prove that some of the toes
atrophied, and were in time lost altogether.

The causes of this evolution of the five-toed foot from the many-toed
fin in the amphibian ancestor must be sought in adaptation to the
entire change of function that the limbs experienced in passing from
an exclusively aquatic to a partly terrestrial life. The many-toed fin
had been used almost solely for motion in the water; it had now also
to support the body in creeping on the solid ground. This led to a
modification both of the skeleton and the muscles of the limbs. The
number of the fin-radii was gradually reduced, and sank finally to
five. But these five remaining radii became much stronger. The soft
cartilaginous radii became bony rods. The rest of the skeleton was
similarly strengthened. Thus from the one-armed lever of the many-toed
fish-fin arose the improved many-armed lever system of the five-toed
amphibian limbs. The movements of the body gained in variety as well
as in strength. The various parts of the skeletal system and
correlated muscular system began to differentiate more and more. In
view of the close correlation of the muscular and nervous systems,
this also made great advance in structure and function. Hence we find,
as a matter of fact, that the brain is much more developed in the
higher Amphibia than in the fishes, the Dipneusta, and the lower
Amphibia.

The first advance in organisation that was occasioned by the adoption
of life on land was naturally the construction of an organ for
breathing air--a lung. This was formed directly from the
floating-bladder inherited from the fishes. At first its function was
insignificant beside that of the gills, the older organ for
water-respiration. Hence we find in the lowest Amphibia, the gilled
Amphibia, that, like the Dipneusta, they pass the greater part of
their life in the water, and breathe water through gills. They only
come to the surface at brief intervals, or creep on to the land, and
then breathe air by their lungs. But some of the tailed Amphibia--the
salamanders--remain entirely in the water when they are young, and
afterwards spend most of their time on land. In the adult state they
only breathe air through lungs. The same applies to the most advanced
of the Amphibia, the Batrachia (frogs and toads); some of them have
entirely lost the gill-bearing larva form.* (* The tree-frog of
Martinique (Hylades martinicensis) loses the gills on the seventh, and
the tail and yelk-sac on the eighth, day of foetal life. On the ninth
or tenth day after fecundation the frog emerges from the egg.) This is
also the case with certain small, serpentine Amphibia, the Caecilia
(which live in the ground like earth-worms).

(FIGURE 2.261. Larva of the Spotted Salamander (Salamandra maculata),
seen from the ventral side. In the centre a yelk-sac still hangs from
the gut. The external gills are gracefully ramified. The two pairs of
legs are still very small.)

The great interest of the natural history of the Amphibia consists
especially in their intermediate position between the lower and higher
Vertebrates. The lower Amphibia approach very closely to the Dipneusta
in their whole organisation, live mainly in the water, and breathe by
gills; but the higher Amphibia are just as close to the Amniotes, live
mainly on land, and breathe by lungs. But in their younger state the
latter resemble the former, and only reach the higher stage by a
complete metamorphosis. The embryonic development of most of the
higher Amphibia still faithfully reproduces the stem-history of the
whole class, and the various stages of the advance that was made by
the lower Vertebrates in passing from aquatic to terrestrial life
during the Devonian or the Carboniferous period are repeated in the
spring by every frog that develops from an egg in our ponds.

(FIGURE 2.262. Larva of the common grass-frog (Rana temporaria), or
"tadpole." m mouth, n a pair of suckers for fastening on to stones, d
skin-fold from which the gill-cover develops; behind it the
gill-cs, from which the branching gills (k) protrude, s
tail-muscles, f cutaneous fin-fringe of the tail.)

The common frog leaves the egg in the shape of a larva, like the
tailed salamander (Figure 2.261), and this is altogether different
from the mature frog (Figure 2.262). The short trunk ends in a long
tail, with the form and structure of a fish's tail (s). There are no
limbs at first. The respiration is exclusively branchial, first
through external (k) and then internal gills. In harmony with this the
heart has the same structure as in the fish, and consists of two
sections--an atrium that receives the venous blood from the body, and
a ventricle that forces it through the arteries into the gills.

We find the larvae of the frog (or tadpoles, Gyrini) in great numbers
in our ponds every spring in this fish-form, using their muscular
tails in swimming, just like the fishes and young Ascidia. When they
have reached a certain size, the remarkable metamorphosis from the
fish-form to the frog begins. A blind sac grows out of the gullet, and
expands into a couple of spacious sacs: these are the lungs. The
simple chamber of the heart is divided into two sections by the
development of a partition, and there are at the same time
considerable changes in the structure of the chief arteries.
Previously all the blood went from the auricle through the aortic
arches into the gills, but now only part of it goes to the gills, the
other part passing to the lungs through the new-formed pulmonary
artery. From this point arterial blood returns to the  auricle of
the heart, while the venous blood gathers in the  auricle. As
both auricles open into a single ventricle, this contains mixed blood.
The dipneust form has now succeeded to the fish-form. In the further
course of the metamorphosis the gills and the branchial vessels
entirely disappear, and the respiration becomes exclusively pulmonary.
Later, the long swimming tail is lost, and the frog now hops to the
land with the legs that have grown meantime.

This remarkable metamorphosis of the Amphibia is very instructive in
connection with our human genealogy, and is particularly interesting
from the fact that the various groups of actual Amphibia have remained
at different stages of their stem-history, in harmony with the
biogenetic law. We have first of all a very low order of Amphibia--the
Sozobranchia ("gilled-amphibia"), which retain their gills throughout
life, like the fishes. In a second order of the salamanders the gills
are lost in the metamorphosis, and when fully grown they have only
pulmonary respiration. Some of the tailed Amphibia still retain the
gill-cs in the side of the neck, though they have lost the gills
themselves (Menopoma). If we force the larvae of our salamanders
(Figure 2.261) and tritons to remain in the water, and prevent them
from reaching the land, we can in favourable circumstances make them
retain their gills. In this fish-like condition they reach sexual
maturity, and remain throughout life at the lower stage of the gilled
Amphibia.

(FIGURE 2.263. Fossil mailed amphibian, from the Bohemian
Carboniferous (Seeleya). (From Fritsch.) The scaly coat is retained on
the .)

We have the reverse of this experiment in a Mexican gilled salamander,
the fish-like axolotl (Siredon pisciformis). It was formerly regarded
as a permanent gilled amphibian persisting throughout life at the
fish-stage. But some of the hundreds of these animals that are kept in
the Botanical Garden at Paris got on to the land for some reason or
other, lost their gills, and changed into a form closely resembling
the salamander (Amblystoma). Other species of the genus became
sexually mature for the first time in this condition. This has been
regarded as an astounding phenomenon, although every common frog and
salamander repeats the metamorphosis in the spring. The whole change
from the aquatic and gill-breathing animal to the terrestrial
lung-breathing form may be followed step by step in this case. But
what we see here in the development of the individual has happened to
the whole class in the course of its stem-history.

The metamorphosis goes farther in a third order of Amphibia, the
Batrachia or Anura, than in the salamander. To this belong the various
kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. These
lose, not only the gills, but also (sooner or later) the tail, during
metamorphosis.

The ontogenetic loss of the gills and the tail in the frog and toad
can only be explained on the assumption that they are descended from
long-tailed Amphibia of the salamander type. This is also clear from
the comparative anatomy of the two groups. This remarkable
metamorphosis is, however, also interesting because it throws a
certain light on the phylogeny of the tail-less apes and man. Their
ancestors also had long tails and gills like the gilled Amphibia, as
the tail and the gill-arches of the human embryo clearly show.

For comparative anatomical and ontogenetic reasons, we must not seek
these amphibian ancestors of ours--as one would be inclined to do,
perhaps--among the tail-less Batrachia, but among the tailed lower
Amphibia.

The vertebrate form that comes next to the Amphibia in the series of
our ancestors is a lizard-like animal, the earlier existence of which
can be confidently deduced from the facts of comparative anatomy and
ontogeny. The living Hatteria of New Zealand (Figure 2.264) and the
extinct Rhyncocephala of the Permian period (Figure 2.265) are closely
related to this important stem-form; we may call them the
Protamniotes, or Primitive Amniotes. All the Vertebrates above the
Amphibia--or the three classes of reptiles, birds, and mammals--differ
so much in their whole organisation from all the lower Vertebrates we
have yet considered, and have so great a resemblance to each other,
that we put them all together in a single group with the title of
Amniotes. In these three classes alone we find the remarkable
embryonic membrane, already mentioned, which we called the amnion; a
cenogenetic adaptation that we may regard as a result of the sinking
of the growing embryo into the yelk-sac.

All the Amniotes known to us--all reptiles, birds, and mammals
(including man)--agree in so many important points of internal
structure and development that their descent from a common ancestor
can be affirmed with tolerable certainty. If the evidence of
comparative anatomy and ontogeny is ever entirely beyond suspicion, it
is certainly the case here. All the peculiarities that accompany and
follow the formation of the amnion, and that we have learned in our
consideration of human embryology; all the peculiarities in the
development of the organs which we will presently follow in detail;
finally, all the principal special features of the internal structure
of the full-grown Amniotes--prove so clearly the common origin of all
the Amniotes from single extinct stem-form that it is difficult to
entertain the idea of their evolution from several independent stems.
This unknown common stem-form is our primitive Amniote (Protamnion).
In outward appearance it was probably something between the salamander
and the lizard.

It is very probable that some part of the Permian period was the age
of the origin of the Protamniotes. This follows from the fact that the
Amphibia are not fully developed until the Carboniferous period, and
that the first fossil reptiles (Palaehatteria, Homoeosaurus,
Proterosaurus) are found towards the close of the Permian period.
Among the important changes of the vertebrate organisation that marked
the rise of the first Amniotes from salamandrine Amphibia during this
period the following three are especially noteworthy: the entire
disappearance of the water-breathing gills and the conversion of the
gill-arches into other organs, the formation of the allantois or
primitive urinary sac, and the development of the amnion.

One of the most salient characteristics of the Amniotes is the
complete loss of the gills. All Amniotes, even if living in water
(such as sea-serpents and whales), breathe air through lungs, never
water through gills. All the Amphibia (with very rare exceptions)
retain their gills for some time when young, and have for a time (if
not permanently) branchial respiration; but after these there is no
question of branchial respiration. The Protamniote itself must have
entirely abandoned water-breathing. Nevertheless, the gill-arches are
preserved by heredity, and develop into totally different (in part
rudimentary) organs--various parts of the bone of the tongue, the
frame of the jaws, the organ of hearing, etc. But we do not find in
the embryos of the Amniotes any trace of gill-leaves, or of real
respiratory organs on the gill-arches.

With this complete abandonment of the gills is probably connected the
formation of another organ, to which we have already referred in
embryology--namely, the allantois or primitive urinary sac (cf.
Chapter 1.15). It is very probable that the urinary bladder of the
Dipneusts is the first structure of the allantois. We find in these a
urinary bladder that proceeds from the lower wall of the hind end of
the gut, and serves as receptacle for the renal secretions. This organ
has been transmitted to the Amphibia, as we can see in the frog.

The formation of the amnion and the allantois and the complete
disappearance of the gills are the chief characteristics that
distinguish the Amniotes from the lower Vertebrates we have hitherto
considered. To these we may add several subordinate features that are
transmitted to all the Amniotes, and are found in these only. One
striking embryonic character of the Amniotes is the great curve of the
head and neck in the embryo. We also find an advance in the structure
of several of the internal organs of the Amniotes which raises them
above the highest of the anamnia. In particular, a partition is formed
in the simple ventricle of the heart, dividing into  and 
chambers. In connection with the complete metamorphosis of the
gill-arches we find a further development of the auscultory organs.
Also, there is a great advance in the structure of the brain,
skeleton, muscular system, and other parts. Finally, one of the most
important changes is the reconstruction of the kidneys. In all the
earlier Vertebrates we have found the primitive kidneys as excretory
organs, and these appear at an early stage in the embryos of all the
higher Vertebrates up to man. But in the Amniotes these primitive
kidneys cease to act at an early stage of embryonic life, and their
function is taken up by the permanent or secondary kidneys, which
develop from the terminal section of the prorenal ducts.

(FIGURE 2.264. The lizard (Hatteria punctata = Sphenodon punctatus) of
New Zealand. The sole surviving proreptile. (From Brehm.))

Taking all these peculiarities of the Amniotes together, it is
impossible to doubt that all the animals of this group--all reptiles,
birds, and mammals--have a common origin, and form a single
blood-related stem. Our own race belongs to this stem. Man is, in
every feature of his organisation and embryonic development, a true
Amniote, and has descended from the Protamniote with all the other
Amniotes. Though they appeared at the end (possibly even in the
middle) of the Paleozoic age, the Amniotes only reached their full
development during the Mesozoic age. The birds and mammals made their
first appearance during this period. Even the reptiles show their
greatest growth at this time, so that it is called "the reptile age."
The extinct Protamniote, the ancestor of the whole group, belongs in
its whole organisation to the reptile class.

The genealogical tree of the amniote group is clearly indicated in its
chief lines by their paleontology, comparative anatomy, and ontogeny.
The group succeeding the Protamniote divided into two branches. The
branch that will claim our whole interest is the class of the Mammals.
The other branch, which developed in a totally different direction,
and only comes in contact with the Mammals at its root, is the
combined group of the reptiles and birds; these two classes may, with
Huxley, be conveniently grouped together as the Sauropsida. Their
common stem-form is an extinct lizard-like reptile of the order of the
Rhyncocephalia. From this have been developed in various directions
the serpents, crocodiles, tortoises, etc.--in a word, all the members
of the reptile class. But the remarkable class of the birds has also
been evolved directly from a branch of the reptile group, as is now
established beyond question. The embryos of the reptiles and birds are
identical until a very late stage, and have an astonishing resemblance
even later. Their whole structure agrees so much that no anatomist now
questions the descent of the birds from the reptiles. On the other
hand, the mammal line has descended from the group of the
Sauromammalia, a different branch of the Proreptilia. It is connected
at its deepest roots with the reptile line, but it then diverges
completely from it and follows a distinctive development. Man is the
highest outcome of this class, the "crown of creation." The hypothesis
that the three higher Vertebrate classes represent a single
Amniote-stem, and that the common root of this stem is to be found in
the amphibian class, is now generally admitted.

(FIGURE 2.265. Homoeosaurus pulchellus, a Jurassic proreptile from
Kehlheim. (From Zittel.))

The instructive group of the Permian Tocosauria, the common root from
which the divergent stems of the Sauropsids and mammals have issued,
merits our particular attention as the stem-group of all the Amniotes.
Fortunately a living representative of this extinct ancestral group
has been preserved to our day; this is the remarkable lizard of New
Zealand, Hatteria punctata (Figure 2.264). Externally it differs
little from the ordinary lizard; but in many important points of
internal structure, especially in the primitive construction of the
vertebral column, the skull, and the limbs, it occupies a much lower
position, and approaches its amphibian ancestors, the Stegocephala.
Hence Hatteria is the phylogenetically oldest of all living reptiles,
an isolated survivor from the Permian period, closely resembling the
common ancestor of the Amniotes. It must differ so little from this
extinct form, our hypothetical Protamniote, that we put it next to the
Proreptilia. The remarkable Permian Palaehatteria, that Credner
discovered in the Plauen terrain at Dresden in 1888, belongs to the
same group (Figure 2.266). The Jurassic genus Homoeosaurus (Figure
2.265), of which well-preserved skeletons are found in the Solenhofen
schists, is perhaps still more closely related to them.

Unfortunately, the numerous fossil remains of Permian and Triassic
Tocosauria that we have found in the last two decades are, for the
most part, very imperfectly preserved. Very often we can make only
precarious inferences from these skeletal fragments as to the anatomic
characters of the soft parts that went with the bony skeleton of the
extinct Tocosauria. Hence it has not yet been possible to arrange
these important fossils with any confidence in the ancestral series
that descend from the Protamniotes to the Sauropsids on the one side
and the Mammals on the other. Opinions are particularly divided as to
the place in classification and the phylogenetic significance of the
remarkable Theromorpha. Cope gives this name to a very interesting and
extensive group of extinct terrestrial reptiles, of which we have only
fossil remains from the Permian and Triassic strata. Forty years ago
some of these Therosauria (fresh-water animals) were described by Owen
as Anomodontia. But during the last twenty years the distinguished
American paleontologists, Cope and Osborn, have greatly increased our
knowledge of them, and have claimed that the stem-forms of the Mammals
must be sought in this order. As a matter of fact, the Theromorpha are
nearer to the Mammals in the chief points of structure than any other
reptiles. This is especially true of the Thereodontia, to which the
Pureosauria and Pelycosauria belong (Figure 2.267). The whole
structure of their pelvis and hind-feet has attained the same form as
in the Monotremes, the lowest Mammals. The formation of the scapula
and the quadrate bone shows an approach to the Mammals such as we find
in no other group of reptiles. The teeth also are already divided into
incisors, canines, and molars. Nevertheless, it is very doubtful
whether the Theromorpha really are in the ancestral line of the
Sauromammals, or lead direct from the Tocosauria to the earliest
Mammals. Other experts on this group believe that it is an independent
legion of the reptiles, connected, perhaps, at its lowest root, with
the Sauromammals, but developed quite independently of the
Mammals--though parallel to them in many ways.

One of the most important of the zoological facts that we rely on in
our investigation of the genealogy of the human race is the position
of man in the Mammal class. However different the views of zoologists
may have been as to this position in detail, and as to his relations
to the apes, no scientist has ever doubted that man is a true mammal
in his whole organisation and development. Linne drew attention to
this fact in the first edition of his famous Systema Naturae (1735).
As will be seen in any museum of anatomy or any manual of comparative
anatomy; the human frame has all the characteristics that are common
to the Mammals and distinguish them conspicuously from all other
animals.

(FIGURE 2.266. Skull of a Permian lizard (Palaehatteria longicaudata).
(From Credner.) n nasal bone, pf frontal bone, l lachrymal bone, po
postorbital bone, sq covering bone, i cheek-bone, vo vomer, im
inter-maxillary.)

If we examine this undoubted fact from the point of view of phylogeny,
in the light of the theory of descent, it follows at once that man is
of a common stem with all the other Mammals, and comes from the same
root as they. But the various features in which the Mammals agree and
by which they are distinguished are of such a character as to make a
polyphyletic hypothesis quite inadmissible. It is impossible to
entertain the idea that all the living and extinct Mammals come from a
number of separate roots. If we accept the general theory of
evolution, we are bound to admit the monophyletic hypothesis of the
descent of all the Mammals (including man) from a single mammalian
stem-form. We may call this long-extinct root-form and its earliest
descendants (a few genera of one family) "primitive mammals" or
"stem-mammals" (Promammalia). As we have already seen, this root-form
developed from the primitive Proreptile stem in a totally different
direction from the birds, and soon separated from the main stem of the
reptiles. The differences between the Mammals and the reptiles and
birds are so important and characteristic that we can assume with
complete confidence this division of the vertebrate stem at the
commencement of the development of the Amniotes. The reptiles and
birds, which we group together as the Sauropsids, generally agree in
the characteristic structure of the skull and brain, and this is
notably different from that of the Mammals. In most of the reptiles
and birds the skull is connected with the first cervical vertebra (the
atlas) by a single, and in the Mammals (and Amphibia) by a double,
condyle at the back of the head. In the former the lower jaw is
composed of several pieces, and connected with the skull so that it
can move by a special maxillary bone (the quadratum); in the Mammals
the lower jaw consists of one pair of bony pieces, which articulate
directly with the temporal bone. Further, in the Sauropsids the skin
is clothed with scales or feathers; in the Mammals with hair. The red
blood-cells of the former have a nucleus; those of the latter have
not. In fine, two quite characteristic features of the Mammals, which
distinguish them not only from the birds and reptiles, but from all
other animals, are the possession of a complete diaphragm and of
mammary glands that produce the milk for the nutrition of the young.
It is only in the Mammals that the diaphragm forms a transverse
partition of the body-cavity, completely separating the pectoral from
the abdominal cavity. It is only in the mammals that the mother
suckles its young, and this ly gives the name to the whole class
(mamma = breast).

(FIGURE 2.267. Skull of a Triassic theromorphum (Galesaurus
planiceps), from the Karoo formation in South Africa. (From Owen.) a
from the , b from below, c from above, d tricuspid tooth. N
nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal
bone, A eye-pits, S temple-pits. Pa Parietal eye, Bo joint at back of
head, Pt pterygoid-bone, Md lower jaw.)

From these pregnant facts of comparative anatomy and ontogeny it
follows absolutely that the whole of the Mammals belong to a single
natural stem, which branched off at an early date from the
reptile-root. It follows further with the same absolute certainty that
the human race is also a branch of this stem. Man shares all the
characteristics I have described with all the Mammals, and differs in
them from all other animals. Finally, from these facts we deduce with
the same confidence those advances in the vertebrate organisation by
which one branch of the Sauromammals was converted into the stem-form
of the Mammals. Of these advances the chief were: (1) The
characteristic modification of the skull and the brain; (2) the
development of a hairy coat; (3) the complete formation of the
diaphragm; and (4) the construction of the mammary glands and
adaptation to suckling. Other important changes of structure proceeded
step by step with these.

The epoch at which these important advances were made, and the
foundation of the Mammal class was laid, may be put with great
probability in the first section of the Mesozoic or secondary age--the
Triassic period. The oldest fossil remains of mammals that we know
were found in strata that belong to the earliest Triassic period--the
upper Kueper. One of the earliest forms is the genus Dromatherium,
from the North American Triassic (Figure 2.268). Their teeth still
strikingly recall those of the Pelycosauria. Hence we may assume that
this small and probably insectivorous mammal belonged to the
stem-group of the Promammals. We do not find any positive trace of the
third and most advanced division of the Mammals--the Placentals. These
(including man) are much younger, and we do not find indisputable
fossil remains of them until the Cenozoic age, or the Tertiary period.
This paleontological fact is very important, because it fully
harmonises with the evolutionary succession of the Mammal orders that
is deduced from their comparative anatomy and ontogeny.

The latter science teaches us that the whole Mammal class divides into
three main groups or sub-classes, which correspond to three successive
phylogenetic stages. These three stages, which also represent three
important stages in our human genealogy, were first distinguished in
1816 by the eminent French zoologist, Blainville, and received the
names of Ornithodelphia, Didelphia, and Monodelphia, according to the
construction of the female organs (delphys = uterus or womb). Huxley
afterwards gave them the names of Prototheria, Metatheria, and
Epitheria. But the three sub-classes differ so widely from each other,
not only in the construction of the sexual organs, but in many other
respects also, that we may confidently draw up the following important
phylogenetic thesis: The Monodelphia or Placentals descend from the
Didelphia or Marsupials; and the latter, in turn, are descended from
the Monotremes or Ornithodelphia.

Thus we must regard as the twenty-first stage in our genealogical tree
the earliest and lowest chief group of the Mammals--the sub-class of
the Monotremes ("cloaca-animals," Ornithodelphia, or Prototheria,
Figures 2.269 and 2.270). They take their name from the cloaca which
they share with all the lower Vertebrates. This cloaca is the common
outlet for the passage of the excrements, the urine, and the sexual
products. The urinary ducts and sexual canals open into the hindmost
part of the gut, while in all the other Mammals they are separated
from the rectum and anus. The latter have a special uro-genital outlet
(porus urogenitalis). The bladder also opens into the cloaca in the
Monotremes, and, indeed, apart from the two urinary ducts; in all the
other Mammals the latter open directly into the bladder. It was proved
by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like
the reptiles, while all the other Mammals are viviparous. In 1894
Richard Semon further proved that these large eggs, rich in food-yelk,
have a partial segmentation and discoid gastrulation, as I had
hypothetically assumed in 1879; here again they resemble their
reptilian ancestors. The construction of the mammary gland is also
peculiar in the Monotremes. In them the glands have no teats for the
young animal to suck, but there is a special part of the breast
pierced with holes like a sieve, from which the milk issues, and the
young Monotreme must lick it off. Further, the brain of the Monotremes
is very little advanced. It is feebler than that of any of the other
Mammals. The fore-brain or cerebrum, in particular, is so small that
it does not cover the cerebellum. In the skeleton (Figure 2.270) the
formation of the scapula among other parts is curious; it is quite
different from that of the other Mammals, and rather agrees with that
of the reptiles and Amphibia. Like these, the Monotremes have a
strongly developed caracoideum. From these and other less prominent
characteristics it follows absolutely that the Monotremes occupy the
lowest place among the Mammals, and represent a transitional group
between the Tocosauria and the rest of the Mammals. All these
remarkable reptilian characters must have been possessed by the
stem-form of the whole mammal class, the Promammal of the Triassic
period, and have been inherited from the Proreptiles.

(FIGURE 2.268. Lower jaw of a Primitive Mammal or Promammal
(Dromatherium silvestre) from the North American Triassic. i incisors,
c canine, p premolars, m molars. (From Doderlein.))

During the Triassic and Jurassic periods the sub-class of the
Monotremes was represented by a number of different stem-mammals.
Numerous fossil remains of them have lately been discovered in the
Mesozoic strata of Europe, Africa, and America. To-day there are only
two surviving specimens of the group, which we place together in the
family of the duck-bills, Ornithostoma. They are confined to Australia
and the neighbouring island of Van Diemen's Land (or Tasmania); they
become scarcer every year, and will soon, like their blood-relatives,
be counted among the extinct animals. One form lives in the rivers,
and builds subterraneous dwellings on the banks; this is the
Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and
broad flat jaws, which look very much like the bill of a duck (Figures
2.269 and 2.270). The other form, the land duck-bill, or spiny
ant-eater (Echidna hystrix), is very much like the anteaters in its
habits and the peculiar construction of its thin snout and very long
tongue; it is covered with needles, and can roll itself up like a
hedgehog. A cognate form (Parechidna Bruyni) has lately been found in
New Guinea.

These modern Ornithostoma are the scattered survivors of the vast
Mesozoic group of Monotremes; hence they have the same interest in
connection with the stem history of the Mammals as the living
stem-reptiles (Hatteria) for that of the reptiles, and the isolated
Acrania (Amphioxus) for the phylogeny of the Vertebrate stem.

The Australian duck-bills are distinguished externally by a toothless
bird-like beak or snout. This absence of real bony teeth is a late
result of adaptation, as in the toothless Placentals (Edentata,
armadillos and ant-eaters). The extinct Monotremes, to which the
Promammalia belonged, must have had developed teeth, inherited from
the reptiles. Lately small rudiments of real molars have been
discovered in the young of the Ornithorhyncus, which has horny plates
in the jaws instead of real teeth.

(FIGURE 2.269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus
paradoxus).

FIGURE 2.270. Skeleton of the Ornithorhyncus.)

The living Ornithostoma and the stem-forms of the Marsupials (or
Didelphia) must be regarded as two widely diverging lines from the
Promammals. This second sub-class of the Mammals is very interesting
as a perfect intermediate stage between the other two. While the
Marsupials retain a great part of the characteristics of the
Monotremes, they have also acquired some of the chief features of the
Placentals. Some features are also peculiar to the Marsupials, such as
the construction of the male and female sexual organs and the form of
the lower jaw. The Marsupials are distinguished by a peculiar
hook-like bony process that bends from the corner of the lower jaw and
points inwards. As most of the Placentals have not this process, we
can, with some probability, recognise the Marsupial from this feature
alone. Most of the mammal remains that we have from the Jurassic and
Cretaceous deposits are merely lower jaws, and most of the jaws found
in the Jurassic deposits at Stonesfield and Purbeck have the peculiar
hook-like process that characterises the lower jaw of the Marsupial.
On the strength of this paleontological fact, we may suppose that they
belonged to Marsupials. Placentals do not seem to have existed at the
middle of the Mesozoic age--not until towards its close (in the
Cretaceous period). At all events, we have no fossil remains of
indubitable Placentals from that period.

The existing Marsupials, of which the plant-eating kangaroo and the
carnivorous opossum (Figure 2.272) are the best known, differ a good
deal in structure, shape, and size, and correspond in many respects to
the various orders of Placentals. Most of them live in Australia, and
a small part of the Australian and East Malayan islands. There is now
not a single living Marsupial on the mainland of Europe, Asia, or
Africa. It was very different during the Mesozoic and even during the
Cenozoic age. The sedimentary deposits of these periods contain a
great number and variety of marsupial remains, sometimes of a colossal
size, in various parts of the earth, and even in Europe. We may infer
from this that the existing Marsupials are the remnant of an extensive
earlier group that was distributed all over the earth. It had to give
way in the struggle for life to the more powerful Placentals during
the Tertiary period. The survivors of the group were able to keep
alive in Australia and South America because the one was completely
separated from the other parts of the earth during the whole of the
Tertiary period, and the other during the greater part of it.

(FIGURE 2.271. Lower jaw of a Promammal (Dryolestes priscus), from the
Jurassic of the Felsen strata. (From Marsh.))

From the comparative anatomy and ontogeny of the existing Marsupials
we may draw very interesting conclusions as to their intermediate
position between the earlier Monotremes and the later Placentals. The
defective development of the brain (especially the cerebrum), the
possession of marsupial bones, and the simple construction of the
allantois (without any placenta as yet) were inherited by the
Marsupials, with many other features, from the Monotremes, and
preserved. On the other hand, they have lost the independent bone
(caracoideum) at the shoulder-blade. But we have a more important
advance in the disappearance of the cloaca; the rectum and anus are
separated by a partition from the uro-genital opening (sinus
urogenitalis). Moreover, all the Marsupials have teats on the mammary
glands, at which the new-born animal sucks. The teats pass into the
cavity of a pouch or pocket on the ventral side of the mother, and
this is supported by a couple of marsupial bones. The young are born
in a very imperfect condition, and carried by the mother for some time
longer in her pouch, until they are fully developed (Figure 2.272). In
the giant kangaroo, which is as tall as a man, the embryo only
develops for a month in the uterus, is then born in a very imperfect
state, and finishes its growth in the mother's pouch (marsupium); it
remains in this about nine months, and at first hangs continually on
to the teat of the mammary gland.

(FIGURE 2.272. The crab-eating Opossum (Philander cancrivorus). The
female has three young in the pouch. (From Brehm.)

From these and other characteristics (especially the peculiar
construction of the internal and external sexual organs in male and
female) it is clear that we must conceive the whole sub-class of the
Marsupials as one stem group, which has been developed from the
Promammalia. From one branch of these Marsupials (possibly from more
than one) the stem-forms of the higher Mammals, the Placentals, were
afterwards evolved. Of the existing forms of the Marsupials, which
have undergone various modifications through adaptation to different
environments, the family of the opossums (Didelphida or Pedimana)
seems to be the oldest and nearest to the common stem-form of the
whole class. To this family belong the crab-eating opossum of Brazil
(Figure 2.272) and the opossum of Virginia, on the embryology of which
Selenka has given us a valuable work (cf. Figures 1.63 to 1.67 and
1.131 to 1.135). These Didelphida climb trees like the apes, grasping
the branches with their hand-shaped hind feet. We may conclude from
this that the stem-forms of the Primates, which we must regard as the
earliest Lemurs, were evolved directly from the opossum. We must not
forget, however, that the conversion of the five-toed foot into a
prehensile hand is polyphyletic. By the same adaptation to climbing
trees the habit of grasping their branches with the feet has in many
different cases brought about that opposition of the  or great
toe to the other toes which makes the hand prehensile. We see this in
the climbing lizards (chameleon), the birds, and the tree-dwelling
mammals of various orders.

Some zoologists have lately advanced the opposite opinion, that the
Marsupials represent a completely independent sub-class of the
Mammals, with no direct relation to the Placentals, and developing
independently of them from the Monotremes. But this opinion is
untenable if we examine carefully the whole organisation of the three
sub-classes, and do not lay the chief stress on incidental features
and secondary adaptations (such as the formation of the marsupium). It
is then clear that the Marsupials--viviparous Mammals without
placenta--are a necessary transition from the oviparous Monotremes to
the higher Placentals with chorion-villi. In this sense the Marsupial
class certainly contains some of man's ancestors.

</section>
<section id="8" fatherid="0" title="OUR APE ANCESTORS" lv="CHAPTER 2.23." priority="0" level="1">
<section id="9" fatherid="8" title="" lv="" priority="0" level="2">
The long series of animal forms which we must regard as the ancestors
of our race has been confined within narrower and narrower circles as
our phylogenetic inquiry has progressed. The great majority of known
animals do not fall in the line of our ancestry, and even within the
vertebrate stem only a small number are found to do so. In the most
advanced class of the stem, the mammals, there are only a few families
that belong directly to our genealogical tree. The most important of
these are the apes and their predecessors, the half-apes, and the
earliest Placentals (Prochoriata).

The Placentals (also called Choriata, Monodelphia, Eutheria or
Epitheria) are distinguished from the lower mammals we have just
considered, the Monotremes and Marsupials, by a number of striking
peculiarities. Man has all these distinctive features; that is a very
significant fact. We may, on the ground of the most careful
comparative-anatomical and ontogenetic research, formulate the thesis:
"Man is in every respect a true Placental." He has all the
characteristics of structure and development that distinguish the
Placentals from the two lower divisions of the mammals, and, in fact,
from all other animals. Among these characteristics we must especially
notice the more advanced development of the brain. The fore-brain or
cerebrum especially is much more developed in them than in the lower
animals. The corpus callosum, which forms a sort of wide bridge
connecting the two hemispheres of the cerebrum, is only fully formed
in the Placentals; it is very rudimentary in the Marsupials and
Monotremes. It is true that the lowest Placentals are not far removed
from the Marsupials in cerebral development; but within the placental
group we can trace an unbroken gradation of progressive development of
the brain, rising gradually from this lowest stage up to the elaborate
psychic organ of the apes and man. The human soul--a physiological
function of the brain--is in reality only a more advanced ape-soul.

The mammary glands of the Placentals are provided with teats like
those of the Marsupials; but we never find in the Placentals the pouch
in which the latter carry and suckle their young. Nor have they the
marsupial bones in the ventral wall at the anterior border of the
pelvis, which the Marsupials have in common with the Monotremes, and
which are formed by a partial ossification of the sinews of the inner
oblique abdominal muscle. There are merely a few insignificant
remnants of them in some of the Carnivora. The Placentals are also
generally without the hook-shaped process at the angle of the lower
jaw which is found in the Marsupials.

(FIGURE 2.273. Foetal membranes of the human embryo (diagrammatic). m
the thick muscular wall of the womb. plu placenta the inner layer
(plu apostrophe) of which penetrates into the chorion-villi (chz) with
its processes. chf tufted, chl smooth chorion. a amnion, ah amniotic
cavity, as amniotic sheath of the umbilical cord (which passes under
into the navel of the embryo--not given here), dg vitelline duct, ds
yelk sac, dv, dr decidua (vera and reflexa). The uterine cavity (uh)
opens below into the vagina and above on the  into an oviduct
(t). (From Kolliker.))

However, the feature that characterises the Placentals above all
others, and that has given its name to the whole sub-class, is the
formation of the placenta. We have already considered the formation
and significance of this remarkable embryonic organ when we traced the
development of the chorion and the allantois in the human embryo
(Chapter 1.15). The urinary sac or the allantois, the curious vesicle
that grows out of the hind part of the gut, has essentially the same
structure and function in the human embryo as in that of all the other
Amniotes (cf. Figures 1.194 to 1.196). There is a quite secondary
difference, on which great stress has wrongly been laid, in the fact
that in man and the higher apes the original cavity of the allantois
quickly degenerates, and the rudiment of it sticks out as a solid
projection from the primitive gut. The thin wall of the allantois
consists of the same two layers or membranes as the wall of the
gut--the gut-gland layer within and the gut-fibre layer without. In
the gut-fibre layer of the allantois there are large blood-vessels,
which serve for the nutrition, and especially the respiration, of the
embryo--the umbilical vessels (Chapter 1.15). In the reptiles and
birds the allantois enlarges into a spacious sac, which encloses the
embryo with the amnion, and does not combine with the outer foetal
membrane (the chorion). This is the case also with the lowest mammals,
the oviparous Monotremes and most of the Marsupials. It is only in
some of the later Marsupials (Peramelida) and all the Placentals that
the allantois develops into the distinctive and remarkable structure
that we call the placenta.

The placenta is formed by the branches of the blood-vessels in the
wall of the allantois growing into the hollow ectodermic tufts (villi)
of the chorion, which run into corresponding depressions in the mucous
membrane of the womb. The latter also is richly permeated with
blood-vessels which bring the mother's blood to the embryo. As the
partition in the villi between the maternal blood-vessels and those of
the foetus is extremely thin, there is a direct exchange of fluid
between the two, and this is of the greatest importance in the
nutrition of the young mammal. It is true that the maternal vessels do
not entirely pass into the foetal vessels, so that the two kinds of
blood are simply mixed. But the partition between them is so thin that
the nutritive fluid easily transudes through it. By means of this
transudation or diosmosis the exchange of fluids takes place without
difficulty. The larger the embryo is in the placentals, and the longer
it remains in the womb, the more necessary it is to have special
structures to meet its great consumption of food.

In this respect there is a very conspicuous difference between the
lower and higher mammals. In the Marsupials, in which the embryo is
only a comparatively short time in the womb and is born in a very
immature condition, the vascular arrangements in the yelk-sac and the
allantois suffice for its nutrition, as we find them in the
Monotremes, birds, and reptiles. But in the Placentals, where
gestation lasts a long time, and the embryo reaches its full
development under the protection of its enveloping membranes, there
has to be a new mechanism for the direct supply of a large quantity of
food, and this is admirably met by the formation of the placenta.

Branches of the blood-vessels penetrate into the chorion-villi from
within, starting from the gut-fibre layer of the allantois, and
bringing the blood of the foetus through the umbilical vessels (Figure
2.273 chz). On the other hand, a thick network of blood-vessels
develops in the mucous membrane that clothes the inner surface of the
womb, especially in the region of the depressions into which the
chorion-villi penetrate (plu). This network of arteries contains
maternal blood, brought by the uterine vessels. As the connective
tissue between the enlarged capillaries of the uterus disappears, wide
cavities filled with maternal blood appear, and into these the
chorion-villi of the embryo penetrate. The sum of these vessels of
both kinds, that are so intimately correlated at this point, together
with the connective and enveloping tissue, is the placenta. The
placenta consists, therefore, properly speaking, of two different
though intimately connected parts--the foetal placenta (Figure 2.273
chz) within and the maternal or uterine placenta (plu) without. The
latter is made up of the mucous coat of the uterus and its
blood-vessels, the former of the tufted chorion and the umbilical
vessels of the embryo (cf. Figure 1.196).

(FIGURE 2.274. Skull of a fossil lemur (Adapis parisiensis,), from the
Miocene at Quercy. A lateral view from the , half natural size. B
lower jaw, C lower molar, i incisors, c canines, p premolars, m
molars.)

The manner in which these two kinds of vessels combine in the
placenta, and the structure, form, and size of it, differ a good deal
in the various Placentals; to some extent they give us valuable data
for the natural classification, and therefore the phylogeny, of the
whole of this sub-class. On the ground of these differences we divide
it into two principal sections; the lower Placentals or Indecidua, and
the higher Placentals or Deciduata.

To the Indecidua belong three important groups of mammals: the Lemurs
(Prosimiae), the Ungulates (tapirs, horses, pigs, ruminants, etc.),
and the Cetacea (dolphins and whales). In these Indecidua the villi
are distributed over the whole surface of the chorion (or its greater
part) either singly or in groups. They are only loosely connected with
the mucous coat of the uterus, so that the whole foetal membrane with
its villi can be easily withdrawn from the uterine depressions like a
hand from a glove. There is no real coalescence of the two placentas
at any part of the surface of contact. Hence at birth the foetal
placenta alone comes away; the uterine placenta is not torn away with
it.

The formation of the placenta is very different in the second and
higher section of the Placentals, the Deciduata. Here again the whole
surface of the chorion is thickly covered with the villi in the
beginning. But they afterwards disappear from one part of the surface,
and grow proportionately thicker on the other part. We thus get a
differentiation between the smooth chorion (chorion laeve, Figure
2.273 chl) and the thickly-tufted chorion (chorion frondosum, Figure
2.273 chf). The former has only a few small villi or none at all; the
latter is thickly covered with large and well-developed villi; this
alone now constitutes the placenta. In the great majority of the
Deciduata the placenta has the same shape as in man (Figures 1.197 and
1.200)--namely a thick, circular disk like a cake; so we find in the
Insectivora, Chiroptera, Rodents, and Apes. This discoplacenta lies on
one side of the chorion. But in the Sarcotheria (both the Carnivora
and the seals, Pinnipedia) and in the elephant and several other
Deciduates we find a zonoplacenta; in these the rich mass of villi
runs like a girdle round the middle of the ellipsoid chorion, the two
poles of it being free from them.

(FIGURE 2.275. The Slender Lori (Stenops gracilis) of Ceylon, a
tail-less lemur.)

Still more characteristic of the Deciduates is the peculiar and very
intimate connection between the chorion frondosum and the
corresponding part of the mucous coat of the womb, which we must
regard as a real coalescence of the two. The villi of the chorion push
their branches into the blood-filled tissues of the coat of the
uterus, and the vessels of each loop together so intimately that it is
no longer possible to separate the foetal from the maternal placenta;
they form henceforth a compact and apparently simple placenta. In
consequence of this coalescence, a whole piece of the lining of the
womb comes away at birth with the foetal membrane that is interlaced
with it. This piece is called the "falling-away" membrane (decidua).
It is also called the serous (spongy) membrane, because it is pierced
like a sieve or sponge. All the higher Placentals that have this
decidua are classed together as the "Deciduates." The tearing away of
the decidua at birth naturally causes the mother to lose a quantity of
blood, which does not happen in the Indecidua. The last part of the
uterine coat has to be repaired by a new growth after birth in the
Deciduates. (Cf. Figures 1.199 and 1.200.)

In the various orders of the Deciduates, the placenta differs
considerably both in outer form and internal structure. The extensive
investigations of the last ten years have shown that there is more
variation in these respects among the higher mammals than was formerly
supposed. The physiological work of this important embryonic organ,
the nutrition of the foetus during its long sojourn in the womb, is
accomplished in the various groups of the Placentals by very different
and sometimes very elaborate structures. They have lately been fully
described by Hans Strahl.

The phylogeny of the placenta has become more intelligible from the
fact that we have found a number of transitional forms of it. Some of
the Marsupials (Perameles) have the beginning of a placenta. In some
of the Lemurs (Tarsius) a discoid placenta with decidua is developed.

While these important results of comparative embryology have been
throwing further light on the close blood-relationship of man and the
anthropoid apes in the last few years (Chapter 1.15), the great
advance of paleontology has at the same time been affording us a
deeper insight into the stem-history of the Placental group. In the
seventh chapter of my Systematic Phylogeny of the Vertebrates I
advanced the hypothesis that the Placentals form a single stem with
many branches, which has been evolved from an older group of the
Marsupials (Prodidelphia). The four great legions of the
Placentals--Rodents, Ungulates, Carnassia, and Primates--are sharply
separated to-day by important features of organisation. But if we
consider their extinct ancestors of the Tertiary period, the
differences gradually disappear, the deeper we go in the Cenozoic
deposits; in the end we find that they vanish altogether. The
primitive stem-forms of the Rodents (Esthonychida), the Ungulates
(Chondylarthra), the Carnassia (Ictopsida), and the Primates
(Lemuravida) are so closely related at the beginning of the Tertiary
period that we might group them together as different families of one
order, the Proplacentals (Mallotheria or Prochoriata).

Hence the great majority of the Placentals have no direct and close
relationship to man, but only the legion of the Primates. This is now
generally divided into three orders--the half-apes (Prosimiae), apes
(Simiae), and man (Anthropi). The lemurs or half-apes are the
stem-group, descending from the older Mallotheria of the Cretaceous
period. From them the apes were evolved in the Tertiary period, and
man was formed from these towards its close.

The Lemurs (Prosimiae) have few living representatives. But they are
very interesting, and are the last survivors of a once extensive
group. We find many fossil remains of them in the older Tertiary
deposits of Europe and North America, in the Eocene and Miocene. We
distinguish two sub-orders, the fossil Lemuravida and the modern
Lemurogona. The earliest and most primitive forms of the Lemuravida
are the Pachylemurs (Hypopsodina); they come next to the earliest
Placentals (Prochoriata), and have the typical full dentition, with
forty-four teeth (3.1.4.3. over 3.1.4.3.). The Necrolemurs (Adapida,
Figure 2.274) have only forty teeth, and have lost an incisor in each
jaw (2.1.4.3. over 2.1.4.3.). The dentition is still further reduced
in the Lemurogona (Autolemures), which usually have only thirty-six
teeth (2.1.3.3. over 2.1.3.3.). These living survivors are scattered
far over the southern part of the Old World. Most of the species live
in Madagascar, some in the Sunda Islands, others on the mainland of
Asia and Africa. They are gloomy and melancholic animals; they live a
quiet life, climbing trees, and eating fruit and insects. They are of
different kinds. Some are closely related to the Marsupials
(especially the opossum). Others (Macrotarsi) are nearer to the
Insectivora, others again (Chiromys) to the Rodents. Some of the
lemurs (Brachytarsi) approach closely to the true apes. The numerous
fossil remains of half-apes and apes that have been recently found in
the Tertiary deposits justify us in thinking that man's ancestors were
represented by several different species during this long period. Some
of these were almost as big as men, such as the diluvial lemurogonon
Megaladapis of Madagascar.

(FIGURE 2.276. The white-nosed ape (Cercopithecus petaurista).)

Next to the lemurs come the true apes (Simiae), the twenty-sixth stage
in our ancestry. It has been beyond question for some time now that
the apes approach nearest to man in every respect of all the animals.
Just as the lowest apes come close to the lemurs, so the highest come
next to man. When we carefully study the comparative anatomy of the
apes and man, we can trace a gradual and uninterrupted advance in the
organisation of the ape up to the purely human frame, and, after
impartial examination of the "ape problem" that has been discussed of
late years with such passionate interest, we come infallibly to the
important conclusion, first formulated by Huxley in 1863: "Whatever
systems of organs we take, the comparison of their modifications in
the series of apes leads to the same result: that the anatomic
differences that separate man from the gorilla and chimpanzee are not
as great as those that separate the gorilla from the lower apes."
Translated into phylogenetic language, this "pithecometra-law,"
formulated in such masterly fashion by Huxley, is quite equivalent to
the popular saying: "Man is descended from the apes."

(FIGURE 2.277. The drill-baboon (Cynocephalus leucophaeus) (From
Brehm.))

In the very first exposition of his profound natural classification
(1735) Linne placed the anthropoid mammals at the head of the animal
kingdom, with three genera: man, the ape, and the sloth. He afterwards
called them the "Primates"--the "lords" of the animal world; he then
also separated the lemur from the true ape, and rejected the sloth.
Later zoologists divided the order of Primates. First the Gottingen
anatomist, Blumenbach, founded a special order for man, which he
called Bimana ("two-handed"); in a second order he united the apes and
lemurs under the name of Quadrumana ("four-handed"); and a third order
was formed of the distantly-related Chiroptera (bats, etc.). The
separation of the Bimana and Quadrumana was retained by Cuvier and
most of the subsequent zoologists. It seems to be extremely important,
but, as a matter of fact, it is totally wrong. This was first shown in
1863 by Huxley, in his famous Man's Place in Nature. On the strength
of careful comparative anatomical research he proved that the apes are
just as truly "two-handed" as man; or, if we prefer to reverse it,
that man is as truly four-handed as the ape. He showed convincingly
that the ideas of hand and foot had been wrongly defined, and had been
improperly based on physiological instead of morphological grounds.
The circumstance that we oppose the  to the other four fingers in
our hand, and so can grasp things, seemed to be a special distinction
of the hand in contrast to the foot, in which the corresponding great
toe cannot be opposed in this way to the others. But the apes can
grasp with the hind-foot as well as the fore, and so were regarded as
quadrumanous. However, the inability to grasp that we find in the foot
of civilised man is a consequence of the habit of clothing it with
tight coverings for thousands of years. Many of the bare-footed lower
races of men, especially among the negroes, use the foot very freely
in the same way as the hand. As a result of early habit and continued
practice, they can grasp with the foot (in climbing trees, for
instance) just as well as with the hand. Even new-born infants of our
own race can grasp very strongly with the great toe, and hold a spoon
with it as firmly as with the hand. Hence the physiological
distinction between hand and foot can neither be pressed very far, nor
has it a scientific basis. We must look to morphological characters.

As a matter of fact, it is possible to draw such a sharp morphological
distinction--a distinction based on anatomic structure--between the
fore and hind extremity. In the formation both of the bony skeleton
and of the muscles that are connected with the hand and foot before
and behind there are material and constant differences; and these are
found both in man and the ape. For instance, the number and
arrangement of the smaller bones of the hand and foot are quite
different. There are similar constant differences in the muscles. The
hind extremity always has three muscles (a short flexor muscle, a
short extensor muscle, and a long calf-muscle) that are not found in
the fore extremity. The arrangement of the muscles also is different
before and behind. These characteristic differences between the fore
and hind extremities are found in man as well as the ape. There can be
no doubt, therefore, that the ape's foot deserves that name just as
much as the human foot does, and that all true apes are just as
"bimanous" as man. The common distinction of the apes as
"quadrumanous" is altogether wrong morphologically.

But it may be asked whether, quite apart from this, we can find any
other features that distinguish man more sharply from the ape than the
various species of apes are distinguished from each other. Huxley gave
so complete and demonstrative a reply to this question that the
opposition still raised on many sides is absolutely without
foundation. On the ground of careful comparative anatomical research,
Huxley proved that in all morphological respects the differences
between the highest and lowest apes are greater than the corresponding
differences between the highest apes and man. He thus restored Linne's
order of the Primates (excluding the bats), and divided it into three
sub-orders, the first composed of the half-apes (Lemuridae), the
second of the true apes (Simiadae), the third of men (Anthropidae).

But, as we wish to proceed quite consistently and impartially on the
laws of systematic logic, we may, on the strength of Huxley's own law,
go a good deal farther in this division. We are justified in going at
least one important step farther, and assigning man his natural place
within one of the sections of the order of apes. All the features that
characterise this group of apes are found in man, and not found in the
other apes. We do not seem to be justified, therefore, in founding for
man a special order distinct from the apes.

The order of the true apes (Simiae or Pitheca)--excluding the
lemurs--has long been divided into two principal groups, which also
differ in their geographical distribution. One group (Hesperopitheca,
or western apes) live in America. The other group, to which man
belongs, are the Eopitheca or eastern apes; they are found in Asia and
Africa, and were formerly in Europe. All the eastern apes agree with
man in the features that are chiefly used in zoological classification
to distinguish between the two simian groups, especially in the
dentition. The objection might be raised that the teeth are too
subordinate an organ physiologically for us to lay stress on them in
so important a question. But there is a good reason for it; it is with
perfect justice that zoologists have for more than a century paid
particular attention to the teeth in the systematic division and
arrangement of the orders of mammals. The number, form, and
arrangement of the teeth are much more faithfully inherited in the
various orders than most other characters.

Hence the form of dentition in man is very important. In the fully
developed condition we have thirty-two teeth; of these eight are
incisors, four canine, and twenty molars. The eight incisors, in the
middle of the jaws, have certain characteristic differences above and
below. In the upper jaw the inner incisors are larger than the outer;
in the lower jaw the inner are the smaller. Next to these, at each
side of both jaws, is a canine (or "eye tooth"), which is larger than
the incisors. Sometimes it is very prominent in man, as it is in most
apes and many of the other mammals, and forms a sort of tusk. Next to
this there are five molars above and below on each side, the first two
of which (the "pre-molars") are small, have only one root, and are
included in the change of teeth; the three back ones are much larger,
have two roots, and only come with the second teeth. The apes of the
Old World, or all the living or fossil apes of Asia, Africa, and
Europe, have the same dentition as man.

(FIGURES 2.278 TO 2.282. Skeletons of man and the four anthropoid
apes. (From Huxley.) Cf. Figures 1.203 to 1.209.

FIGURE 2.278. Gibbon (Hylobates).

FIGURE 2.279. Orang (Satyrus).

FIGURE 2.280. Chimpanzee (Anthropithecus).

FIGURE 2.281. Gorilla (Gorilla).

FIGURE 2.282. Man (Homo).)

On the other hand, all the American apes have an additional pre-molar
in each half of the jaw. They have six molars above and below on each
side, or thirty-six teeth altogether. This characteristic difference
between the eastern and western apes has been so faithfully inherited
that it is very instructive for us. It is true that there seems to be
an exception in the case of a small family of South American apes. The
small silky apes (Arctopitheca or Hapalidae), which include the
tamarin (Midas) and the brush-monkey (Jacchus), have only five molars
in each half of the jaw (instead of six), and so seem to be nearer to
the eastern apes. But it is found, on closer examination, that they
have three premolars, like all the western apes, and that only the
last molar has been lost. Hence the apparent exception really confirms
the above distinction.

Of the other features in which the two groups of apes differ, the
structure of the nose is particularly instructive and conspicuous. All
the eastern apes have the same type of nose as man--a comparatively
narrow partition between the two halves, so that the nostrils run
downwards. In some of them the nose protrudes as far as in man, and
has the same characteristic structure. We have already alluded to the
curious long-nosed apes, which have a long, finely-curved nose. Most
of the eastern apes have, it is true, rather flat noses, like, for
instance, the white-nosed monkey (Figure 2.276); but the nasal
partition is thin and narrow in them all. The American apes have a
different type of nose. The partition is very broad and thick at the
bottom, and the wings of the nostrils are not developed, so that they
point outwards instead of downwards. This difference in the form of
the nose is so constantly inherited in both groups that the apes of
the New World are called "flat-nosed" (Platyrrhinae), and those of the
Old World "narrow-nosed" (Catarrhinae). The bony passage of the ear
(at the bottom of which is the tympanum) is short and wide in all the
Platyrrhines, but long and narrow in all the Catarrhines; and in man
this difference also is significant.

This division of the apes into Platyrrhines and Catarrhines, on the
ground of the above hereditary features, is now generally admitted in
zoology, and receives strong support from the geographical
distribution of the two groups in the east and west. It follows at
once, as regards the phylogeny of the apes, that two divergent lines
proceeded from the common stem-form of the ape-order in the early
Tertiary period, one of which spread over the Old, the other over the
New, World. It is certain that all the Platyrrhines come of one stock,
and also all the Catarrhines; but the former are phylogenetically
older, and must be regarded as the stem-group of the latter.

What can we deduce from this with regard to our own genealogy? Man has
just the same characters, the same form of dentition, auditory
passage, and nose, as all the Catarrhines; in this he radically
differs from the Platyrrhines. We are thus forced to assign him a
position among the eastern apes in the order of Primates, or at least
place him alongside of them. But it follows that man is a direct blood
relative of the apes of the Old World, and can be traced to a common
stem-form together with all the Catarrhines. In his whole organisation
and in his origin man is a true Catarrhine; he originated in the Old
World from an unknown, extinct group of the eastern apes. The apes of
the New World, or the Platyrrhines, form a divergent branch of our
genealogical tree, and this is only distantly related at its root to
the human race. We must assume, of course, that the earliest Eocene
apes had the full dentition of the Platyrrhines; hence we may regard
this stem-group as a special stage (the twenty-sixth) in our ancestry,
and deduce from it (as the twenty-seventh stage) the earliest
Catarrhines.

We have now reduced the circle of our nearest relatives to the small
and comparatively scanty group that is represented by the sub-order of
the Catarrhines; and we are in a position to answer the question of
man's place in this sub-order, and say whether we can deduce anything
further from this position as to our immediate ancestors. In answering
this question the comprehensive and able studies that Huxley gives of
the comparative anatomy of man and the various Catarrhines in his
Man's Place in Nature are of great assistance to us. It is quite clear
from these that the differences between man and the highest
Catarrhines (gorilla, chimpanzee, and orang) are in every respect
slighter than the corresponding differences between the highest and
the lowest Catarrhines (white-nosed monkey, macaco, baboon, etc.). In
fact, within the small group of the tail-less anthropoid apes the
differences between the various genera are not less than the
differences between them and man. This is seen by a glance at the
skeletons that Huxley has put together (Figures 2.278 to 2.282).
Whether we take the skull or the vertebral column or the ribs or the
fore or hind limbs, or whether we extend the comparison to the
muscles, blood-vessels, brain, placenta, etc., we always reach the
same result on impartial examination--that man is not more different
from the other Catarrhines than the extreme forms of them (for
instance, the gorilla and baboon) differ from each other. We may now,
therefore, complete the Huxleian law we have already quoted with the
following thesis: "Whatever system of organs we take, a comparison of
their modifications in the series of Catarrhines always leads to the
same conclusion; the anatomic differences that separate man from the
most advanced Catarrhines (orang, gorilla, chimpanzee) are not as
great as those that separate the latter from the lowest Catarrhines
(white-nosed monkey, macaco, baboon)."

We must, therefore, consider the descent of man from other Catarrhines
to be fully proved. Whatever further information on the comparative
anatomy and ontogeny of the living Catarrhines we may obtain in the
future, it cannot possibly disturb this conclusion. Naturally, our
Catarrhine ancestors must have passed through a long series of
different forms before the human type was produced. The chief advances
that effected this "creation of man," or his differentiation from the
nearest related Catarrhines, were: the adoption of the erect posture
and the consequent greater differentiation of the fore and hind limbs,
the evolution of articulate speech and its organ, the larynx, and the
further development of the brain and its function, the soul; sexual
selection had a great influence in this, as Darwin showed in his
famous work.

With an eye to these advances we can distinguish at least four
important stages in our simian ancestry, which represent prominent
points in the historical process of the making of man. We may take,
after the Lemurs, the earliest and lowest Platyrrhines of South
America, with thirty-six teeth, as the twenty-sixth stage of our
genealogy; they were developed from the Lemurs by a peculiar
modification of the brain, teeth, nose, and fingers. From these Eocene
stem-apes were formed the earliest Catarrhines or eastern apes, with
the human dentition (thirty-two teeth), by modification of the nose,
lengthening of the bony channel of the ear, and the loss of four
pre-molars. These oldest stem-forms of the whole Catarrhine group were
still thickly coated with hair, and had long tails--baboons
(Cynopitheca) or tailed apes (Menocerca, Figure 2.276). They lived
during the Tertiary period, and are found fossilised in the Miocene.
Of the actual tailed apes perhaps the nearest to them are the
Semnopitheci.

If we take these Semnopitheci as the twenty-seventh stage in our
ancestry, we may put next to them, as the twenty-eighth, the tail-less
anthropoid apes. This name is given to the most advanced and man-like
of the existing Catarrhines. They were developed from the other
Catarrhines by losing the tail and part of the hair, and by a higher
development of the brain, which found expression in the enormous
growth of the skull. Of this remarkable family there are only a few
genera to-day, and we have already dealt with them (Chapter 1.15)--the
gibbon (Hylobates, Figure 1.203) and orang (Satyrus, Figures 1.204 and
1.205) in South-Eastern Asia and the Archipelago; and the chimpanzee
(Anthropithecus, Figures 1.206 and 1.207) and gorilla (Gorilla, Figure
1.208) in Equatorial Africa.

The great interest that every thoughtful man takes in these nearest
relatives of ours has found expression recently in a fairly large
literature. The most distinguished of these works for impartial
treatment of the question of affinity is Robert Hartmann's little work
on The Anthropoid Apes. Hartmann divides the primate order into two
families: (1) Primarii (man and the anthropoid apes); and (2) Simianae
(true apes, Catarrhines and Platyrrhines). Professor Klaatsch, of
Heidelberg, has advanced a different view in his interesting and
richly illustrated work on The Origin and Development of the Human
Race. This is a substantial supplement to my Anthropogeny, in so far
as it gives the chief results of modern research on the early history
of man and civilisation. But when Klaatsch declares the descent of man
from the apes to be "irrational, narrow-minded, and false," in the
belief that we are thinking of some living species of ape, we must
remind him that no competent scientist has ever held so narrow a view.
All of us look merely--in the sense of Lamarck and Darwin--to the
original unity (admitted by Klaatsch) of the primate stem. This common
descent of all the Primates (men, apes, and lemurs) from one primitive
stem-form, from which the most far-reaching conclusions follow for the
whole of anthropology and philosophy, is admitted by Klaatsch as well
as by myself and all other competent zoologists who accept the theory
of evolution in general. He says explicitly (page 172): "The three
anthropoid apes--gorilla, chimpanzee, and orang--seem to be branches
from a common root, and this was not far from that of the gibbon and
man." That is in the main the opinion that I have maintained
(especially against Virchow) in a number of works ever since 1866. The
hypothetical common ancestor of all the Primates, which must have
lived in the earliest Tertiary period (more probably in the
Cretaceous), was called by me Archiprimus, Klaatsch now calls it
Primatoid. Dubois has proposed the appropriate name of Prothylobates
for the common and much younger stem-form of the anthropomorpha (man
and the anthropoid apes). The actual Hylobates is nearer to it than
the other three existing anthropoids. None of these can be said to be
absolutely the most man-like. The gorilla comes next to man in the
structure of the hand and foot, the chimpanzee in the chief features
of the skull, the orang in brain development, and the gibbon in the
formation of the chest. None of these existing anthropoid apes is
among the direct ancestors of our race; they are scattered survivors
of an ancient branch of the Catarrhines, from which the human race
developed in a particular direction.

(FIGURE 2.283. Skull of the fossil ape-man of Java (Pithecanthropus
erectus), restored by Eugen Dubois.)

Although man is directly connected with this anthropoid family and
originates from it, we may assign an important intermediate form
between the Prothylobates and him (the twenty-ninth stage in our
ancestry), the ape-men (Pithecanthropi). I gave this name in the
History of Creation to the "speechless primitive men" (Alali), which
were men in the ordinary sense as far as the general structure is
concerned (especially in the differentiation of the limbs), but lacked
one of the chief human characteristics, articulate speech and the
higher intelligence that goes with it, and so had a less developed
brain. The phylogenetic hypothesis of the organisation of this
"ape-man" which I then advanced was brilliantly confirmed twenty-four
years afterwards by the famous discovery of the fossil Pithecanthropus
erectus by Eugen Dubois (then military surgeon in Java, afterwards
professor at Amsterdam). In 1892 he found at Trinil, in the residency
of Madiun in Java, in Pliocene deposits, certain remains of a large
and very man-like ape (roof of the skull, femur, and teeth), which he
described as "an erect ape-man" and a survivor of a "stem-form of man"
(Figure 2.283). Naturally, the Pithecanthropus excited the liveliest
interest, as the long-sought transitional form between man and the
ape: we seemed to have found "the missing link." There were very
interesting scientific discussions of it at the last three
International Congresses of Zoology (Leyden, 1895, Cambridge, 1898,
and Berlin, 1901). I took an active part in the discussion at
Cambridge, and may refer the reader to the paper I read there on "The
Present Position of Our Knowledge of the Origin of Man" (translated by
Dr. Gadow with the title of The Last Link).

An extensive and valuable literature has grown up in the last ten
years on the Pithecanthropus and the pithecoid theory connected with
it. A number of distinguished anthropologists, anatomists,
paleontologists, and phylogenists have taken part in the controversy,
and made use of the important data furnished by the new science of
pre-historic research. Hermann Klaatsch has given a good summary of
them, with many fine illustrations, in the above-mentioned work. I
refer the reader to it as a valuable supplement to the present work,
especially as I cannot go any further here into these anthropological
and pre-historic questions. I will only repeat that I think he is
wrong in the attitude of hostility that he affects to take up with
regard to my own views on the descent of man from the apes.

The most powerful opponent of the pithecoid theory--and the theory of
evolution in general--during the last thirty years (until his death in
September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In
the speeches which he delivered every year at various congresses and
meetings on this question, he was never tired of attacking the hated
"ape theory." His constant categorical position was: "It is quite
certain that man does not descend from the ape or any other animal."
This has been repeated incessantly by opponents of the theory,
especially theologians and philosophers. In the inaugural speech that
he delivered in 1894 at the Anthropological Congress at Vienna, he
said that "man might just as well have descended from a sheep or an
elephant as from an ape." Absurd expressions like this only show that
the famous pathological anatomist, who did so much for medicine in the
establishment of cellular pathology, had not the requisite attainments
in comparative anatomy and ontogeny, systematic zoology and
paleontology, for sound judgment in the province of anthropology. The
Strassburg anatomist, Gustav Schwalbe, deserved great praise for
having the moral courage to oppose this dogmatic and ungrounded
teaching of Virchow, and showing its untenability. The recent
admirable works of Schwalbe on the Pithecanthropus, the earliest races
of men, and the Neanderthal skull (1897 to 1901) will supply any
candid and judicious reader with the empirical material with which he
can convince himself of the baselessness of the erroneous dogmas of
Virchow and his clerical friends (J. Ranke, J. Bumuller, etc.).

As the Pithecanthropus walked erect, and his brain (judging from the
capacity of his skull, Figure 2.283) was midway between the lowest men
and the anthropoid apes, we must assume that the next great step in
the advance from the Pithecanthropus to man was the further
development of human speech and reason.

Comparative philology has recently shown that human speech is
polyphyletic in origin; that we must distinguish several (probably
many) different primitive tongues that were developed independently.
The evolution of language also teaches us (both from its ontogeny in
the child and its phylogeny in the race) that human speech proper was
only gradually developed after the rest of the body had attained its
characteristic form. It is probable that language was not evolved
until after the dispersal of the various species and races of men, and
this probably took place at the commencement of the Quaternary or
Diluvial period. The speechless ape-men or Alali certainly existed
towards the end of the Tertiary period, during the Pliocene, possibly
even the Miocene, period.

The third, and last, stage of our animal ancestry is the true or
speaking man (Homo), who was gradually evolved from the preceding
stage by the advance of animal language into articulate human speech.
As to the time and place of this real "creation of man" we can only
express tentative opinions. It was probably during the Diluvial period
in the hotter zone of the Old World, either on the mainland in
tropical Africa or Asia or on an earlier continent (Lemuria--now sunk
below the waves of the Indian Ocean), which stretched from East Africa
(Madagascar, Abyssinia) to East Asia (Sunda Islands, Further India). I
have given fully in my History of Creation, (chapter 28) the weighty
reasons for claiming this descent of man from the anthropoid eastern
apes, and shown how we may conceive the spread of the various races
from this "Paradise" over the whole earth. I have also dealt fully
with the relations of the various races and species of men to each
other.

</section>
<section id="10" fatherid="8" title="SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY" lv="" priority="0" level="2">
FIRST STAGE: THE PROTISTS.

Man's ancestors are unicellular protozoa, originally unnucleated
Monera like the Chromacea, structureless green particles of plasm;
afterwards real nucleated cells (first plasmodomous Protophyta, like
the Palmella; then plasmophagous Protozoa, like the Amoeba).

SECOND STAGE: THE BLASTAEADS.

Man's ancestors are round coenobia or colonies of Protozoa; they
consist of a close association of many homogeneous cells, and thus are
individuals of the second order. They resemble the round
cell-communities of the Magospherae and Volvocina, equivalent to the
ontogenetic blastula: hollow globules, the wall of which consists of a
single layer of ciliated cells (blastoderm).

THIRD STAGE: THE GASTRAEADS.

Man's ancestors are Gastraeads, like the simplest of the actual
Metazoa (Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body
consists merely of a primitive gut, the wall of which is made up of
the two primary germinal layers.

FOURTH STAGE: THE PLATODES.

Man's ancestors have substantially the organisation of simple Platodes
(at first like the cryptocoelic Platodaria, later like the
rhabdocoelic Turbellaria). The leaf-shaped bilateral-symmetrical body
has only one gut-opening, and develops the first trace of a nervous
centre from the ectoderm in the middle line of the back (Figures 2.239
and 2.240).

FIFTH STAGE: THE VERMALIA.

Man's ancestors have substantially the organisation of unarticulated
Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia
(Nemertina, Enteropneusta). Four secondary germinal layers develop,
two middle layers arising between the limiting layers (coeloma). The
dorsal ectoderm forms the vertical plate, acroganglion (Figure 2.243).

SIXTH STAGE: THE PROCHORDONIA.

Man's ancestors have substantially the organisation of a simple
unarticulated Chordonium (Copelata and Ascidia-larvae). The
unsegmented chorda develops between the dorsal medullary tube and the
ventral gut-tube. The simple coelom-pouches divide by a frontal septum
into two on each side; the dorsal pouch (episomite) forms a
muscle-plate; the ventral pouch (hyposomite) forms a gonad. Head-gut
with gill-cs.

SEVENTH STAGE: THE ACRANIA.

Man's ancestors are skull-less Vertebrates, like the Amphioxus. The
body is a series of metamera, as several of the primitive segments are
developed. The head contains in the ventral half the branchial gut,
the trunk the hepatic gut. The medullary tube is still simple. No
skull, jaws, or limbs.

EIGHTH STAGE: THE CYCLOSTOMA.

Man's ancestors are jaw-less Craniotes (like the Myxinoida and
Petromyzonta). The number of metamera increases. The fore-end of the
medullary tube expands into a vesicle and forms the brain, which soon
divides into five cerebral vesicles. In the sides of it appear the
three higher sense-organs: nose, eyes, and auditory vesicles. No jaws,
limbs, or floating bladder.

NINTH STAGE: THE ICHTHYODA.

Man's ancestors are fish-like Craniotes: (1) Primitive fishes
(Selachii); (2) plated fishes (Ganoida); (3) amphibian fishes
(Dipneusta); (4) mailed amphibia (Stegocephala). The ancestors of this
series develop two pairs of limbs: a pair of fore (breast-fins) and of
hind (belly-fins) legs. The gill-arches are formed between the
gill-cs: the first pair form the maxillary arches (the upper and
lower jaws). The floating bladder (lung) and pancreas grow out of the
gut.

TENTH STAGE: THE AMNIOTES.

Man's ancestors are Amniotes or gill-less Vertebrates: (1) Primitive
Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals
(Monotremes); (4) Marsupials; (5) Lemurs (Prosimiae); (6) Western apes
(Platyrrhinae); (7) Eastern apes (Catarrhinae): at first tailed
Cynopitheca; then tail-less anthropoids; later speechless ape-men
(Alali); finally speaking man. The ancestors of these Amniotes develop
an amnion and allantois, and gradually assume the mammal, and finally
the specifically human, form.

</section>
</section>
<section id="11" fatherid="0" title="EVOLUTION OF THE NERVOUS SYSTEM" lv="CHAPTER 2.24." priority="0" level="1">
The previous chapters have taught us how the human body as a whole
develops from the first simple rudiment, a single layer of cells. The
whole human race owes its origin, like the individual man, to a simple
cell. The unicellular stem-form of the race is reproduced daily in the
unicellular embryonic stage of the individual. We have now to consider
in detail the evolution of the various parts that make up the human
frame. I must, naturally, confine myself to the most general and
principal outlines; to make a special study of the evolution of each
organ and tissue is both beyond the scope of this work, and probably
beyond the anatomic capacity of most of my readers to appreciate. In
tracing the evolution of the various organs we shall follow the method
that has hitherto guided us, except that we shall now have to consider
the ontogeny and phylogeny of the organs together. We have seen, in
studying the evolution of the body as a whole, that phylogeny casts a
light over the darker paths of ontogeny, and that we should be almost
unable to find our way in it without the aid of the former. We shall
have the same experience in the study of the organs in detail, and I
shall be compelled to give simultaneously their ontogenetic and
phylogenetic origin. The more we go into the details of organic
development, and the more closely we follow the rise of the various
parts, the more we see the inseparable connection of embryology and
stem-history. The ontogeny of the organs can only be understood in the
light of their phylogeny, just as we found of the embryology of the
whole body. Each embryonic form is determined by a corresponding
stem-form. This is true of details as well as of the whole.

We will consider first the animal and then the vegetal systems of
organs of the body. The first group consists of the psychic and the
motor apparatus. To the former belong the skin, the nervous system,
and the sense-organs. The motor apparatus is composed of the passive
and the active organs of movement (the skeleton and the muscles). The
second or vegetal group consists of the nutritive and the reproductive
apparatus. To the nutritive apparatus belong the alimentary canal with
all its appendages, the vascular system, and the renal (kidney)
system. The reproductive apparatus comprises the different organs of
sex (embryonic glands, sexual ducts, and copulative organs).

As we know from previous chapters (1.11 to 1.13), the animal systems
of organs (the organs of sensation and presentation) develop for the
most part out of the OUTER primary germ-layer, or the cutaneous (skin)
layer. On the other hand, the vegetal systems of organs arise for the
most part from the INNER primary germ-layer, the visceral layer. It is
true that this antithesis of the animal and vegetal spheres of the
body in man and all the higher animals is by no means rigid; several
parts of the animal apparatus (for instance, the greater part of the
muscles) are formed from cells that come originally from the entoderm;
and a great part of the vegetative apparatus (for instance, the
mouth-cavity and the gonoducts) are composed of cells that come from
the ectoderm.

In the more advanced animal body there is so much interlacing and
displacement of the various parts that it is often very difficult to
indicate the sources of them. But, broadly speaking, we may take it as
a positive and important fact that in man and the higher animals the
chief part of the animal organs comes from the ectoderm, and the
greater part of the vegetative organs from the entoderm. It was for
this reason that Carl Ernst von Baer called the one the animal and the
other the vegetative layer (see Chapter 1.3).

The solid foundation of this important thesis is the gastrula, the
most instructive embryonic form in the animal world, which we still
find in the same shape in the most diverse classes of animals. This
form points demonstrably to a common stem-form of all the Metazoa, the
Gastraea; in this long-extinct stem-form the whole body consisted
throughout life of the two primary germinal layers, as is now the case
temporarily in the gastrula; in the Gastraea the simple cutaneous
(skin) layer ACTUALLY represented all the animal organs and functions,
and the simple visceral (gut) layer all the vegetal organs and
functions. This is the case with the modern Gastraeads (Figure 2.233);
and it is also the case potentially with the gastrula.

We shall easily see that the gastraea theory is thus able to throw a
good deal of light, both morphologically and physiologically, on some
of the chief features of embryonic development, if we take up first
the consideration of the chief element in the animal sphere, the
psychic apparatus or sensorium and its evolution. This apparatus
consists of two very different parts, which seem at first to have very
little connection with each other--the outer skin, with all its hairs,
nails, sweat-glands, etc., and the nervous system. The latter
comprises the central nervous system (brain and spinal cord), the
peripheral, cerebral, and spinal nerves, and the sense-organs. In the
fully-formed vertebrate body these two chief elements of the sensorium
lie far apart, the skin being external to, and the central nervous
system in the very centre of, the body. The one is only connected with
the other by a section of the peripheral nervous system and the
sense-organs. Nevertheless, as we know from human embryology, the
medullary tube is formed from the cutaneous layer. The organs that
discharge the most advanced functions of the animal body--the organs
of the soul, or of psychic life--develop from the external skin. This
is a perfectly natural and necessary process. If we reflect on the
historical evolution of the psychic and sensory functions, we are
forced to conclude that the cells which accomplish them must
originally have been located on the outer surface of the body. Only
elementary organs in this superficial position could directly receive
the influences of the environment. Afterwards, under the influence of
natural selection, the cellular group in the skin which was
specifically "sensitive" withdrew into the inner and more protected
part of the body, and formed there the foundation of a central nervous
organ. As a result of increased differentiation, the skin and the
central nervous system became further and further separated, and in
the end the two were only permanently connected by the afferent
peripheral sensory nerves.

(FIGURE 2.284. The human skin in vertical section (from Ecker), highly
magnified, a horny layer of the epidermis, b mucous layer of the
epidermis, c papillae of the corium, d blood-vessels of same, ef ducts
of the sweat-glands (g), h fat-glands in the corium, i nerve, passing
into a tactile corpuscle above.)

The observations of the comparative anatomist are in complete accord
with this view. He tells us that large numbers of the lower animals
have no nervous system, though they exercise the functions of
sensation and will like the higher animals. In the unicellular
Protozoa, which do not form germinal layers, there is, of course,
neither nervous system nor skin. But in the second division of the
animal kingdom also, the Metazoa, there is at first no nervous system.
Its functions are represented by the simple cell-layer of the
ectoderm, which the lower Metazoa have inherited from the Gastraea
(Figure 1.30 e). We find this in the lowest Zoophytes--the Gastraeads,
Physemaria, and Sponges (Figures 2.233 to 2.238). The lowest Cnidaria
(the hydroid polyps) also are little superior to the Gastraeads in
structure. Their vegetative functions are accomplished by the simple
visceral layer, and their animal functions by the simple cutaneous
layer. In these cases the simple cell-layer of the ectoderm is at once
skin, locomotive apparatus, and nervous system.

(FIGURE 2.285. Epidermic cells of a human embryo of two months. (From
Kolliker.))

When we come to the higher Metazoa, in which the sensory functions and
their organs are more advanced, we find a division of labour among the
ectodermic cells. Groups of sensitive nerve cells separate from the
ordinary epidermic cells; they retire into the more protected tissue
of the mesodermic under-skin, and form special neural ganglia there.
Even in the Platodes, especially the Turbellaria, we find an
independent nervous system, which has separated from the outer skin.
This is the "upper pharyngeal ganglion," or acroganglion, situated
above the gullet (Figure 2.241 g). From this rudimentary structure has
been developed the elaborate central nervous system of the higher
animals. In some of the higher worms, such as the earth-worm, the
first rudiment of the central nervous system (Figure 1.74 n) is a
local thickening of the skin-sense layer (hs), which afterwards
separates altogether from the horny plate. In the earliest Platodes
(Cryptocoela) and Vermalia (Gastrotricha) the acroganglion remains in
the epidermis. But the medullary tube of the Vertebrates originates in
the same way. Our embryology has taught us that this first structure
of the central nervous system also develops originally from the outer
germinal layer.

Let us now examine more closely the evolution of the human skin, with
its various appendages, the hairs and glands. This external covering
has, physiologically, a double and important part to play. It is, in
the first place, the common integument that covers the whole surface
of the body, and forms a protective envelope for the other organs. As
such it also effects a certain exchange of matter between the body and
the surrounding atmosphere (exhalation, perspiration). In the second
place, it is the earliest and original sense organ, the common organ
of feeling that experiences the sensation of the temperature of the
environment and the pressure or resistance of bodies that come into
contact.

The human skin (like that of all the higher animals) is composed of
two layers, the outer and the inner or underlying skin. The outer skin
or epidermis, consists of simple ectodermic cells, and contains no
blood-vessels (Figure 2.284 a, b). It develops from the outer germinal
layer, or skin-sense layer. The underlying skin (corium or hypodermis)
consists chiefly of connective tissue, contains numerous blood-vessels
and nerves, and has a totally different origin. It comes from the
outermost parietal stratum of the middle germinal layer, or the
skin-fibre layer. The corium is much thicker than the epidermis. In
its deeper strata (the subcutis) there are clusters of fat-cells
(Figure 2.284 h). Its uppermost stratum (the cutis proper, or the
papillary stratum) forms, over almost the whole surface of the body, a
number of conical microscopic papillae (something like warts), which
push into the overlying epidermis (c). These tactile or sensory
particles contain the finest sensory organs of the skin, the touch
corpuscles. Others contain merely end-loops of the blood-vessels that
nourish the skin (c, d). The various parts of the corium arise by
division of labour from the originally homogeneous cells of the
cutis-plate, the outermost lamina of the mesodermic skin-fibre layer
(Figure 1.145 hpr, and Figures 1.161 and 1.162 cp).

In the same way, all the parts and appendages of the epidermis develop
by differentiation from the homogeneous cells of this horny plate
(Figure 2.285). At an early stage the simple cellular layer of this
horny plate divides into two. The inner and softer stratum (Figure
2.284 b) is known as the mucous stratum, the outer and harder (a) as
the horny (corneous) stratum. This horny layer is being constantly
used up and rubbed away at the surface; new layers of cells grow up in
their place out of the underlying mucous stratum. At first the
epidermis is a simple covering of the surface of the body. Afterwards
various appendages develop from it, some internally, others
externally. The internal appendages are the cutaneous glands--sweat,
fat, etc. The external appendages are the hairs and nails.

The cutaneous glands are originally merely solid cone-shaped growths
of the epidermis, which sink into the underlying corium (Figure 2.286
1). Afterwards a canal (2, 3) is formed inside them, either by the
softening and dissolution of the central cells or by the secretion of
fluid internally. Some of the glands, such as the sudoriferous, do not
ramify (Figure 2.284 efg). These glands, which secrete the
perspiration, are very long, and have a spiral coil at the end, but
they never ramify; so also the wax-glands of the ears. Most of the
other cutaneous glands give out buds and ramify; thus, for instance,
the lachrymal glands of the upper eye-lid that secrete tears (Figure
2.286), and the sebaceous glands which secrete the fat in the skin and
generally open into the hair-follicles. Sudoriferous and sebaceous
glands are found only in mammals. But we find lachrymal glands in all
the three classes of Amniotes--reptiles, birds, and mammals. They are
wanting in the lower aquatic vertebrates.

(FIGURE 2.286. Rudimentary lachrymal glands from a human embryo of
four months. (From Kolliker.) 1 earliest structure, in the shape of a
simple solid cone, 2 and 3 more advanced structures, ramifying and
hollowing out. a solid buds, e cellular coat of the hollow buds, f
structure of the fibrous envelope, which afterwards forms the corium
about the glands.)

The mammary glands (Figures 2.287 and 2.288) are very remarkable; they
are found in all mammals, and in these alone. They secrete the milk
for the feeding of the new-born mammal. In spite of their unusual size
these structures are nothing more than large sebaceous glands in the
skin. The milk is formed by the liquefaction of the fatty milk-cells
inside the branching mammary-gland tubes (Figure 2.287 c), in the same
way as the skin-grease or hair-fat, by the solution of fatty cells
inside the sebaceous glands. The outlets of the mammary glands enlarge
and form sac-like mammary ducts (b); these narrow again (a), and open
in the teats or nipples of the breast by sixteen to twenty-four fine
apertures. The first structure of this large and elaborate gland is a
very simple cone in the epidermis, which penetrates into the corium
and ramifies. In the new-born infant it consists of twelve to eighteen
radiating lobes (Figure 2.288). These gradually ramify, their ducts
become hollow and larger, and rich masses of fat accumulate between
the lobes. Thus is formed the prominent female breast (mamma), on the
top of which rises the teat or nipple (mammilla). The latter is only
developed later on, when the mammary gland is fully-formed; and this
ontogenetic phenomenon is extremely interesting, because the earlier
mammals (the stem-forms of the whole class) have no teats. In them the
milk comes out through a flat portion of the ventral skin that is
pierced like a sieve, as we still find in the lowest living mammals,
the oviparous Monotremes of Australia. The young animal licks the milk
from the mother instead of sucking it. In many of the lower mammals we
find a number of milk-glands at different parts of the ventral
surface. In the human female there is usually only one pair of glands,
at the breast; and it is the same with the apes, bats, elephants, and
several other mammals. Sometimes, however, we find two successive
pairs of glands (or even more) in the human female. Some women have
four or five pairs of breasts, like pigs and hedgehogs (Figure 1.103).
This polymastism points back to an older stem-form. We often find
these accessory breasts in the male also (Figure 1.103 D). Sometimes,
moreover, the normal mammary glands are fully developed and can suckle
in the male; but as a rule they are merely rudimentary organs without
functions in the male. We have already (Chapter 1.11) dealt with this
remarkable and interesting instance of atavism.

(FIGURE 2.287. The female breast (mamma) in vertical section. c
racemose glandular lobes, b enlarged milk-ducts, a narrower outlets,
which open into the nipple. (From H. Meyer.))

While the cutaneous glands are inner growths of the epidermis, the
appendages which we call hairs and nails are external local growths in
it. The nails (Ungues) which form important protective structures on
the back of the most sensitive parts of our limbs, the tips of the
fingers and toes, are horny growths of the epidermis, which we share
with the apes. The lower mammals usually have claws instead of them;
the ungulates, hoofs. The stem-form of the mammals certainly had
claws; we find them in a rudimentary form even in the salamander. The
horny claws are highly developed in most of the reptiles (Figure
2.264), and the mammals have inherited them from the earliest
representatives of this class, the stem-reptiles (Tocosauria). Like
the hoofs (ungulae) of the Ungulates, the nails of apes and men have
been evolved from the claws of the older mammals. In the human embryo
the first rudiment of the nails is found (between the horny and the
mucous stratum of the epidermis) in the fourth month. But their edges
do not penetrate through until the end of the sixth month.

The most interesting and important appendages of the epidermis are the
hairs; on account of their peculiar composition and origin we must
regard them as highly characteristic of the whole mammalian class. It
is true that we also find hairs in many of the lower animals, such as
insects and worms. But these hairs, like the hairs of plants, are
thread-like appendages of the surface, and differ entirely from the
hairs of the mammals in the details of their structure and
development.

The embryology of the hairs is known in all its details, but there are
two different views as to their phylogeny. On the older view the hairs
of the mammals are equivalent or homologous to the feathers of the
bird or the horny scales of the reptile. As we deduce all three
classes of Amniotes from a common stem-group, we must assume that
these Permian stem-reptiles had a complete scaly coat, inherited from
their Carboniferous ancestors, the mailed amphibia (Stegocephala); the
bony scales of their corium were covered with horny scales. In passing
from aquatic to terrestrial life the horny scales were further
developed, and the bony scales degenerated in most of the reptiles. As
regards the bird's feathers, it is certain that they are modifications
of the horny scales of their reptilian ancestors. But it is otherwise
with the hairs of the mammals. In their case the hypothesis has lately
been advanced on the strength of very extensive research, especially
by Friedrich Maurer, that they have been evolved from the cutaneous
sense-organs of amphibian ancestors by modification of functions; the
epidermic structure is very similar in both in its embryonic
rudiments. This modern view, which had the support of the greatest
expert on the vertebrates, Carl Gegenbaur, can be harmonised with the
older theory to an extent, in the sense that both formations, scales
and hairs, were very closely connected originally. Probably the
conical budding of the skin-sense layer grew up UNDER THE PROTECTION
OF THE HORNY SCALE, and became an organ of touch subsequently by the
cornification of the hairs; many hairs are still sensory organs
(tactile hairs on the muzzle and cheeks of many mammals: pubic hairs).

This middle position of the genetic connection of scales and hairs was
advanced in my Systematic Phylogeny of the Vertebrates (page 433). It
is confirmed by the similar arrangement of the two cutaneous
formations. As Maurer pointed out, the hairs, as well as the cutaneous
sense-organs and the scales, are at first arranged in regular
longitudinal series, and they afterwards break into alternate groups.
In the embryo of a bear two inches long, which I owe to the kindness
of Herr von Schmertzing (of Arva Varallia, Hungary), the back is
covered with sixteen to twenty alternating longitudinal rows of scaly
protuberances (Figure 2.289). They are at the same time arranged in
regular transverse rows, which converge at an acute angle from both
sides towards the middle of the back. The tip of the scale-like wart
is turned inwards. Between these larger hard scales (or groups of
hairs) we find numbers of rudimentary smaller hairs.

The human embryo is, as a rule, entirely clothed with a thick coat of
fine wool during the last three or four weeks of gestation. This
embryonic woollen coat (Lanugo) generally disappears in part during
the last weeks of foetal life but in any case, as a rule, it is lost
immediately after birth, and is replaced by the thinner coat of the
permanent hair. These permanent hairs grow out of hair-follicles,
which are formed from the root-sheaths of the disappearing
wool-fibres. The embryonic wool-coat usually, in the case of the human
embryo, covers the whole body, with the exception of the palms of the
hands and soles of the feet. These parts are always bare, as in the
case of apes and of most other mammals. Sometimes the wool-coat of the
embryo has a striking effect, by its colour, on the later permanent
hair-coat. Hence it happens occasionally, for instance, among our
Indo-Germanic races, that children of blond parents seem--to the
dismay of the latter--to be covered at birth with a dark brown or even
a black woolly coat. Not until this has disappeared do we see the
permanent blond hair which the child has inherited. Sometimes the
darker coat remains for weeks, and even months, after birth. This
remarkable woolly coat of the human embryo is a legacy from the apes,
our ancient long-haired ancestors.

(FIGURE 2.288. Mammary gland of a new-born infant, a original central
gland, b small and c large buds of same. (From Langer.))

It is not less noteworthy that many of the higher apes approach man in
the thinness of the hair on various parts of the body. With most of
the apes, especially the higher Catarrhines (or narrow-nosed apes),
the face is mostly, or entirely, bare, or at least it has hair no
longer or thicker than that of man. In their case, too, the back of
the head is usually provided with a thicker growth of hair; this is
lacking, however, in the case of the bald-headed chimpanzee
(Anthropithecus calvus). The males of many species of apes have a
considerable beard on the cheeks and chin; this sign of the masculine
sex has been acquired by sexual selection. Many species of apes have a
very thin covering of hair on the breast and the upper side of the
limbs--much thinner than on the back or the under side of the limbs.
On the other hand, we are often astonished to find tufts of hair on
the shoulders, back, and extremities of members of our Indo-Germanic
and of the Semitic races. Exceptional hair on the face, as on the
whole body, is hereditary in certain families of hairy men. The
quantity and the quality of the hair on head and chin are also
conspicuously transmitted in families. These extraordinary variations
in the total and partial hairy coat of the body, which are so
noticeable, not only in comparing different races of men, but also in
comparing different families of the same race, can only be explained
on the assumption that in man the hairy coat is, on the whole, a
rudimentary organ, a useless inheritance from the more thickly-coated
apes. In this man resembles the elephant, rhinoceros, hippopotamus,
whale, and other mammals of various orders, which have also, almost
entirely or for the most part, lost their hairy coats by adaptation.

(FIGURE 2.289. Embryo of a bear (Ursus arctos), twice natural size. A
seen from ventral side, B from the .)

The particular process of adaptation by which man lost the growth of
hair on most parts of his body, and retained or augmented it at some
points, was most probably sexual selection. As Darwin luminously
showed in his Descent of Man, sexual selection has been very active in
this respect. As the male anthropoid apes chose the females with the
least hair, and the females favoured the males with the finest growths
on chin and head, the general coating of the body gradually
degenerated, and the hair of the beard and head was more strongly
developed. The growth of hair at other parts of the body (arm-pit,
pubic region) was also probably due to sexual selection. Moreover,
changes of climate, or habits, and other adaptations unknown to us,
may have assisted the disappearance of the hairy coat.

The fact that our coat of hair is inherited directly from the
anthropoid apes is proved in an interesting way, according to Darwin,
by the direction of the rudimentary hairs on our arms, which cannot be
explained in any other way. Both on the upper and the lower part of
the arm they point towards the elbow. Here they meet at an obtuse
angle. This curious arrangement is found only in the anthropoid
apes--gorilla, chimpanzee, orang, and several species of
gibbons--besides man (Figures 1.203 and 1.207). In other species of
gibbon the hairs are pointed towards the hand both in the upper and
lower arm, as in the rest of the mammals. We can easily explain this
remarkable peculiarity of the anthropoids and man on the theory that
our common ancestors were accustomed (as the anthropoid apes are
to-day) to place their hands over their heads, or across a branch
above their heads, during rain. In this position, the fact that the
hairs point downwards helps the rain to run off. Thus the direction of
the hair on the lower part of our arm reminds us to-day of that useful
custom of our anthropoid ancestors.

The nervous system in man and all the other Vertebrates is, when fully
formed, an extremely complex apparatus, that we may compare, in
anatomic structure and physiological function, with an extensive
telegraphic system. The chief station of the system is the central
marrow or central nervous system, the innumerable ganglionic cells or
neurona (Figure 1.9) of which are connected by branching processes
with each other and with numbers of very fine conducting wires. The
latter are the peripheral and ubiquitous nerve-fibres; with their
terminal apparatus, the sense-organs, etc., they constitute the
conducting marrow or peripheral nervous system. Some of them--the
sensory nerve-fibres--conduct the impressions from the skin and other
sense-organs to the central marrow; others--the motor
nerve-fibres--convey the commands of the will to the muscles.

The central nervous system or central marrow (medulla centralis) is
the real organ of psychic action in the narrower sense. However we
conceive the intimate connection of this organ and its functions, it
is certain that its characteristic actions, which we call sensation,
will, and thought, are inseparably dependent on the normal development
of the material organ in man and all the higher animals. We must,
therefore, pay particular attention to the evolution of the latter. As
it can give us most important information regarding the nature of the
"soul," it should be full of interest. If the central marrow develops
in just the same way in the human embryo as in the embryo of the other
mammals, the evolution of the human psychic organ from the central
organ of the other mammals, and through them from the lower
vertebrates, must be beyond question. No one can doubt the momentous
bearing of these embryonic phenomena.

(FIGURE 2.290. Human embryo, three months old, natural size, from the
dorsal side: brain and spinal cord exposed. (From Kolliker.) h
cerebral hemispheres (fore brain), m corpora quadrigemina (middle
brain), c cerebellum (hind brain): under the latter is the triangular
medulla oblongata (after brain).

FIGURE 2.291. Central marrow of a human embryo, four months old,
natural size, from the back. (From Kolliker.) h large hemispheres, v
quadrigemina, c cerebellum, mo medulla oblongata: underneath it the
spinal cord.)

In order to understand them fully we must first say a word or two of
the general form and the anatomic composition of the mature human
central marrow. Like the central nervous system of all the other
Craniotes, it consists of two parts, the head-marrow or brain (medulla
capitis or encephalon) and the spinal-marrow (medulla spinalis or
notomyelon). The one is enclosed in the bony skull, the other in the
bony vertebral column. Twelve pairs of cerebral nerves proceed from
the brain, and thirty-one pairs of spinal nerves from the spinal cord,
to the rest of the body (Figure 1.171). On general anatomic
investigation the spinal marrow is found to be a cylindrical cord,
with a spindle-shaped bulb both in the region of the neck above (at
the last cervical vertebra) and the region of the loins (at the first
lumbar vertebra) below (Figure 2.291). At the cervical bulb the strong
nerves of the upper limbs, and at the lumbar bulb those of the lower
limbs, proceed from the spinal cord. Above, the latter passes into the
brain through the medulla oblongata (Figure 2.291 mo). The spinal cord
seems to be a thick mass of nervous matter, but it has a narrow canal
at its axis, which passes into the further cerebral ventricles above,
and is filled, like these, with a clear fluid.

The brain is a large nerve-mass, occupying the greater part of the
skull, of most elaborate structure. On general examination it divides
into two parts, the cerebrum and cerebellum. The cerebrum lies in
front and above, and has the familiar characteristic convolutions and
furrows on its surface (Figures 2.292 and 2.293). On the upper side it
is divided by a deep longitudinal fissure into two halves, the
cerebral hemispheres; these are connected by the corpus callosum. The
large cerebrum is separated from the small cerebellum by a deep
transverse furrow. The latter lies behind and below, and has also
numbers of furrows, but much finer and more regular, with convolutions
between, at its surface. The cerebellum also is divided by a
longitudinal fissure into two halves, the "small hemispheres"; these
are connected by a worm-shaped piece, the vermis cerebelli, above, and
by the broad pons Varolii below (Figure 2.292 VI).

(FIGURE 2.292. The human brain, seen from below. (From H. Meyer.)
Above (in front) is the cerebrum with its extensive branching furrows;
below (behind) the cerebellum with its narrow parallel furrows. The
Roman numbers I to XII indicate the roots of the twelve pairs of
cerebral nerves in a series towards the rear.)

But comparative anatomy and ontogeny teach us that in man and all the
other Craniotes the brain is at first composed, not of these two, but
of three, and afterwards five, consecutive parts. These are found in
just the same form--as five consecutive vesicles--in the embryo of all
the Craniotes, from the Cyclostoma and fishes to man. But, however
much they agree in their rudimentary condition, they differ
considerably afterwards. In man and the higher mammals the first of
these ventricles, the cerebrum, grows so much that in its mature
condition it is by far the largest and heaviest part of the brain. To
it belong not only the large hemispheres, but also the corpus callosum
that unites them, the olfactory lobes, from which the olfactory nerves
start, and most of the structures that are found at the roof and
bottom of the large lateral ventricles inside the two hemispheres,
such as the corpora striata. On the other hand, the optic thalami,
which lie between the latter, belong to the second division, which
develops from the "intermediate brain "; to the same section belong
the single third cerebral ventricle and the structures that are known
as the corpora geniculata, the infundibulum, and the pineal gland.
Behind these parts we find, between the cerebrum and cerebellum, a
small ganglion composed of two prominences, which is called the corpus
quadrigeminum on account of a superficial transverse fissure cutting
across (Figures 2.290 m and 2.291 v). Although this quadrigeminum is
very insignificant in man and the higher mammals, it forms a special
third section, greatly developed in the lower vertebrates, the "middle
brain." The fourth section is the "hind-brain" or little brain
(cerebellum) in the narrower sense, with the single median part, the
vermis, and the pair of lateral parts, the "small hemispheres" (Figure
2.291 c). Finally, we have the fifth and last section, the medulla
oblongata (Figure 2.291 mo), which contains the single fourth cerebral
cavity and the contiguous parts (pyramids, olivary bodies, corpora
restiformia). The medulla oblongata passes straight into the medulla
spinalis (spinal cord). The narrow central canal of the spinal cord
continues above into the quadrangular fourth cerebral cavity of the
medulla oblongata, the floor of which is the quadrangular depression.
From here a narrow duct, called "the aqueduct of Sylvius," passes
through the corpus quadrigeminum to the third cerebral ventricle,
which lies between the two optic thalami; and this in turn is
connected with the pairs of lateral ventricles which lie to the 
and  in the large hemispheres. Thus all the cavities of the
central marrow are directly interconnected. All these parts of the
brain have an infinitely complex structure in detail, but we cannot go
into this. Although it is much more elaborate in man and the higher
Vertebrates than in the lower classes, it develops in them all from
the same rudimentary structure, the five simple cerebral vesicles of
the embryonic brain.

But before we consider the development of the complicated structure of
the brain from this simple series of vesicles, let us glance for a
moment at the lower animals, which have no brain. Even in the
skull-less vertebrate, the Amphioxus, we find no independent brain, as
we have seen. The whole central marrow is merely a simple cylindrical
cord which runs the length of the body, and ends equally simply at
both extremities--a plain medullary tube. All that we can discover is
a small vesicular bulb at the foremost part of the tube, a degenerate
rudiment of a primitive brain. We meet the same simple medullary tube
in the first structure of the ascidia larva, in the same
characteristic position, above the chorda. On closer examination we
find here also a small vesicular swelling at the fore end of the tube,
the first trace of a differentiation of it into brain and spinal cord.
It is probable that this differentiation was more advanced in the
extinct Provertebrates, and the brain-bulb more pronounced (Figures
1.98 to 1.102). The brain is phylogenetically older than the spinal
cord, as the trunk was not developed until after the head. If we
consider the undeniable affinity of the Ascidiae to the Vermalia, and
remember that we can trace all the Chordonia to lower Vermalia, it
seems probable that the simple central marrow of the former is
equivalent to the simple nervous ganglion, which lies above the gullet
in the lower worms, and has long been known as the "upper pharyngeal
ganglion" (ganglion pharyngeum superius); it would be better to call
it the primitive or vertical brain (acroganglion).

Probably this upper pharyngeal ganglion of the lower worms is the
structure from which the complex central marrow of the higher animals
has been evolved. The medullary tube of the Chordonia has been formed
by the lengthening of the vertical brain on the dorsal side. In all
the other animals the central nervous system has been developed in a
totally different way from the upper pharyngeal ganglion; in the
Articulates, especially, a pharyngeal ring, with ventral marrow, has
been added. The Molluscs also have a pharyngeal ring, but it is not
found in the Vertebrates. In these the central marrow has been
prolonged down the dorsal side; in the Articulates down the ventral
side. This fact proves of itself that there is no direct relationship
between the Vertebrates and the Articulates. The unfortunate attempts
to derive the dorsal marrow of the former from the ventral marrow of
the latter have totally failed (cf. Chapter 2.20).

(FIGURE 2.293. The human brain, seen from the . (From H. Meyer.)
The furrows of the cerebrum are indicated by thick, and those of the
cerebellum by finer lines. Under the latter we can see the medulla
oblongata. f1 to f2 frontal convolutions, C central convolutions, S
fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular
gyrus, Po parieto-occipital fissure.)

When we examine the embryology of the human nervous system, we must
start from the important fact, which we have already seen, that the
first structure of it in man and all the higher Vertebrates is the
simple medullary tube, and that this separates from the outer germinal
layer in the middle line of the sole-shaped embryonic shield. As the
reader will remember, the straight medullary furrow first appears in
the middle of the sandal-shaped embryonic shield. At each side of it
the parallel borders curve over in the form of dorsal or medullary
swellings. These bend together with their free borders, and thus form
the closed medullary tube (Figures 1.133 to 1.137). At first this tube
lies directly underneath the horny plate; but it afterwards travels
inwards, the upper edges of the provertebral plates growing together
between the horny plate and the tube, joining above the latter, and
forming a completely closed canal. As Gegenbaur very properly
observes, "this gradual imbedding in the inner part of the body is a
process acquired with the progressive differentiation and the higher
potentiality that this secures; by this process the organ of greater
value to the organism is buried within the frame." (Cf. Figures 1.143
to 1.146).

(FIGURES 2.294 TO 2.296. Central marrow of the human embryo from the
seventh week, 4/5 inch long. (From Kolliker.)

FIGURE 2.294. The brain from above, v fore brain, z intermediate
brain, m middle brain, h hind brain, n after brain.

FIGURE 2.295. The brain with the uppermost part of the cord, from the
.

FIGURE 2.296. Back view of the whole embryo: brain and spinal cord
exposed.)

In the Cyclostoma--a stage above the Acrania--the fore end of the
cylindrical medullary tube begins early to expand into a pear-shaped
vesicle; this is the first outline of an independent brain. In this
way the central marrow of the Vertebrates divides clearly into its two
chief sections, brain and spinal cord. The simple vesicular form of
the brain, which persists for some time in the Cyclostoma, is found
also at first in all the higher Vertebrates (Figure 1.153 hb). But in
these it soon passes away, the one vesicle being divided into several
successive parts by transverse constrictions. There are first two of
these constrictions, dividing the brain into three consecutive
vesicles (fore brain, middle brain, and hind brain, Figure 1.154 v, m,
h). Then the first and third are sub-divided by fresh constrictions,
and thus we get five successive sections (Figure 1.155).

In all the Craniotes, from the Cyclostoma up to man, the same parts
develop from these five original cerebral vesicles, though in very
different ways. The first vesicle, the fore brain (Figure 1.155 v),
forms by far the largest part of the cerebrum--namely, the large
hemispheres, the olfactory lobes, the corpora striata, the callosum,
and the fornix. From the second vesicle, the intermediate brain (z),
originate especially the optic thalami, the other parts that surround
the third cerebral ventricle, and the infundibulum and pineal gland.
The third vesicle, the middle brain (m), produces the corpora
quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the
hind brain (h), develops the greater part of the cerebellum--namely,
the vermis and the two small hemispheres. Finally, the fifth vesicle,
the after brain (n), forms the medulla oblongata, with the
quadrangular pit (the floor of the fourth ventricle), the pyramids,
olivary bodies, etc.

We must certainly regard it as a comparative-anatomical and
ontogenetic fact of the greatest significance that in all the
Craniotes, from the lowest Cyclostomes and fishes up to the apes and
man, the brain develops in just the same way in the embryo. The first
rudiment of it is always a simple vesicular enlargement of the fore
end of the medullary tube. In every case, first three, then five,
vesicles develop from this bulb, and the permanent brain with all its
complex anatomic structures, of so great a variety in the various
classes of Vertebrates, is formed from the five primitive vesicles.
When we compare the mature brain of a fish, an amphibian, a reptile, a
bird, and a mammal, it seems incredible that we can trace the various
parts of these organs, that differ so much internally and externally,
to common types. Yet all these different Craniote brains have started
with the same rudimentary structure. To convince ourselves of this we
have only to compare the corresponding stages of development of the
embryos of these different animals.

(FIGURE 2.297. Head of a chick embryo (hatched fifty-eight hours),
from the back, magnified forty times. (From Mihalkovics.) vw anterior
wall of the fore brain. vh its ventricle. au optic vesicles, mh middle
brain, kh hind brain, nh after brain, hz heart (seen from below), vw
vitelline veins, us primitive segment, rm spinal cord.)

This comparison is extremely instructive. If we extend it through the
whole series of the Craniotes, we soon discover this interesting fact:
In the Cyclostomes (the Myxinoida and Petromyzonta), which we have
recognised as the lowest and earliest Craniotes, the whole brain
remains throughout life at a very low stage, which is very brief and
passing in the embryos of the higher Craniotes; they retain the five
original sections of the brain unchanged. In the fishes we find an
essential and considerable modification of the five vesicles; it is
clearly the brain of the Selachii in the first place, and subsequently
the brain of the Ganoids, from which the brain of the rest of the
fishes on the one hand and of the Dipneusts and Amphibia, and through
these of the higher Vertebrates, on the other hand, must be derived.
In the fishes and Amphibia (Figure 2.300) there is a preponderant
development of the middle brain, and also the after brain, the first,
second, and fourth sections remaining very primitive. It is just the
reverse in the higher Vertebrates, in which the first and third
sections, the cerebrum and cerebellum, are exceptionally developed;
while the middle brain and after brain remain small. The corpora
quadrigemina are mostly covered by the cerebrum, and the oblongata by
the cerebellum. But we find a number of stages of development within
the higher Vertebrates themselves. From the Amphibia upwards the brain
(and with it the psychic life) develops in two different directions;
one of these is followed by the reptiles and birds, and the other by
the mammals. The development of the first section, the fore brain, is
particularly characteristic of the mammals. It is only in them that
the cerebrum becomes so large as to cover all the other parts of the
brain (Figures 2.293 and 2.301 to 2.304).

There are also notable variations in the relative position of the
cerebral vesicles. In the lower Craniotes they lie originally almost
in the same plane. When we examine the brain laterally, we can cut
through all five vesicles with a straight line. But in the Amniotes
there is a considerable curve in the brain along with the bending of
the head and neck; the whole of the upper dorsal surface of the brain
develops much more than the under ventral surface. This causes a
curve, so that the parts come to lie as follows: The fore brain is
 in front and below, the intermediate brain a little higher, and
the middle brain highest of all; the hind brain lies a little lower,
and the after brain lower still. We find this only in the
Amniotes--the reptiles, birds, and mammals.

(FIGURE 2.298. Brain of three craniote embryos in vertical section. A
of a shark (Heptarchus), B of a serpent (Coluber), C of a goat
(Capra). a fore brain, b intermediate brain, c middle brain, d hind
brain, e after brain, s primitive c. (From Gegenbaur.)

FIGURE 2.299. Brain of a shark (Scyllium), back view. g fore-brain, h
olfactory lobes, which send the large olfactory nerves to the nasal
capsule (o), d intermediate brain, b middle brain; behind this the
insignificant structure of the hind brain, a after brain. (From
Gegenbaur.)

FIGURE 2.300. Brain and spinal cord of the frog. A from the dorsal, B
from the ventral side. a olfactory lobes before the (b) fore brain, i
infundibulum at the base of the intermediate brain, c middle brain, d
hind brain, s quadrangular pit in the after brain, m spinal cord (very
short in the frog), m apostrophe roots of the spinal nerves, t
terminal fibres of the spinal cord. (From Gegenbaur.)

FIGURE 2.301. Brain of an ox-embryo, two inches in length. (From
Mihalkovics, magnified three times.) Left view; the lateral wall of
the  hemisphere has been removed, st corpora striata, ml
Monro-foramen, ag arterial plexus, ah Ammon's horn, mh middle brain,
kh cerebellum. dv roof of the fourth ventricle, bb pons Varolii, na
medulla oblongata.)

Thus, while the brain of the mammals agrees a good deal in general
growth with that of the birds and reptiles, there are some striking
differences between the two. In the Sauropsids (birds and reptiles)
the middle brain and the middle part of the hind brain are well
developed. In the mammals these parts do not grow, and the fore-brain
develops so much that it overlies the other vesicles. As it continues
to grow towards the rear, it at last covers the whole of the rest of
the brain, and also encloses the middle parts from the sides (Figures
2.301 to 2.303). This process is of great importance, because the fore
brain is the organ of the higher psychic life, and in it those
functions of the nerve-cells are discharged which we sum up in the
word "soul." The highest achievements of the animal body--the
wonderful manifestations of consciousness and the complex molecular
processes of thought--have their seat in the fore brain. We can remove
the large hemispheres, piece by piece, from the mammal without killing
it, and we then see how the higher functions of consciousness,
thought, will, and sensation, are gradually destroyed, and in the end
completely extinguished. If the animal is fed artificially, it may be
kept alive for a long time, as the destruction of the psychic organs
by no means involves the extinction of the faculties of digestion,
respiration, circulation, urination--in a word, the vegetative
functions. It is only conscious sensation, voluntary movement,
thought, and the combination of various higher psychic functions that
are affected.

(FIGURE 2.302. Brain of a human embryo, twelve weeks old. (From
Mihalkovics, natural size.) Seen from behind and above. ms
mantle-furrow, mh corpora quadrigemina (middle brain), vs anterior
medullary ala, kh cerebellum, vv fourth ventricle, na medulla
oblongata.)

The fore brain, the organ of these functions, only attains this high
level of development in the more advanced Placentals, and thus we have
the simple explanation of the intellectual superiority of the higher
mammals. The soul of most of the lower Placentals is not much above
that of the reptiles, but among the higher Placentals we find an
uninterrupted gradation of mental power up to the apes and man. In
harmony with this we find an astonishing variation in the degree of
development of their fore brain, not only qualitatively, but also
quantitatively. The mass and weight of the brain are much greater in
modern mammals, and the differentiation of its various parts more
important, than in their extinct Tertiary ancestors. This can be shown
paleontologically in any particular order. The brains of the living
ungulates are (relatively to the size of the body) four to six times
(in the highest groups even eight times) as large as those of their
earlier Tertiary ancestors, the well-preserved skulls of which enable
us to determine the size and weight of the brain.

(FIGURE 2.303. Brain of a human embryo, twenty-four weeks old, halved
in the median plane:  hemisphere seen from inside. (From
Mihalkovics, natural size.) rn olfactory nerve. tr funnel of the
intermediate brain, vc anterior commissure, ml Monro-foramen, gw
fornix, ds transparent sheath, bl corpus callosum, br fissure at its
border, hs occipital fissure, zh cuneus, sf occipital transverse
fissure, zb pineal gland, mh corpora quadrigemina, kh cerebellum.

In the lower mammals the surface of the cerebral hemispheres is quite
smooth and level, as in the rabbit (Figure 2.304). Moreover, the fore
brain remains so small that it does not cover the middle brain. At a
stage higher the middle brain is covered, but the hind brain remains
free. Finally, in the apes and man, the latter also is covered by the
fore brain. We can trace a similar gradual development in the fissures
and convolutions that are found on the surface of the cerebrum of the
higher mammals (Figures 2.292 and 2.293). If we compare different
groups of mammals in regard to these fissures and convolutions, we
find that their development proceeds step by step with the advance of
mental life.

Of late years great attention has been paid to this special branch of
cerebral anatomy, and very striking individual differences have been
detected within the limits of the human race. In all human beings of
special gifts and high intelligence the convolutions and fissures are
much more developed than in the average man; and they are more
developed in the latter than in idiots and others of low mental
capacity. There is a similar gradation among the mammals in the
internal structure of the fore brain. In particular the corpus
callosum, that unites the two cerebral hemispheres, is only developed
in the Placentals. Other structures--for instance, in the lateral
ventricles--that seem at first to be peculiar to man, are also found
in the higher apes, and these alone. It was long thought that man had
certain distinctive organs in his cerebrum which were not found in any
other animal. But careful examination has discovered that this is not
the case, but that the characteristic features of the human brain are
found in a rudimentary form in the lower apes, and are more or less
fully developed in the higher apes. Huxley has convincingly shown, in
his Man's Place in Nature (1863), that the differences in the
formation of the brain within the ape-group constitute a deeper gulf
between the lower and higher apes than between the higher apes and
man.

The comparative anatomy and physiology of the brain of the higher and
lower mammals are very instructive, and give important information in
connection with the chief questions of psychology.

(FIGURE 2.304. Brain of the rabbit. A from the dorsal, B from the
ventral side, lo olfactory lobes, I fore brain, h hypophysis at the
base of the intermediate brain, III middle brain, IV hind brain, V
after brain, 2 optic nerve, 3 oculo-motor nerve, 5 to 8 cerebral
nerves. In A the roof of the  hemisphere (I) is removed, so that
we can see the corpora striata in the lateral ventricle. (From
Gegenbaur.))

The central marrow (brain and spinal cord) develops from the medullary
tube in man just as in all the other mammals, and the same applies to
the conducting marrow or "peripheral nervous system." It consists of
the SENSORY nerves, which conduct centripetally the impressions from
the skin and the sense-organs to the central marrow, and of the MOTOR
nerves, which convey centrifugally the movements of the will from the
central marrow to the muscles. All these peripheral nerves grow out of
the medullary tube (Figure 1.171), and are, like it, products of the
skin-sense layer.

The complete agreement in the structure and development of the psychic
organs which we find between man and the highest mammals, and which
can only be explained by their common origin, is of profound
importance in the monistic psychology. This is only seen in its full
light when we compare these morphological facts with the corresponding
physiological phenomena, and remember that every psychic action
requires the complete and normal condition of the correlative brain
structure for its full and normal exercise. The very complex molecular
movements inside the neural cells, which we describe comprehensively
as "the life of the soul," can no more exist in the vertebrate, and
therefore in man, without their organs than the circulation without
the heart and blood. And as the central marrow develops in man from
the same medullary tube as that of the other vertebrates, and as man
shares the characteristic structure of his cerebrum (the organ of
thought) with the anthropoid apes, his psychic life also must have the
same origin as theirs.

If we appreciate the full weight of these morphological and
physiological facts, and put a proper phylogenetic interpretation on
the observations of embryology, we see that the older idea of the
personal immortality of the human soul is scientifically untenable.
Death puts an end, in man as in any other vertebrate, to the
physiological function of the cerebral neurona, the countless
microscopic ganglionic cells, the collective activity of which is
known as "the soul." I have shown this fully in the eleventh chapter
of my Riddle of the Universe.

</section>
<section id="12" fatherid="0" title="EVOLUTION OF THE SENSE-ORGANS" lv="CHAPTER 2.25." priority="0" level="1">
The sense-organs are indubitably among the most important and
interesting parts of the human body; they are the organs by means of
which we obtain our knowledge of objects in the surrounding world.
Nihil est in intellectu quod non prius fuerit in sensu. They are the
first sources of the life of the soul. There is no other part of the
body in which we discover such elaborate anatomical structures,
co-operating with a definite purpose; and there is no other organ in
which the wonderful and purposive structure seems so clearly to compel
us to admit a Creator and a preconceived plan. Hence we find special
efforts made by dualists to draw our attention here to the "wisdom of
the Creator" and the design visible in his works. As a matter of fact,
you will discover, on mature reflection, that on this theory the
Creator is at bottom only playing the part of a clever mechanic or
watch-maker; all these familiar teleological ideas of Creator and
creation are based, in the long run, on a similar childlike
anthropomorphism.

However, we must grant that at the first glance the teleological
theory seems to give the simplest and most satisfactory explanation of
these purposive structures. If we merely examine the structure and
functions of the most advanced sense-organs, it seems impossible to
explain them without postulating a creative act. Yet evolution shows
us quite clearly that this popular idea is totally wrong. With its
assistance we discover that the purposive and remarkable sense-organs
were developed, like all other organs, without any preconceived
design--developed by the same mechanical process of natural selection,
the same constant correlation of adaptation and heredity, by which the
other purposive structures in the animal frame were slowly and
gradually brought forth in the struggle for life.

Like most other Vertebrates, man has six sensory organs, which serve
for eight different classes of sensations. The skin serves for
sensations of pressure and temperature. This is the oldest, lowest,
and vaguest of the sense-organs; it is distributed over the surface of
the body. The other sensory activities are localised. The sexual sense
is bound up with the skin of the external sexual organs, the sense of
taste with the mucous lining of the mouth (tongue and palate), and the
sense of smell with the mucous lining of the nasal cavity. For the two
most advanced and most highly differentiated sensory functions there
are special and very elaborate mechanical structures--the eye for the
sense of sight, and the ear for the sense of hearing and space
(equilibrium).

Comparative anatomy and physiology teach us that there are no
differentiated sense-organs in the lower animals; all their sensations
are received by the surface of the skin. The undifferentiated
skin-layer or ectoderm of the Gastraea is the simple stratum of cells
from which the differentiated sense-organs of all the Metazoa
(including the Vertebrates) have been evolved. Starting from the
assumption that necessarily only the superficial parts of the body,
which are in direct touch with the outer world, could be concerned in
the origin of sensations, we can see at once that the sense-organs
also must have arisen there. This is really the case. The chief part
of all the sense-organs originates from the skin-sense layer, partly
directly from the horny plate, partly from the brain, the foremost
part, of the medullary tube, after it has separated from the horny
plate. If we compare the embryonic development of the various
sense-organs, we see that they all make their appearance in the
simplest conceivable form; the wonderful contrivances that make the
higher sense-organs among the most remarkable and elaborate structures
in the body develop only gradually. In the phylogenetic explanation of
them comparative anatomy and ontogeny achieve their greatest triumphs.
But at first all the sense-organs are merely parts of the skin in
which sensory nerves expand. These nerves themselves were originally
of a homogeneous character. The different functions or specific
energies of the differentiated sense-nerves were only gradually
developed by division of labour. At the same time, their simple
terminal expansions in the skin were converted into extremely complex
organs.

The great instructiveness of these historical facts in connection with
the life of the soul is not difficult to see. The whole philosophy of
the future will be transformed as soon as psychology takes cognisance
of these genetic phenomena and makes them the basis of its
speculations. When we examine impartially the manuals of psychology
that have been published by the most distinguished speculative
philosophers and are still widely distributed, we are astonished at
the naivete with which the authors raise their airy metaphysical
speculations, regardless of the momentous embryological facts that
completely refute them. Yet the science of evolution, in conjunction
with the great advance of the comparative anatomy and physiology of
the sense-organs, provides the one sound empirical basis of a natural
psychology.

(FIGURE 2.305. Head of a shark (Scyllium), from the ventral side. m
mouth, o olfactory pits, r nasal groove, n nasal fold in natural
position, n apostrophe nasal fold drawn up. (The dots are openings of
the mucous canals.) (From Gegenbaur.))

In respect of the terminal expansions of the sensory nerves, we can
distribute the human sense-organs in three groups, which correspond to
three stages of development. The first group comprises those organs
the nerves of which spread out quite simply in the free surface of the
skin itself (organs of the sense of pressure, warmth, and sex). In the
second group the nerves spread out in the mucous coat of cavities
which are at first depressions in or invaginations of the skin (organs
of the sense of smell and taste). The third group is formed of the
very elaborate organs, the nerves of which spread out in an internal
vesicle, separated from the skin (organs of the sense of sight,
hearing, and space).

(FIGURES 2.306 AND 2.307. Head of a chick embryo, three days old:
2.306 front view, 2.307 from the . n rudimentary nose (olfactory
pits), l rudimentary eyes (optic pits), g rudimentary ear (auscultory
pit), v fore brain, gl eye-c, o process of upper jaw, u process of
lower jaw of the first gill-arch.

FIGURE 2.308. Head of a chick embryo, four days old, from below. n
nasal pit, o upper-jaw process of the first gill-arch, u lower-jaw
process of same, k double apostrophe second gill-arch, sp choroid
fissure of eye, s gullet.

FIGURES 2.309 AND 2.310. Heads of chick embryos: 2.309 from the end of
the fourth, 2.310 from the beginning of the fifth week. Letters as in
Figure 2.308, except: in inner, an outer, nasal process, nf nasal
furrow, st frontal process, m mouth. (From Kolliker.) Figures 2.306 to
2.310 are magnified to the same extent.)

There is little to be said of the development of the lower
sense-organs. We have already considered (Chapter 2.24) the organ of
touch and temperature in the skin. I need only add that in the corium
of man and all the higher Vertebrates countless microscopic
sense-organs develop, but the precise relation of these to the
sensations of pressure or resistance, of warmth and cold, has not yet
been explained. Organs of this kind, in or on which sensory cutaneous
nerves terminate, are the "tactile corpuscles" (or the Pacinian
corpuscles) and end-bulbs. We find similar corpuscles in the organs of
the sexual sense, the male penis and the female clitoris; they are
processes of the skin, the development of which we will consider later
(together with the rest of the sexual parts, Chapter 2.29). The
evolution of the organ of taste, the tongue and palate, will also be
treated later, together with that of the alimentary canal to which
these parts belong (Chapter 2.27). I will only point out for the
present that the mucous coat of the tongue and palate, in which the
gustatory nerve ends, originates from a part of the outer skin. As we
have seen, the whole of the mouth-cavity is formed, not as a part of
the gut-tube proper, but as a pit-like fold in the outer skin (Chapter
1.13). Its mucous lining is therefore formed, not from the visceral,
but from the cutaneous layer, and the taste-cells at the surface of
the tongue and palate are not products of the gut-fibre layer, but of
the skin-sense layer.

This applies also to the mucous lining of the olfactory organ, the
nose. However, the development of this organ is much more interesting.
Although the nose seems superficially to be simple and single, it
really consists, in man and all other Gnathostomes, of two completely
separated halves, the  and  cavities. They are divided by a
vertical partition, so that the  nostril leads into the 
cavity alone and the  nostril into the  cavity. They open
internally (and separately) by the posterior nasal apertures into the
pharynx, so that we can get direct into the gullet through the nasal
passages without touching the mouth. This is the way the air usually
passes in respiration; the mouth being closed, it goes through the
nose into the gullet, and through the larynx and bronchial tubes into
the lungs. The nasal cavities are separated from the mouth by the
horizontal bony palate, to which is attached behind (as a dependent
process) the soft palate with the uvula. In the upper and hinder parts
of the nasal cavities the olfactory nerve, the first pair of cerebral
nerves, expands in the mucous coat which clothes them. The terminal
branches of it spread partly over the septum (partition), partly on
the side walls of the internal cavities, to which are attached the
turbinated bones. These bones are much more developed in many of the
higher mammals than in man, but there are three of them in all
mammals. The sensation of smell arises by the passage of a current of
air containing odorous matter over the mucous lining of the cavities,
and stimulating the olfactory cells of the nerve-endings.

Man has all the features which distinguish the olfactory organ of the
mammals from that of the lower Vertebrates. In all essential points
the human nose entirely resembles that of the Catarrhine apes, some of
which have quite a human external nose (compare the face of the
long-nosed apes). However, the first structure of the olfactory organ
in the human embryo gives no indication of the future ample
proportions of our catarrhine nose. It has the form in which we find
it permanently in the fishes--a couple of simple depressions in the
skin at the outer surface of the head. We find these blind olfactory
pits in all the fishes; sometimes they lie near the eyes, sometimes
more forward at the point of the muzzle, sometimes lower down, near
the mouth (Figure 2.249).

(FIGURE 2.311. Frontal section of the mouth and throat of a human
embryo, neck half-inch long. "Invented" by Wilhelm His. The vertical
section (in the frontal plane, from  to ) is so constructed
that we see the nasal pits in the upper third of the figure and the
eyes at the sides: in the middle third the primitive gullet with the
gill-cs (gill-arches in section); in the lower third the pectoral
cavity with the bronchial tubes and the rudimentary lungs.)

This first rudimentary structure of the double nose is the same in all
the Gnathostomes; it has no connection with the primitive mouth. But
even in a section of the fishes a connection of this kind begins to
make its appearance, a furrow in the surface of the skin running from
each side of the nasal pit to the nearest corner of the mouth. This
furrow, the nasal groove or furrow (Figure 2.305 r), is very
important. In many of the sharks, such as the Scyllium, a special
process of the frontal skin, the nasal fold or internal nasal process,
is formed internally over the groove (n, n apostrophe). In contrast to
this the outer edge of the furrow rises in an "external nasal
process." As the two processes meet and coalesce over the nasal groove
in the Dipneusts and Amphibia, it is converted into a canal, the nasal
canal. Henceforth we can penetrate from the external pits through the
nasal canals direct into the mouth, which has been formed quite
independently. In the Dipneusts and the lower Amphibia the internal
aperture of the nasal canals lies in front (behind the lips); in the
higher Amphibia it is  behind. Finally, in the three higher
classes of Vertebrates the primary mouth-cavity is divided by the
formation of the horizontal palate-roof into two distinct
cavities--the upper (secondary) nasal cavity and the lower (secondary)
mouth-cavity. The nasal cavity in turn is divided by the construction
of the vertical septum into two halves-- and .

(FIGURE 2.312. Diagrammatic section of the mouth-nose cavity. While
the palate-plates (p) divide the original mouth-cavity into the lower
secondary mouth (m) and the upper nasal cavity, the latter in turn is
divided by the vertical partition (e) into two halves (n, n). (From
Gegenbaur.))

Comparative anatomy shows us to-day, in the series of the double-nosed
Vertebrates, from the fishes up to man, all the different stages in
the development of the nose, which the advanced olfactory organ of the
higher mammals has passed through at various periods in the course of
its phylogeny. It first appears in the embryo of man and the higher
Vertebrates, in which the double fish-nose persists throughout life.
At an early stage, before there is any trace of the characteristic
human face, a pair of small pits are formed in the head over the
original mouth-cavity; these were first discovered by Baer, and
ly called the "olfactory pits" (Figures 2.306 n and 2.307 n).
These primitive nasal pits are quite separate from the rudimentary
mouth, which also originates as a pit-like depression in the skin, in
front of the blind fore end of the gut. Both the pair of nasal pits
and the single mouth-pit (Figure 2.310 m) are clothed with the horny
plate. The original separation of the former from the latter is,
however, presently abolished, a process forming above the
mouth-pit--the "frontal process" (Figure 2.309 st). Its outer edge
rises to the  and  in the shape of two lateral processes;
these are the inner nasal processes or folds (in). Opposite to these a
parallel ridge is formed on either side between the eye and the nasal
pit; these are the outer nasal processes (an). Thus between the inner
and outer nasal processes a groove-like depression is formed on either
side, which leads from the nasal pit towards the mouth-pit (m); this
groove is, as the reader will guess, the same nasal furrow or groove
that we have already seen in the shark (Figure 2.305 r). As the
parallel edges of the inner and outer nasal processes bend towards
each other and join above the nasal groove, this is converted into a
tube, the primitive nasal canal. Hence the nose of man and all the
other Amniotes consists at this embryonic stage of a couple of narrow
tubes, the nasal canals, which lead from the outer surface of the
forehead into the rudimentary mouth. This transitory condition
resembles that in which we find the nose permanently in the Dipneusts
and Amphibia.

A cone-shaped structure, which grows from below towards the lower ends
of the two nasal processes and joins with them, plays an important
part in the conversion of the open nasal groove into the closed canal.
This is the upper-jaw process (Figures 2.306 to 2.310 o). Below the
mouth-pit are the gill-arches, which are separated by the gill-cs.
The first of these gill-arches, and the most important for our
purpose, which we may call the maxillary (jaw) arch, forms the
skeleton of the jaws. Above at the basis a small process grows out of
this first gill-arch; this is the upper-jaw process. The first
gill-arch itself develops a cartilage at one of its inner sides, the
"Meckel cartilage" (named after its discoverer), on the outer surface
of which the lower jaw is formed (Figures 2.306 to 2.310 u). The
upper-jaw process forms the chief part of the skeleton of that jaw,
the palate bone, and the pterygoid bone. On its outer side is
afterwards formed the upper-jaw bone, in the narrower sense, while the
middle part of the skeleton of the upper jaw, the intermaxillary,
develops from the foremost part of the frontal process.

The two upper-jaw processes are of great importance in the further
development of the face. From them is formed, growing into the
primitive mouth-cavity, the important horizontal partition (the
palate) that divides the former into two distinct cavities. The upper
cavity, into which the nasal canals open, now develops into the nasal
cavity, the air-passage and the organ of smell. The lower cavity forms
the permanent secondary mouth (Figure 2.312 m), the food-passage and
the organ of taste. Both the upper and lower cavities open behind into
the gullet (pharynx). The hard palate that separates them is formed by
the joining of two lateral halves, the horizontal plates of the two
upper-jaw processes, or the palate-plates (p). When these do not,
sometimes, completely join in the middle, a longitudinal c
remains, through which we can penetrate from the mouth straight into
the nasal cavity. This is the malformation known as "wolf's throat."
"Hare-lip" is the lesser form of the same defect. At the same time as
the horizontal partition of the hard palate a vertical partition is
formed by which the single nasal cavity is divided into two
sections--a  and  half (Figure 2.312 n, n).

(FIGURES 2.313 AND 2.314. Upper part of the body of a human embryo,
two-thirds of an inch long, of the sixth week; Figure 2.313 from the
, Figure 2.314 from the front. The origin of the nose and the
upper lip from two lateral and originally separate halves can be
clearly seen. Nose and upper lip are large in proportion to the rest
of the face, and especially to the lower lip. (From Kollmann.))

The double nose has now acquired the characteristic form that man
shares with the other mammals. Its further development is easy to
follow; it consists of the formation of the inner and outer processes
of the walls of the two cavities. The external nose is not formed
until long after all these essential parts of the internal organ of
smell. The first traces of it in the human embryo are found about the
middle of the second month (Figures 2.313 to 2.316). As can be seen in
any human embryo during the first month, there is at first no trace of
the external nose. It only develops afterwards from the foremost nasal
part of the primitive skull, growing forwards from behind. The
characteristic human nose is formed very late. Much stress is at times
laid on this organ as an exclusive privilege of man. But there are
apes that have similar noses, such as the long-nosed ape.

(FIGURE 2.315. Face of a human embryo, seven weeks old, (From
Kollmann.) Joining of the nasal processes (e outer, i inner) with the
upper-jaw process (o), n nasal wall, a ear-opening.)

The evolution of the eye is not less interesting and instructive than
that of the nose. Although this noblest of the sensory organs is one
of the most elaborate and purposive on account of its optic perfection
and remarkable structure, it nevertheless develops, without
preconceived design, from a simple process of the outer germinal
layer. The fully-formed human eye is a round capsule, the eye-ball
(Figure 2.317). This lies in the bony cavity of the skull, surrounded
by protective fat and motor muscles. The greater part of it is taken
up with a semi-fluid, transparent gelatinous substance, the corpus
vitreum. The crystalline lens is fitted into the anterior surface of
the ball (Figure 2.317 l). It is a lenticular, bi-convex, transparent
body, the most important of the refractive media in the eye. Of this
group we have, besides the corpus vitreum and the lens, the watery
fluid (humor aqueus) that is found in front of the lens (at the letter
m in Figure 2.317). These three transparent refractive media, by which
the rays of light that enter the eye are broken up and re-focussed,
are enclosed in a solid round capsule, composed of several different
coats, something like the concentric layers of an onion. The outermost
and thickest of these envelopes is the white sclerotic coat of the
eye. It consists of tough white connective tissue. In front of the
lens a circular, strongly-curved, transparent plate is fitted into the
sclerotic, like the glass of a watch--the cornea (b). At its outer
surface the cornea is covered with a very thin layer of the epidermis;
this is known as the conjunctiva. It goes from the cornea over the
inner surface of the eye-lids, the upper and lower folds which we draw
over the eye in closing it. At the inner corner of the eye we have a
rudimentary organ in the shape of the relic of a third (inner)
eye-lid, which is greatly developed, as "nictitating (winking)
membrane," in the lower Vertebrates (Chapter 1.5). Underneath the
upper eye-lid are the lachrymal glands, the product of which, the
lachrymal fluid, keeps the outer surface of the eye smooth and clean.

Immediately under the sclerotic we find a very delicate, dark-red
membrane, very rich in blood-vessels--the choroid coat--and inside
this the retina (o), the expansion of the optic nerve (i). The latter
is the second cerebral nerve. It proceeds from the optic thalami (the
second cerebral vesicle) to the eye; penetrates its outer envelopes,
and then spreads out like a net between the choroid and the corpus
vitreum. Between the retina and the choroid there is a very delicate
membrane, which is usually (but wrongly) associated with the latter.
This is the black pigment-membrane (n). It consists of a single
stratum of graceful, hexagonal, regularly-joined cells, full of
granules of black colouring matter. This pigment membrane clothes, not
only the inner surface of the choroid proper, but also the hind
surface of its anterior muscular continuation, which covers the edge
of the lens in front as a circular membrane, and arrests the rays of
light at the sides. This is the well-known iris of the eye (h),
coloured differently in different individuals (blue, grey, brown,
etc.); it forms the anterior border of the choroid. The circular
opening that is  in the middle is the pupil, through which the
rays of light penetrate into the eye. At the point where the iris
leaves the anterior border of the choroid proper the latter is very
thick, and forms a delicate crown of folds (g), which surrounds the
edge of the lens with about seventy large and many smaller rays
(corona ciliaris.)

At a very early stage a couple of pear-shaped vesicles develop from
the foremost part of the first cerebral vesicle in the embryo of man
and the other Craniotes (Figures 1.155 a and 2.297 au). These growths
are the primary optic vesicles. They are at first directed outwards
and forwards, but presently grow downward, so that, after the complete
separation of the five cerebral vesicles, they lie at the base of the
intermediate brain. The inner cavities of these pear-shaped vesicles,
which soon attain a considerable size, are openly connected with the
ventricle of the intermediate brain by their hollow stems. They are
covered externally by the epidermis.

(FIGURE 2.316. Face of a human embryo, eight weeks old (From Ecker.))

At the point where this comes into direct contact with the most curved
part of the primary optic vesicle there is a thickening (l) and also a
depression (o) of the horny plate (Figure 2.318, I). This pit, which
we may call the lens-pit, is converted into a closed sac, the
thick-walled lens-vesicle (2, l), the thick edges of the pit joining
together above it. In the same way in which the medullary tube
separates from the outer germinal layer, we now see this lens-sac
sever itself entirely from the horny plate (h), its source of origin.
The hollow of the sac is afterwards filled with the cells of its thick
walls, and thus we get the solid crystalline lens. This is, therefore,
a purely epidermic structure. Together with the lens the small
underlying piece of corium-plate also separates from the skin.

As the lens separates from the corneous plate and grows inwards, it
necessarily hollows out the contiguous primary optic vesicle (Figure
2.318, 1 to 3). This is done in just the same way as the invagination
of the blastula, which gives rise to the gastrula in the amphioxus
(Figure 2.38 C to F). In both cases the hollowing of the closed
vesicle on one side goes so far that at last the inner, folded part
touches the outer, not folded part, and the cavity disappears. As in
the gastrula the first part is converted into the entoderm and the
latter into the ectoderm, so in the invagination of the primary optic
vesicle the retina (r) is formed from the first (inner) part, and the
black pigment membrane (u) from the latter (outer, non-invaginated)
part. The hollow stem of the primary optic vesicle is converted into
the optic nerve. The lens (l), which has so important a part in this
process, lies at first directly on the invaginated part, or the retina
(r). But they soon separate, a new structure, the corpus vitreum (gl),
growing between them. While the lenticular sac is being detached and
is causing the invagination of the primary optic vesicle, another
invagination is taking place from below; this proceeds from the
superficial part of the skin-fibre layer--the corium of the head.
Behind and under the lens a last-shaped process rises from the
cutis-plate (Figure 2.319 g), hollows out the cup-shaped optic vesicle
from below, and presses between the lens (l) and the retina (i). In
this way the optic vesicle acquires the form of a hood.

(FIGURE 2.317. The human eye in section. a sclerotic coat, b cornea, c
conjunctiva, d circular veins of the iris, e choroid coat, f ciliary
muscle, g corona ciliaris, h iris, i optic nerve, k anterior border of
the retina, l crystalline lens, m inner covering of the cornea
(aqueous membrane), n pigment membrane, o retina, p Petit's canal, q
yellow spot of the retina. (From Helmholtz.))

Finally, a complete fibrous envelope, the fibrous capsule of the
eye-ball, is formed about the secondary optic vesicle and its stem
(the secondary optic nerve). It originates from the part of the
head-plates which immediately encloses the eye. This fibrous envelope
takes the form of a closed round vesicle, surrounding the whole of the
ball and pushing between the lens and the horny plate at its outer
side. The round wall of the capsule soon divides into two different
membranes by surface-cleavage. The inner membrane becomes the choroid
or vascular coat, and in front the ciliary corona and iris. The outer
membrane is converted into the white protective or sclerotic coat--in
front, the transparent cornea. The eye is now formed in all its
essential parts. The further development--the complicated
differentiation and composition of the various parts--is a matter of
detail.

(FIGURE 2.318. Eye of the chick embryo in longitudinal section (1.
from an embryo sixty-five hours old; 2. from a somewhat older embryo;
3. from an embryo four days old). h horny plate, o lens-pit, l lens
(in 1. still part of the epidermis, in 2. and 3. separated from it), x
thickening of the horny plate at the point where the lens has severed
itself, gl corpus vitreum, r retina, u pigment membrane. (From
Remak.))

The chief point in this remarkable evolution of the eye is the
circumstance that the optic nerve, the retina, and the pigment
membrane originate really from a part of the brain--an outgrowth of
the intermediate brain--while the lens, the chief refractive body,
develops from the outer skin. From the skin--the horny plate--also
arises the delicate conjunctiva, which afterwards covers the outer
surface of the eyeball. The lachrymal glands are ramified growths from
the conjunctiva (Figure 2.286). All these important parts of the eye
are products of the outer germinal layer. The remaining parts--the
corpus vitreum (with the vascular capsule of the lens), the choroid
(with the iris), and the sclerotic (with the cornea)--are formed from
the middle germinal layer.

The outer protection of the eye, the eye-lids, are merely folds of the
skin, which are formed in the third month of human embryonic life. In
the fourth month the upper eye-lid reaches the lower, and the eye
remains covered with them until birth. As a rule, they open wide
shortly before birth (sometimes only after birth). Our craniote
ancestors had a third eye-lid, the nictitating membrane, which was
drawn over the eye from its inner angle. It is still found in many of
the Selachii and Amniotes. In the apes and man it has degenerated, and
there is now only a small relic of it at the inner corner of the eye,
the semi-lunar fold, a useless rudimentary organ (Chapter 1.5). The
apes and man have also lost the Harderian gland that opened under the
nictitating membrane; we find this in the rest of the mammals, and the
birds, reptiles, and amphibia.

The peculiar embryonic development of the vertebrate eye does not
enable us to draw any definite conclusions as to its obscure
phylogeny; it is clearly cenogenetic to a great extent, or obscured by
the reduction and curtailment of its original features. It is probable
that many of the earlier stages of its phylogeny have disappeared
without leaving a trace. It can only be said positively that the
peculiar ontogeny of the complicated optic apparatus in man follows
just the same laws as in all the other Vertebrates. Their eye is a
part of the fore brain, which has grown forward towards the skin, not
an original cutaneous sense-organ, as in the Invertebrates.

(FIGURE 2.319. Horizontal transverse section of the eye of a human
embryo, four weeks old (magnified one hundred times). (From Kolliker.)
t lens (the dark wall of which is as thick as the diameter of the
central cavity), g corpus vitreum (connected by a stem, g, with the
corium), v vascular loop (pressing behind the lens inside the corpus
vitreum by means of this stem g), i retina (inner thicker, invaginated
layer of the primary optic vesicle), a pigment membrane (outer, thin,
non-invaginated layer of same), h space between retina and pigment
membrane (remainder of the cavity of the primary optic vesicle).

FIGURE 2.320. The human ear ( ear, seen from the front, natural
size), a shell of ear, b external passage, c tympanum, d tympanic
cavity, e Eustachian tube, f, g, h the three bones of the ear (f
hammer, g anvil, h stirrup), i utricle, k the three semi-circular
canals, l the sacculus, m cochlea, n auscultory nerve.)

The vertebrate ear resembles the eye and nose in many important
respects, but is different in others, in its development. The
auscultory organ in the fully-developed man is like that of the other
mammals, and especially the apes, in the main features. As in them, it
consists of two chief parts--an apparatus for conducting sound
(external and middle ear) and an apparatus for the sensation of sound
(internal ear). The external ear opens in the shell at the side of the
head (Figure 2.320 a). From this point the external passage (b), about
an inch in length, leads into the head. The inner end of it is closed
by the tympanum, a vertical, but not quite up, thin membrane of
an oval shape (c). This tympanum separates the external passage from
the tympanic cavity (d). This is a small cavity, filled with air, in
the temporal bone; it is connected with the mouth by a special tube.
This tube is rather longer, but much narrower, than the outer passage,
leads inwards obliquely from the anterior wall of the tympanic cavity,
and opens in the throat below, behind the nasal openings. It is called
the Eustachian tube (e); it serves to equalise the pressure of the air
within the tympanic cavity and the outer atmosphere that enters by the
external passage. Both the Eustachian tube and the tympanic cavity are
lined with a thin mucous coat, which is a direct continuation of the
mucous lining of the throat. Inside the tympanic cavity there are
three small bones which are known (from their shape) as the hammer,
anvil, and stirrup (Figure 2.320, f, g, h). The hammer (f) is the
outermost, next to the tympanum. The anvil (g) fits between the other
two, above and inside the hammer. The stirrup (h) lies inside the
anvil, and touches with its base the outer wall of the internal ear,
or auscultory vesicle. All these parts of the external and middle ear
belong to the apparatus for conducting sound. Their chief task is to
convey the waves of sound through the thick wall of the head to the
inner-lying auscultory vesicle. They are not found at all in the
fishes. In these the waves of sound are conveyed directly by the wall
of the head to the auscultory vesicle.

The internal apparatus for the sensation of sound, which receives the
waves of sound from the conducting apparatus, consists in man and all
other mammals of a closed auscultory vesicle filled with fluid and an
auditory nerve, the ends of which expand over the wall of this
vesicle. The vibrations of the sound-waves are conveyed by these media
to the nerve-endings. In the labyrinthic water that fills the
auscultory vesicle there are small stones at the points of entry of
the acoustic nerves, which are composed of groups of microscopic
calcareous crystals (otoliths). The auscultory organ of most of the
Invertebrates has substantially the same composition. It usually
consists of a closed vesicle, filled with fluid, and containing
otoliths, with the acoustic nerve expanding on its wall. But, while
the auditory vesicle is usually of a simple round or oval shape in the
Invertebrates, it has in the Vertebrates a special and curious
structure, the labyrinth. This thin-membraned labyrinth is enclosed in
a bony capsule of the same shape, the osseous labyrinth (Figure
2.321), and this lies in the middle of the petrous bone of the skull.
The labyrinth is divided into two vesicles in all the Gnathostomes.
The larger one is called the utriculus, and has three arched
appendages, called the "semi-circular canals" (c, d, e). The smaller
vesicle is called the sacculus, and is connected with a peculiar
appendage, with (in man and the higher mammals) a spiral form
something like a snail's shell, and therefore called the cochlea (=
snail, b). On the thin wall of this delicate labyrinth the acoustic
nerve, which comes from the after-brain, spreads out in most elaborate
fashion. It divides into two main branches--a cochlear nerve (for the
cochlea) and a vestibular nerve (for the rest of the labyrinth). The
former seems to have more to do with the quality, the latter with the
quantity, of the acoustic sensations. Through the cochlear nerves we
learn the height and timbre, through the vestibular nerves the
intensity, of tones.

(FIGURE 2.321. The bony labyrinth of the human ear ( side). a
vestibulum, b cochlea, c upper canal, d posterior canal, e outer
canal, f oval fenestra, g round fenestra. (From Meyer.)

FIGURE 2.322. Development of the auscultory labyrinth of the chick, in
five successive stages (A to E). (Vertical transverse sections of the
skull.) fl auscultory pits, lv auscultory vesicles, lr labyrinthic
appendage, c rudimentary cochlea, csp posterior canal, cse external
canal, jv jugular vein. (From Reissner.))

The first structure of this highly elaborate organ is very simple in
the embryo of man and all the other Craniotes; it is a pit-like
depression in the skin. At the back part of the head at both sides,
near the after brain, a small thickening of the horny plate is formed
at the upper end of the second gill-c (Figure 2.322 A fl). This
sinks into a sort of pit, and severs from the epidermis, just as the
lens of the eye does. In this way is formed at each side, directly
under the horny plate of the back part of the head, a small vesicle
filled with fluid, the primitive auscultory vesicle, or the primary
labyrinth. As it separates from its source, the horny plate, and
presses inwards and backwards into the skull, it changes from round to
pear-shaped (Figures 2.322 B lv and 2.323 o). The outer part of it is
lengthened into a thin stem, which at first still opens outwards by a
narrow canal. This is the labyrinthic appendage (Figure 2.322 lr). In
the lower Vertebrates it develops into a special cavity filled with
calcareous crystals, which remains open permanently in some of the
primitive fishes, and opens outwards in the upper part of the skull.
But in the mammals the labyrinthic appendage degenerates. In these it
has only a phylogenetic interest as a rudimentary organ, with no
actual physiological significance. The useless relic of it passes
through the wall of the petrous bone in the shape of a narrow canal,
and is called the vestibular aqueduct.

It is only the inner and lower bulbous part of the separated
auscultory vesicle that develops into the highly complex and
differentiated structure that is afterwards known as the secondary
labyrinth. This vesicle divides at an early stage into an upper and
larger and a lower and smaller section. From the one we get the
utriculus with the semi-circular canals; from the other the sacculus
and the cochlea (Figure 2.320 c). The canals are formed in the shape
of simple pouch-like involutions of the utricle (cse and csp). The
edges join together in the middle part of each fold, and separate from
the utricle, the two ends remaining in open connection with its
cavity. All the Gnathostomes have these three canals like man, whereas
among the Cyclostomes the lampreys have only two and the hag-fishes
only one. The very complex structure of the cochlea, one of the most
elaborate and wonderful outcomes of adaptation in the mammal body,
develops originally in very simple fashion as a flask-like projection
from the sacculus. As Hasse and Retzius have pointed out, we find the
successive ontogenetic stages of its growth represented permanently in
the series of the higher Vertebrates. The cochlea is wanting even in
the Monotremes, and is restricted to the rest of the mammals and man.

The auditory nerve, or eighth cerebral nerve, expands with one branch
in the cochlea, and with the other in the remaining parts of the
labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal
branch of a cerebro-spinal nerve, the motor ventral branch of which
acts for the muscles of the face (nervus facialis). It has therefore
originated phylogenetically from an ordinary cutaneous nerve, and so
is of quite different origin from the optic and olfactory nerves,
which both represent direct outgrowths of the brain. In this respect
the auscultory organ is essentially different from the organs of sight
and smell. The acoustic nerve is formed from ectodermic cells of the
hind brain, and develops from the nervous structure that appears at
its dorsal limit. On the other hand, all the membranous,
cartilaginous, and osseous coverings of the labyrinth are formed from
the mesodermic head-plates.

(FIGURE 2.323. Primitive skull of the human embryo, four weeks old,
vertical section,  half seen internally. v, z, m, h, n the five
pits of the cranial cavity, in which the five cerebral vesicles lie
(fore, intermediate, middle, hind, and after brains), o pear-shaped
primary auscultory vesicle (appearing through), a eye (appearing
through), no optic nerve, p canal of the hypophysis, t central
prominence of the skull. (From Kolliker.))

The apparatus for conducting sound which we find in the external and
middle ear of mammals develops quite separately from the apparatus for
the sensation of sound. It is both phylogenetically and
ontogenetically an independent secondary formation, a later accession
to the primary internal ear. Nevertheless, its development is not less
interesting, and is explained with the same ease by comparative
anatomy. In all the fishes and in the lowest Vertebrates there is no
special apparatus for conducting sound, no external or middle ear;
they have only a labyrinth, an internal ear, which lies within the
skull. They are without the tympanum and tympanic cavity, and all its
appendages. From many observations made in the last few decades it
seems that many of the fishes (if not all) cannot distinguish tones;
their labyrinth seems to be chiefly (if not exclusively) an organ for
the sense of space (or equilibrium). If it is destroyed, the fishes
lose their balance and fall. In the opinion of recent physiologists
this applies also to many of the Invertebrates (including the nearer
ancestors of the Vertebrates). The round vesicles which are considered
to be their auscultory vesicles, and which contain an otolith, are
supposed to be merely organs of the sense of space ("static vesicles
or statocysts").

The middle ear makes its first appearance in the amphibian class,
where we find a tympanum, tympanic cavity, and Eustachian tube; these
animals, and all terrestrial Vertebrates, certainly have the faculty
of hearing. All these essential parts of the middle ear originate from
the first gill-c and its surrounding part; in the Selachii this
remains throughout life an open squirting-hole, and lies between the
first and second gill-arch. In the embryo of the higher Vertebrates it
closes up in the centre, and thus forms the tympanic membrane. The
outlying remainder of the first gill-c is the rudiment of the
external meatus. From its inner part we get the tympanic cavity, and,
further inward still, the Eustachian tube. Connected with this is the
development of the three bones of the mammal ear from the first two
gill-arches; the hammer and anvil are formed from the first, the
stirrup from the upper end of the second, gill-arch.

(FIGURE 2.324. The rudimentary muscles of the ear in the human skull.
a raising muscle (M. attollens), b drawing muscle (M. attrahens), c
withdrawing muscle (M. retrahens), d large muscle of the helix (M.
helicis major), e small muscle of the helix (M. helicis minor), f
muscle of the angle of the ear (M. tragicus), g anti-angular muscle
(M. antitragicus). (From H. Meyer.))

Finally, the shell (pinna or concha) and external meatus (passage to
the tympanum) of the outer ear are developed in a very simple fashion
from the skin that borders the external aperture of the first
gill-c. The shell rises in the shape of a circular fold of the
skin, in which cartilage and muscles are afterwards formed (Figures
2.313 and 2.315). This organ is only found in the mammalian class. It
is very rudimentary in the lowest section, the Monotremes. In the
others it is found at very different stages of development, and
sometimes of degeneration. It is degenerate in most of the aquatic
mammals. The majority of them have lost it altogether--for instance,
the walruses and whales and most of the seals. On the other hand, the
pinna is well developed in the great majority of the Marsupials and
Placentals; it receives and collects the waves of sound, and is
equipped with a very elaborate muscular apparatus, by means of which
the pinna can be turned freely in any direction and its shape be
altered. It is well known how readily domestic animals--horses, cows,
dogs, hares, etc.--point their ears and move them in different
directions. Most of the apes do the same, and our earlier ape
ancestors were also able to do it. But our later simian ancestors,
which we have in common with the anthropoid apes, abandoned the use of
these muscles, and they gradually became rudimentary and useless.
However, we possess them still (Figure 2.324). In fact, some men can
still move their ears a little backward and forward by means of the
drawing and withdrawing muscles (b and c); with practice this faculty
can be much improved. But no man can now lift up his ears by the
raising muscle (a), or change the shape of them by the small inner
muscles (d, e, f, g). These muscles were very useful to our ancestors,
but are of no consequence to us. This applies to most of the
anthropoid apes as well.

We also share with the higher anthropoid apes (gorilla, chimpanzee,
and orang) the characteristic form of the human outer ear, especially
the folded border, the helix and the lobe. The lower apes have pointed
ears, without folded border or lobe, like the other mammals. But
Darwin has shown that at the upper part of the folded border there is
in many men a small pointed process, which most of us do not possess.
In some individuals this process is well developed. It can only be
explained as the relic of the original point of the ear, which has
been turned inwards in consequence of the curving of the edge. If we
compare the pinna of man and the various apes in this respect, we find
that they present a connected series of degenerate structures. In the
common catarrhine ancestors of the anthropoids and man the
degeneration set in with the folding together of the pinna. This
brought about the helix of the ear, in which we find the significant
angle which represents the relic of the salient point of the ear in
our earlier simian ancestors. Here again, therefore, comparative
anatomy enables us to trace with certainty the human ear to the
similar, but more developed, organ of the lower mammals. At the same
time, comparative physiology shows that it was a more or less useful
implement in the latter, but it is quite useless in the anthropoids
and man. The conducting of the sound has scarcely been affected by the
loss of the pinna. We have also in this the explanation of the
extraordinary variety in the shape and size of the shell of the ear in
different men; in this it resembles other rudimentary organs.

</section>
<section id="13" fatherid="0" title="EVOLUTION OF THE ORGANS OF MOVEMENT" lv="CHAPTER 2.26." priority="0" level="1">
The peculiar structure of the locomotive apparatus is one of the
features that are most distinctive of the vertebrate stem. The chief
part of this apparatus is formed, as in all the higher animals, by the
active organs of movement, the muscles; in consequence of their
contractility they have the power to draw up and shorten themselves.
This effects the movement of the various parts of the body, and thus
the whole body is conveyed from place to place. But the arrangement of
these muscles and their relation to the solid skeleton are different
in the Vertebrates from the Invertebrates.

(FIGURE 2.325. The human skeleton. From the .

FIGURE 2.326. The human skeleton. Front.)

In most of the lower animals, especially the Platodes and Vermalia, we
find that the muscles form a simple, thin layer of flesh immediately
underneath the skin. This muscular layer is very closely connected
with the skin itself; it is the same in the Mollusc stem. Even in the
large division of the Articulates, the classes of crabs, spiders,
myriapods, and insects, we find a similar feature, with the difference
that in this case the skin forms a solid armour--a rigid cutaneous
skeleton made of chitine (and often also of carbonate of lime). This
external chitine coat undergoes a very elaborate articulation both on
the trunk and the limbs of the Articulates, and in consequence the
muscular system also, the contractile fibres of which are attached
inside the chitine tubes, is highly articulated. The Vertebrates form
a direct contrast to this. In these alone a solid internal skeleton is
developed, of cartilage or bone, to which the muscles are attached.
This bony skeleton is a complex lever apparatus, or PASSIVE apparatus
of movement. Its rigid parts, the arms of the levers, or the bones,
are brought together by the actively mobile muscles, as if by
drawing-ropes. This admirable locomotorium, especially its solid
central axis, the vertebral column, is a special feature of the
Vertebrates, and has given the name to the group.

(FIGURE 2.327. The human vertebral column (standing up, from the
 side). (From H. Meyer.))

In order to get a clear idea of the chief features of the development
of the human skeleton, we must first examine its composition in the
adult frame (Figure 2.325, the human skeleton seen from the ;
Figure 2.326, front view of the whole skeleton). As in other mammals,
we distinguish first between the axial or dorsal skeleton and the
skeleton of the limbs. The axial skeleton consists of the vertebral
column (the skeleton of the trunk) and the skull (skeleton of the
head); the latter is a peculiarly modified part of the former. As
appendages of the vertebral column we have the ribs, and of the skull
we have the hyoid bone, the lower jaw, and the other products of the
gill-arches.

The skeleton of the limbs or extremities is composed of two groups of
parts--the skeleton of the extremities proper and the zone-skeleton,
which connects these with the vertebral column. The zone-skeleton of
the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the
legs (or hind legs) is the pelvic zone.

(FIGURE 2.328. A piece of the axial rod (chorda dorsalis), from a
sheep embryo. a cuticular sheath, b cells. (From Kolliker.))

The vertebral column (Figure 2.327) in man is composed of thirty-three
to thirty-five ring-shaped bones in a continuous series (above each
other, in man's up position). These vertebrae are separated from
each other by elastic ligaments, and at the same time connected by
joints, so that the whole column forms a firm and solid, but flexible
and elastic, axial skeleton, moving freely in all directions. The
vertebrae differ in shape and connection at the various parts of the
trunk, and we distinguish the following groups in the series,
beginning at the top: Seven cervical vertebrae, twelve dorsal
vertebrae, five lumbar vertebrae, five sacral vertebrae, and four to
six caudal vertebrae. The uppermost, or those next to the skull, are
the cervical vertebrae (Figure 2.327); they have a hole in each of the
lateral processes. There are seven of these vertebrae in man and
almost all the other mammals, even if the neck is as long as that of
the camel or giraffe, or as short as that of the mole or hedgehog.
This constant number, which has few exceptions (due to adaptation), is
a strong proof of the common descent of the mammals; it can only be
explained by faithful heredity from a common stem-form, a primitive
mammal with seven cervical vertebrae. If each species had been created
separately, it would have been better to have given the long-necked
mammals more, and the short-necked animals less, cervical vertebrae.
Next to these come the dorsal (or pectoral) vertebrae, which number
twelve to thirteen (usually twelve) in man and most of the other
mammals. Each dorsal vertebra (Figure 1.165) has at the side,
connected by joints, a couple of ribs, long bony arches that lie in
and protect the wall of the chest. The twelve pairs of ribs, together
with the connecting intercostal muscles and the sternum, which joins
the ends of the  and  ribs in front, form the chest (thorax).
In this strong and elastic frame are the lungs, and between them the
heart. Next to the dorsal vertebrae comes a short but stronger section
of the column, formed of five large vertebrae. These are the lumbar
vertebrae (Figure 1.166); they have no ribs and no holes in the
transverse processes. To these succeeds the sacral bone, which is
fitted between the two halves of the pelvic zone. The sacrum is formed
of five vertebrae, completely blended together. Finally, we have at
the end a small rudimentary caudal column, the coccyx. This consists
of a varying number (usually four, more rarely three, or five or six)
of small degenerated vertebrae, and is a useless rudimentary organ
with no actual physiological significance. Morphologically, however,
it is of great interest as an irrefragable proof of the descent of man
and the anthropoids from long-tailed apes. On no other theory can we
explain the existence of this rudimentary tail. In the earlier stages
of development the tail of the human embryo protrudes considerably. It
afterwards atrophies; but the relic of the atrophied caudal vertebrae
and of the rudimentary muscles that once moved it remains permanently.
Sometimes, in fact, the external tail is preserved. The older
anatomists say that the tail is usually one vertebra longer in the
human female than in the male (or four against five); Steinbach says
it is the reverse.

(FIGURE 2.329. Three dorsal vertebrae, from a human embryo, eight
weeks old, in lateral longitudinal section. v cartilaginous vertebral
body, li inter-vertebral disks, ch chorda. (From Kolliker.)

(FIGURE 2.330. A dorsal vertebra of the same embryo, in lateral
transverse section. cv cartilaginous vertebral body, ch chorda, pr
transverse process, a vertebral arch (upper arch), c upper end of the
rib (lower arch). (From Kolliker.))

In the human vertebral column there are usually thirty-three
vertebrae. It is interesting to find, however, that the number often
changes, one or two vertebrae dropping out or an additional one
appearing. Often, also, a mobile rib is formed at the last cervical or
the first lumbar vertebra, so that there are then thirteen dorsal
vertebrae, besides six cervical and four lumbar. In this way the
contiguous vertebrae of the various sections of the column may take
each other's places.

In order to understand the embryology of the human vertebral column we
must first carefully consider the shape and connection of the
vertebrae. Each vertebra has, in general, the shape of a seal-ring
(Figures 1.164 to 1.166). The thicker portion, which is turned towards
the ventral side, is called the body of the vertebra, and forms a
short osseous disk; the thinner part forms a semi-circular arch, the
vertebral arch, and is turned towards the back. The arches of the
successive vertebrae are connected by thin intercrural ligaments in
such a way that the cavity they collectively enclose represents a long
canal. In this vertebral canal we find the trunk part of the central
nervous system, the spinal cord. Its head part, the brain, is enclosed
by the skull, and the skull itself is merely the uppermost part of the
vertebral column, distinctively modified. The base or ventral side of
the vesicular cranial capsule corresponds originally to a number of
developed vertebral bodies; its vault or dorsal side to their combined
upper vertebral arches.

(FIGURE 2.331. Intervertebral disk of a new-born infant, transverse
section. a rest of the chorda. (From Kolliker.))

While the solid, massive bodies of the vertebrae represent the real
central axis of the skeleton, the dorsal arches serve to protect the
central marrow they enclose. But similar arches develop on the ventral
side for the protection of the viscera in the breast and belly. These
lower or ventral vertebral arches, proceeding from the ventral side of
the vertebral bodies, form, in many of the lower Vertebrates, a canal
in which the large blood-vessels are enclosed on the lower surface of
the vertebral column (aorta and caudal vein). In the higher
Vertebrates the majority of these vertebral arches are lost or become
rudimentary. But at the thoracic section of the column they develop
into independent strong osseous arches, the ribs (costae). In reality
the ribs are merely large and independent lower vertebral arches,
which have lost their original connection with the vertebral bodies.

If we turn from this anatomic survey of the composition of the column
to the question of its development, I may refer the reader to earlier
pages with regard to the first and most important points (Chapter
1.14). It will be remembered that in the human embryo and that of the
other vertebrates we find at first, instead of the segmented column,
only a simple unarticulated cartilaginous rod. This solid but flexible
and elastic rod is the axial rod (or the chorda dorsalis). In the
lowest Vertebrate, the Amphioxus, it retains this simple form
throughout life, and permanently represents the whole internal
skeleton (Figure 2.210 i). In the Tunicates, also, the nearest
Invertebrate relatives of the Vertebrates, we meet the same
chorda--transitorily in the passing larva tail of the Ascidia,
permanently in the Copelata (Figure 2.225 c). Undoubtedly both the
Tunicates and Acrania have inherited the chorda from a common
unsegmented stem-form; and these ancient, long-extinct ancestors of
all the chordonia are our hypothetical Prochordonia.

Long before there is any trace of the skull, limbs, etc., in the
embryo of man or any of the higher Vertebrates--at the early stage in
which the whole body is merely a sole-shaped embryonic shield--there
appears in the middle line of the shield, directly under the medullary
furrow, the simple chorda. (Cf. Figures 1.131 to 1.135 ch). It follows
the long axis of the body in the shape of a cylindrical axial rod of
elastic but firm composition, equally pointed at both ends. In every
case the chorda originates from the dorsal wall of the primitive gut;
the cells that compose it (Figure 2.328 b) belong to the entoderm
(Figures 2.216 to 2.221). At an early stage the chorda develops a
transparent structureless sheath, which is secreted from its cells
(Figure 2.328 a). This chordalemma is often called the "inner
chorda-sheath," and must not be confused with the real external
sheath, the mesoblastic perichorda.

(FIGURE 2.332. Human skull.

FIGURE 2.333. Skull of a new-born child. (From Kollmann.) Above, in
the three bones of the roof of the skull, we see the lines that
radiate from the central points of ossification; in front, the frontal
bone; behind, the occipital bone; between the two the large parietal
bone, p. s the scurf bone, w mastoid fontanelle, f petrous bone, t
tympanic bone, l lateral part, b bulla, j cheek-bone, a large wing of
cuneiform bone, k fontanelle of cuneiform bone.)

But this unsegmented primary axial skeleton is soon replaced by the
segmented secondary axial skeleton, which we know as the vertebral
column. The provertebral plates (Figure 1.124 s) differentiate from
the innermost, median part of the visceral layer of the coelom-pouches
at each side of the chorda. As they grow round the chorda and enclose
it they form the skeleton plate or skeletogenetic layer--that is to
say, the skeleton-forming stratum of cells, which provides the mobile
foundation of the permanent vertebral column and skull (scleroblast).
In the head-half of the embryo the skeletal plate remains a
continuous, simple, undivided layer of tissue, and presently enlarges
into a thin-walled capsule enclosing the brain, the primordial skull.
In the trunk-half the provertebral plate divides into a number of
homogeneous, cubical, successive pieces; these are the several
primitive vertebrae. They are not numerous at first, but soon increase
as the embryo grows longer (Figures 1.153 to 1.155).

(FIGURE 2.334. Head-skeleton of a primitive fish, n nasal pit, eth
cribriform bone region, orb orbit of eye, la wall of auscultory
labyrinth, occ occipital region of primitive skull, cv vertebral
column, a fore, bc hind-lip cartilage, o primitive upper jaw
(palato-quadratum), u primitive lower jaw, II hyaloid bone, III to
VIII first to sixth branchial arches. (From Gegenbaur.)

FIGURE 2.335. Roofs of the skulls of nine Primates (Cattarrhines),
seen from above and reduced to a common size. 1 European, 2 Brazilian,
3 Pithecanthropus, 4 Gorilla, 5 Chimpanzee, 6 Orang, 7 Gibbon, 8
Tailed ape, 9 Baboon.)

In all the Craniotes the soft, indifferent cells of the mesoderm,
which originally compose the skeletal plate, are afterwards converted
for the most part into cartilaginous cells, and these secrete a firm
and elastic intercellular substance between them, and form
cartilaginous tissue. Like most of the other parts of the skeleton,
the membranous rudiments of the vertebrae soon pass into a
cartilaginous state, and in the higher Vertebrates this is afterwards
replaced by the hard osseous tissue with its characteristic stellate
cells (Figure 1.6). The primary axial skeleton remains a simple chorda
throughout life in the Acrania, the Cyclostomes, and the lowest
fishes. In most of the other Vertebrates the chorda is more or less
replaced by the cartilaginous tissue of the secondary perichorda that
grows round it. In the lower Craniotes (especially the fishes) a more
or less considerable part of the chorda is preserved in the bodies of
the vertebrae. In the mammals it disappears for the most part. By the
end of the second month in the human embryo the chorda is merely a
slender thread, running through the axis of the thick, cartilaginous
vertebral column (Figures 1.182 ch and 2.329 ch). In the cartilaginous
vertebral bodies themselves, which afterwards ossify, the slender
remnant of the chorda presently disappears (Figure 2.330 ch). But in
the elastic inter-vertebral disks, which develop from the skeletal
plate between each pair of vertebral bodies (Figure 2.329 li), a relic
of the chorda remains permanently. In the new-born child there is a
large pear-shaped cavity in each intervertebral disk, filled with a
gelatinous mass of cells (Figure 2.331 a). Though less sharply
defined, this gelatinous nucleus of the elastic cartilaginous disks
persists throughout life in the mammals, but in the birds and most
reptiles the last trace of the chorda disappears. In the subsequent
ossification of the cartilaginous vertebra the first deposit of bony
matter ("first osseous nucleus") takes place in the vertebral body
immediately round the remainder of the chorda, and soon displaces it
altogether. Then there is a special osseous nucleus formed in each
half of the vertebral arch. The ossification does not reach the point
at which the three nuclei are joined until after birth. In the first
year the two osseous halves of the arches unite; but it is much
later--in the second to the eighth year--that they connect with the
osseous vertebral bodies.

(FIGURE 2.336. Skeleton of the breast-fin of Ceratodus (biserial
feathered skeleton). A, B, cartilaginous series of the fin-stem. rr
cartilaginous fin-radii. (From Gunther.)

FIGURE 2.337. Skeleton of the breast-fin of an early Selachius
(Acanthias). The radii of the median fin-border (B) have disappeared
for the most part; a few only (R) are . R, R, radii of the lateral
fin-border, mt metapterygium, ms mesopterygium, p propterygium. (From
Gegenbaur.)

FIGURE 2.338. Skeleton of the breast-fin of a young Selachius. The
radii of the median fin-border have wholly disappeared. The shaded
part on the  is the section that persists in the five-fingered
hand of the higher Vertebrates. (b the three basal pieces of the fin:
mt metapterygium, rudiment of the humerus, ms mesopterygium, p
propterygium.) (From Gegenbaur.))

The bony skull (cranium), the head-part of the secondary axial
skeleton, develops in just the same way as the vertebral column. The
skull forms a bony envelope for the brain, just as the vertebral canal
does for the spinal cord; and as the brain is only a peculiarly
differentiated part of the head, while the spinal cord represents the
longer trunk-section of the originally homogeneous medullary tube, we
shall expect to find that the osseous coat of the one is a special
modification of the osseous envelope of the other. When we examine the
adult human skull in itself (Figure 2.332), it is difficult to
conceive how it can be merely the modified fore part of the vertebral
column. It is an elaborate and extensive bony structure, composed of
no less than twenty bones of different shapes and sizes. Seven of them
form the spacious shell that surrounds the brain, in which we
distinguish the solid ventral base below and the curved dorsal vault
above. The other thirteen bones form the facial skull, which is
especially the bony envelope of the higher sense-organs, and at the
same time encloses the entrance of the alimentary canal. The lower jaw
is articulated at the base of the skull (usually regarded as the XXI
cranial bone). Behind the lower jaw we find the hyoid bone at the root
of the tongue, also formed from the gill-arches, and a part of the
lower arches that have developed as "head-ribs" from the ventral side
of the base of the cranium.

Although the fully-developed skull of the higher Vertebrates, with its
peculiar shape, its enormous size, and its complex composition, seems
to have nothing in common with the ordinary vertebrae, nevertheless
even the older comparative anatomists came to recognise at the end of
the eighteenth century that it is really nothing else originally than
a series of modified vertebrae. When Goethe in 1790 "picked up the
skull of a slain victim from the sand of the Jewish cemetery at
Venice, he noticed at once that the bones of the face also could be
traced to vertebrae (like the three hind-most cranial vertebrae)." And
when Oken (without knowing anything of Goethe's discovery) found at
Ilenstein, "a fine bleached skull of a hind, the thought flashed
across him like lightning: 'It is a vertebral column.'"

(FIGURE 2.339. Skeleton of the fore leg of an amphibian. h upper-arm
(humerus), ru lower arm (r radius, u ulna), rcicu apostrophe,
wrist-bones of first series (r radiale, i intermedium, c centrale, u
apostrophe ulnare). 1, 2, 3, 4, 5 wrist-bones of the second series.
(From Gegenbaur.)

FIGURE 2.340. Skeleton of gorilla's hand. (From Huxley.)

FIGURE 2.341. Skeleton of human hand, back. (From Meyer.))

This famous vertebral theory of the skull has interested the most
distinguished zoologists for more than a century: the chief
representatives of comparative anatomy have devoted their highest
powers to the solution of the problem, and the interest has spread far
beyond their circle. But it was not until 1872 that it was happily
solved, after seven years' labour, by the comparative anatomist who
surpassed all other experts of this science in the second half of the
nineteenth century by the richness of his empirical knowledge and the
acuteness and depth of his philosophic speculations. Carl Gegenbaur
has shown, in his classic Studies of the Comparative Anatomy of the
Vertebrates (third section), that we find the most solid foundation
for the vertebral theory of the skull in the head-skeleton of the
Selachii. Earlier anatomists had wrongly started from the mammal
skull, and had compared the several bones that compose it with the
several parts of the vertebra (Figure 2.333) they thought they could
prove in this way that the fully-formed mammal skull was made of from
three to six vertebrae.

The older theory was refuted by simple and obvious facts, which were
first pointed out by Huxley. Nevertheless, the fundamental idea of
it--the belief that the skull is formed from the head-part of the
perichordal axial skeleton, just as the brain is from the simple
medullary tube, by differentiation and modification--remained. The
work now was to discover the proper way of supplying this philosophic
theory with an empirical foundation, and it was reserved for Gegenbaur
to achieve this. He first opened out the phylogenetic path which here,
as in all morphological questions, leads most confidently to the goal.
He showed that the primitive fishes (Figures 2.249 to 2.251), the
ancestors of all the Gnathostomes, still preserve permanently in the
form of their skull the structure out of which the transformed skull
of the higher Vertebrates, including man, has been evolved. He further
showed that the branchial arches of the Selachii prove that their
skull originally consisted of a large number of (at least nine or ten)
provertebrae, and that the cerebral nerves that proceed from the base
of the brain entirely confirm this. These cerebral nerves are (with
the exception of the first and second pair, the olfactory and optic
nerves) merely modifications of spinal nerves, and are essentially
similar to them in their peripheral expansion. The comparative anatomy
of these cerebral nerves, their origin and their expansion, furnishes
one of the strongest arguments for the new vertebral theory of the
skull.

(FIGURE 2.342. Skeleton of the hand or fore foot of six mammals. I
man, II dog, III pig, IV ox, V tapir, VI horse. r radius, u ulna, a
scaphoideum, b lunare, a triquetrum, d trapezium, e trapezoid, f
capitatum, g hamatum, p pisiforme. 1 , 2 index finger, 3 middle
finger, 4 ring finger, 5 little finger. (From Gegenbaur.))

We have not space here to go into the details of Gegenbaur's theory of
the skull. I must be content to refer the reader to the great work I
have mentioned, in which it is thoroughly established from the
empirico-philosophical point of view. He has also given a
comprehensive and up-to-date treatment of the subject in his
Comparative Anatomy of the Vertebrates (1898). Gegenbaur indicates as
original "cranial ribs," or "lower arches of the cranial vertebrae,"
at each side of the head of the Selachii (Figure 2.334), the following
pairs of arches: I and II, two lip-cartilages, the anterior (a) of
which is composed of an upper piece only, the posterior (bc) from an
upper and lower piece; III, the maxillary arches, also consisting of
two pieces on each side--the primitive upper jaw (os palato-quadratum,
o) and the primitive lower jaw (u); IV, the hyaloid bone (II);
finally, V to X, six branchial arches in the narrower sense (III to
VIII). From the anatomic features of these nine to ten cranial ribs or
"lower vertebral arches" and the cranial nerves that spread over them,
it is clear that the apparently simple cartilaginous primitive skull
of the Selachii was originally formed from so many (at least nine)
somites or provertebrae. The blending of these primitive segments into
a single capsule is, however, so ancient that, in virtue of the law of
curtailed heredity, the original division seems to have disappeared;
in the embryonic development it is very difficult to detect it in
isolated traces, and in some respects quite impossible. It is claimed
that several (three to six) traces of provertebrae have been
discovered in the anterior (pre-chordal) part of the Selachii-skull;
this would bring up the number of cranial somites to twelve or
sixteen, or even more.

(FIGURES 2.343 TO 2.345. Arm and hand of three anthropoids.

FIGURE 2.343. Chimpanzee (Anthropithecus niger).

FIGURE 2.344. Veddah of Ceylon (Homo veddalis).

FIGURE 2.345. European (Homo mediterraneus). (From Paul and Fritz
Sarasin.))

In the primitive skull of man (Figure 2.323) and the higher
Vertebrates, which has been evolved from that of the Selachii, five
consecutive sections are discoverable at a certain early period of
development, and one might be induced to trace these to five primitive
vertebrae; but these sections are due entirely to adaptation to the
five primitive cerebral vesicles, and correspond, like these, to a
large number of metamera. That we have in the primitive skull of the
mammals a greatly modified and transformed organ, and not at all a
primitive formation, is clear from the circumstance that its original
soft membranous form only assumes the cartilaginous character for the
most part at the base and the sides, and remains membranous at the
roof. At this part the bones of the subsequent osseous skull develop
as external coverings over the membranous structure, without an
intermediate cartilaginous stage, as there is at the base of the
skull. Thus a large part of the cranial bones develop originally as
covering bones from the corium, and only secondarily come into close
touch with the primitive skull (Figure 2.333). We have previously seen
how this very rudimentary beginning of the skull in man is formed
ontogenetically from the "head-plates," and thus the fore end of the
chorda is enclosed in the base of the skull. (Cf. Figs 1.145 and
Chapters 1.13 and 1.14.)

The phylogeny of the skull has made great progress during the last
three decades through the joint attainments of comparative anatomy,
ontogeny, and paleontology. By the judicious and comprehensive
application of the phylogenetic method (in the sense of Gegenbaur) we
have found the key to the great and important problems that arise from
the thorough comparative study of the skull. Another school of
research, the school of what is called "exact craniology" (in the
sense of Virchow), has, meantime, made fruitless efforts to obtain
this result. We may gratefully acknowledge all that this descriptive
school has done in the way of accurately describing the various forms
and measurements of the human skull, as compared with those of other
mammals. But the vast empirical material that it has accumulated in
its extensive literature is mere dead and sterile erudition until it
is vivified and illumined by phylogenetic speculation.

Virchow confined himself to the most careful analysis of large numbers
of human skulls and those of anthropoid mammals. He saw only the
differences between them, and sought to express these in figures.

Without adducing a single solid reason, or offering any alternative
explanation, he rejected evolution as an unproved hypothesis. He
played a most unfortunate part in the controversy as to the
significance of the fossil human skulls of Spy and Neanderthal, and
the comparison of them with the skull of the Pithecanthropus (Figure
2.283). All the interesting features of these skulls that clearly
indicated the transition from the anthropoid to the man were declared
by Virchow to be chance pathological variations. He said that the roof
of the skull of Pithecanthropus (Figure 2.335, 3) must have belonged
to an ape, because so pronounced an orbital stricture (the horizontal
constriction between the outer edge of the eye-orbit and the temples)
is not found in any human being. Immediately afterwards Nehring showed
in the skull of a Brazilian Indian (Figure 2.335, 2), found in the
Sambaquis of Santos, that this stricture can be even deeper in man
than in many of the apes. It is very instructive in this connection to
compare the roofs of the skulls (seen from above) of different
primates. I have, therefore, arranged nine such skulls in Figure
2.335, and reduced them to a common size.

(FIGURE 2.346. Transverse section of a fish's tail (from the tunny).
(From Johannes Muller.) a upper (dorsal) lateral muscles, a
apostrophe, b apostrophe lower (ventral) lateral muscles, d vertebral
bodies, b sections of incomplete conical mantle, B attachment lines of
the inter-muscular ligaments (from the side).)

We turn now to the branchial arches, which were regarded even by the
earlier natural philosophers as "head-ribs." (Cf. Figures 1.167 to
1.170). Of the four original gill-arches of the mammals the first lies
between the primitive mouth and the first gill-c. From the base of
this arch is formed the upper-jaw process, which joins with the inner
and outer nasal processes on each side, in the manner we have
previously explained, and forms the chief parts of the skeleton of the
upper jaw (palate bone, pterygoid bone, etc.) (Cf. Chapter 2.25.) The
remainder of the first branchial arch, which is now called, by way of
contrast, the "upper-jaw process," forms from its base two of the
ear-ossicles (hammer and anvil), and as to the rest is converted into
a long strip of cartilage that is known, after its discoverer, as
"Meckel's cartilage," or the promandibula. At the outer surface of the
latter is formed from the cellular matter of the corium, as covering
or accessory bone, the permanent bony lower jaw. From the first part
or base of the second branchial arch we get, in the mammals, the third
ossicle of the ear, the stirrup; and from the succeeding parts we get
(in this order) the muscle of the stirrup, the styloid process of the
temporal bone, the styloid-hyoid ligament, and the little horn of the
hyoid bone. The third branchial arch is only cartilaginous at the
foremost part, and here the body of the hyoid bone and its larger horn
are formed at each side by the junction of its two halves. The fourth
branchial arch is only found transitorily in the mammal embryo as a
rudimentary organ, and does not develop special parts; and there is no
trace in the embryo of the higher Vertebrates of the posterior
branchial arches (fifth and sixth pair), which are permanent in the
Selachii. They have been lost long ago. Moreover, the four gill-cs
of the human embryo are only interesting as rudimentary organs, and
they soon close up and disappear. The first alone (between the first
and second branchial arches) has any permanent significance; from it
are developed the tympanic cavity and the Eustachian tube. (Cf.
Figures 1.169 and 2.320.)

It was Carl Gegenbaur again who solved the difficult problem of
tracing the skeleton of the limbs of the Vertebrates to a common type.
Few parts of the vertebrate body have undergone such infinitely varied
modifications in regard to size, shape, and adaptation of structure as
the limbs or extremities; yet we are in a position to reduce them all
to the same hereditary standard. We may generally distinguish three
groups among the Vertebrates in relation to the formation of their
limbs. The lowest and earliest Vertebrates, the Acrania and
Cyclostomes, had, like their invertebrate ancestors, no pairs of
limbs, as we see in the Amphioxus and the Cyclostomes to-day (Figures
2.210 and 2.247). The second group is formed of the two classes of the
true fishes and the Dipneusts; here there are always two pairs of
limbs at first, in the shape of many-toed fins--one pair of
breast-fins or fore legs, and one pair of belly-fins or hind legs
(Figures 2.248 to 2.259). The third group comprises the four higher
classes of Vertebrates--the amphibia, reptiles, birds, and mammals; in
these quadrupeds there are at first the same two pairs of limbs, but
in the shape of five-toed feet. Frequently we find less than five
toes, and sometimes the feet are wholly atrophied (as in the
serpents). But the original stem-form of the group had five toes or
fingers before and behind (Figures 2.263 to 2.265).

The true primitive form of the pairs of limbs, such as they were found
in the primitive fishes of the Silurian period, is preserved for us in
the Australian dipneust, the remarkable Ceratodus (Figure 2.257). Both
the breast-fin and the belly-fin are flat oval paddles, in which we
find a biserial cartilaginous skeleton (Figure 2.336). This consists,
firstly, of a much segmented fin-rod or "stem" (A, B), which runs
through the fin from base to tip; and secondly of a double row of thin
articulated fin-radii (r, r), which are attached to both sides of the
fin-rod, like the feathers of a feathered leaf. This primitive fin,
which Gegenbaur first recognised, is attached to the vertebral column
by a simple zone in the shape of a cartilaginous arch. It has probably
originated from the branchial arches.* (* While Gegenbaur derives the
fins from two pairs of posterior separated branchial arches, Balfour
holds that they have been developed from segments of a pair of
originally continuous lateral fins or folds of the skin.)

We find the same biserial primitive fin more or less preserved in the
fossilised remains of the earliest Selachii (Figure 2.248), Ganoids
(Figure 2.253), and Dipneusts (Figure 2.256). It is also found in
modified form in some of the actual sharks and pikes. But in the
majority of the Selachii it has already degenerated to the extent that
the radii on one side of the fin-rod have been partly or entirely
lost, and are retained only on the other (Figure 2.337). We thus get
the uniserial fin, which has been transmitted from the Selachii to the
rest of the fishes (Figure 2.338).

(FIGURE 2.347. Human skeleton. (Cf. Figure 2.326.)

FIGURE 2.348. Skeleton of the giant gorilla. (Cf. Figure 1.209.))

Gegenbaur has shown how the five-toed leg of the Amphibia, that has
been inherited by the three classes of Amniotes, was evolved from the
uniserial fish-fin.* (* The limb of the four higher classes of
Vertebrates is now explained in the sense that the original fin-rod
passes along its outer (ulnar or fibular) side, and ends in the fifth
toe. It was formerly believed to go along the inner (radial or tibial)
side, and end in the first toe, as Figure 2.339 shows.) In the
dipneust ancestors of the Amphibia the radii gradually atrophy, and
are lost, for the most part, on the other side of the fin-rod as well
(the lighter cartilages in Figure 2.338). Only the four lowest radii
(shaded in the illustration) are preserved; and these are the four
inner toes of the foot (first to fourth). The little or fifth toe is
developed from the lower end of the fin-rod. From the middle and upper
part of the fin-rod was developed the long stem of the limb--the
important radius and ulna (Figure 2.339 r and u) and humerus (h) of
the higher Vertebrates.

In this way the five-toed foot of the Amphibia, which we first meet in
the Carboniferous Stegocephala (Figure 2.260), and which was inherited
from them by the reptiles on one side and the mammals on the other,
was formed by gradual degeneration and differentiation from the
many-toed fish-fin (Figure 2.341). The reduction of the radii to four
was accompanied by a further differentiation of the fin-rod, its
transverse segmentation into upper and lower halves, and the formation
of the zone of the limb, which is composed originally of three limbs
before and behind in the higher Vertebrates. The simple arch of the
original shoulder-zone divides on each side into an upper (dorsal)
piece, the shoulder-blade (scapula), and a lower (ventral) piece; the
anterior part of the latter forms the primitive clavicle
(procoracoideum), and the posterior part the coracoideum. In the same
way the simple arch of the pelvic zone breaks up into an upper
(dorsal) piece, the iliac-bone (os ilium), and a lower (ventral)
piece; the anterior part of the latter forms the pubic bone (os
pubis), and the posterior the ischial bone (os ischii).

There is also a complete agreement between the fore and hind limb in
the stem or shaft. The first section of the stem is supported by a
single strong bone--the humerus in the fore, the femur in the hind
limb. The second section contains two bones: in front the radius (r)
and ulna (u), behind the tibia and fibula. (Cf. the skeletons in
Figures 2.260, 2.265, 2.270, 2.278 to 2.282, and 2.348.) The
succeeding numerous small bones of the wrist (carpus) and ankle
(tarsus) are also similarly arranged in the fore and hind extremities,
and so are the five bones of the middle-hand (metacarpus) and
middle-foot (metatarsus). Finally, it is the same with the toes
themselves, which have a similar characteristic composition from a
series of bony pieces before and behind. We find a complete parallel
in all the parts of the fore leg and the hind leg.

When we thus learn from comparative anatomy that the skeleton of the
human limbs is composed of just the same bones, put together in the
same way, as the skeleton in the four higher classes of Vertebrates,
we may at once infer a common descent of them from a single stem-form.
This stem-form was the earliest amphibian that had five toes on each
foot. It is particularly the outer parts of the limbs that have been
modified by adaptation to different conditions. We need only recall
the immense variations they offer within the mammal class. We have the
slender legs of the deer and the strong springing legs of the
kangaroo, the climbing feet of the sloth and the digging feet of the
mole, the fins of the whale and the wings of the bat. It will readily
be granted that these organs of locomotion differ as much in regard to
size, shape, and special function as can be conceived. Nevertheless,
the bony skeleton is substantially the same in every case. In the
different limbs we always find the same characteristic bones in
essentially the same rigidly hereditary connection; this is as
splendid a proof of the theory of evolution as comparative anatomy can
discover in any organ of the body. It is true that the skeleton of the
limbs of the various mammals undergoes many distortions and
degenerations besides the special adaptations (Figure 2.342). Thus we
find the first finger or the  atrophied in the fore-foot (or
hand) of the dog (II). It has entirely disappeared in the pig (III)
and tapir (V). In the ruminants (such as the ox, IV) the second and
fifth toes are also atrophied, and only the third and fourth are well
developed (VI, 3). Nevertheless, all these different fore-feet, as
well as the hand of the ape (Figure 2.340) and of man (Figure 2.341),
were originally developed from a common pentadactyle stem-form. This
is proved by the rudiments of the degenerated toes, and by the
similarity of the arrangement of the wrist-bones in all the pentanomes
(Figure 2.342 a to p).

If we candidly compare the bony skeleton of the human arm and hand
with that of the nearest anthropoid apes, we find an almost perfect
identity. This is especially true of the chimpanzee. In regard to the
proportions of the various parts, the lowest living races of men (the
Veddahs of Ceylon, Figure 2.344) are midway between the chimpanzee
(Figure 2.343) and the European (Figure 2.345). More considerable are
the differences in structure and the proportions of the various parts
between the different genera of anthropoid apes (Figures 2.278 to
2.282); and still greater is the morphological distance between these
and the lowest apes (the Cynopitheca). Here, again, impartial and
thorough anatomic comparison confirms the accuracy of Huxley's
pithecometra principle (Chapter 1.15).

The complete unity of structure which is thus revealed by the
comparative anatomy of the limbs is fully confirmed by their
embryology. However different the extremities of the four-footed
Craniotes may be in their adult state, they all develop from the same
rudimentary structure. In every case the first trace of the limb in
the embryo is a very simple protuberance that grows out of the side of
the hyposoma. These simple structures develop directly into fins in
the fishes and Dipneusts by differentiation of their cells. In the
higher classes of Vertebrates each of the four takes the shape in its
further growth of a leaf with a stalk, the inner half becoming
narrower and thicker and the outer half broader and thinner. The inner
half (the stalk of the leaf) then divides into two sections--the upper
and lower parts of the limb. Afterwards four shallow indentations are
formed at the free edge of the leaf, and gradually deepen; these are
the intervals between the five toes (Figure 1.174). The toes soon make
their appearance. But at first all five toes, both of fore and hind
feet, are connected by a thin membrane like a swimming-web; they
remind us of the original shaping of the foot as a paddling fin. The
further development of the limbs from this rudimentary structure takes
place in the same way in all the Vertebrates according to the laws of
heredity.

The embryonic development of the muscles, or ACTIVE organs of
locomotion, is not less interesting than that of the skeleton, or
PASSIVE organs. But the comparative anatomy and ontogeny of the
muscular system are much more difficult and inaccessible, and
consequently have hitherto been less studied. We can therefore only
draw some general phylogenetic conclusions therefrom.

It is incontestable that the musculature of the Vertebrates has been
evolved from that of lower Invertebrates; and among these we have to
consider especially the unarticulated Vermalia. They have a simple
cutaneous muscular layer, developing from the mesoderm. This was
afterwards replaced by a pair of internal lateral muscles, that
developed from the middle wall of the coelom-pouches; we still find
the first rudiments of the muscles arising from the muscle-plate of
these in the embryos of all the Vertebrates (cf. Figures 1.124, 1.158
to 1.160, 2.222 to 2.224 mp). In the unarticulated stem-forms of the
Chordonia, which we have called the Prochordonia, the two
coelom-pouches, and therefore also the muscle-plates of their walls,
were not yet segmented. A great advance was made in the articulation
of them, as we have followed it step by step in the Amphioxus (Figures
1.124 and 1.158). This segmentation of the muscles was the momentous
historical process with which vertebration, and the development of the
vertebrate stem, began. The articulation of the skeleton came after
this segmentation of the muscular system, and the two entered into
very close correlation.

The episomites or dorsal coelom-pouches of the Acrania, Cyclostomes,
and Selachii (Figure 1.161 h) first develop from their inner or median
wall (from the cell-layer that lies directly on the skeletal plate
sk and the medullary tube nr) a strong muscle-plate (mp). By
dorsal growth (w) it also reaches the external wall of the
coelom-pouches, and proceeds from the dorsal to the ventral wall. From
these segmental muscle-plates, which are chiefly concerned in the
segmentation of the Vertebrates, proceed the lateral muscles of the
stem, as we find in the simplest form in the Amphioxus (Figure 2.210).
By the formation of a horizontal frontal septum they divide on each
side into an upper and lower series of myotomes, dorsal and ventral
lateral muscles. This is seen with typical regularity in the
transverse section of the tail of a fish (Figure 2.346). From these
earlier lateral muscles of the trunk develop the greater part of the
subsequent muscles of the trunk, and also the much later "muscular
buds" of the limbs.* (* The ontogeny of the muscles is mostly
cenogenetic. The greater part of the muscles of the head (or the
visceral muscles) belong originally to the hyposoma of the vertebrate
organism, and develop from the wall of the hyposomites or ventral
coelom-pouches. This also applies originally to the primary muscles of
the limbs, as these too belong phylogenetically to the hyposoma. (Cf.
Chapter 1.14))

</section>
<section id="14" fatherid="0" title="THE EVOLUTION OF THE ALIMENTARY SYSTEM" lv="CHAPTER 2.27." priority="0" level="1">
The chief of the vegetal organs of the human frame, to the evolution
of which we now turn our attention, is the alimentary canal. The gut
is the oldest of all the organs of the metazoic body, and it leads us
back to the earliest age of the formation of organs--to the first
section of the Laurentian period. As we have already seen, the result
of the first division of labour among the homogeneous cells of the
earliest multicellular animal body was the formation of an alimentary
cavity. The first duty and first need of every organism is
self-preservation. This is met by the functions of the nutrition and
the covering of the body. When, therefore, in the primitive globular
Blastaea the homogeneous cells began to effect a division of labour,
they had first to meet this twofold need. One half were converted into
alimentary cells and enclosed a digestive cavity, the gut. The other
half became covering cells, and formed an envelope round the
alimentary tube and the whole body. Thus arose the primary germinal
layers--the inner, alimentary, or vegetal layer, and the outer,
covering, or animal layer. (Cf. Chapter 2.19.)

When we try to construct an animal frame of the simplest conceivable
type, that has some such primitive alimentary canal and the two
primary layers constituting its wall, we inevitably come to the very
remarkable embryonic form of the gastrula, which we have found with
extraordinary persistence throughout the whole range of animals, with
the exception of the unicellulars--in the Sponges, Cnidaria, Platodes,
Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and
Vertebrates. In all these stems the gastrula recurs in the same very
simple form. It is certainly a remarkable fact that the gastrula is
found in various animals as a larva-stage in their individual
development, and that this gastrula, though much disguised by
cenogenetic modifications, has everywhere essentially the same
palingenetic structure (Figures 1.30 to 1.35). The elaborate
alimentary canal of the higher animals develops ontogenetically from
the same simple primitive gut of the gastrula.

This gastraea theory is now accepted by nearly all zoologists. It was
first supported and partly modified by Professor Ray-Lankester; he
proposed three years afterwards (in his essay on the development of
the Molluscs, 1875) to give the name of archenteron to the primitive
gut and blastoporus to the primitive mouth.

Before we follow the development of the human alimentary canal in
detail, it is necessary to say a word about the general features of
its composition in the fully-developed man. The mature alimentary
canal in man is constructed in all its main features like that of all
the higher mammals, and particularly resembles that of the
Catarrhines, the narrow-nosed apes of the Old World. The entrance into
it, the mouth, is armed with thirty-two teeth, fixed in rows in the
upper and lower jaws. As we have seen, our dentition is exactly the
same as that of the Catarrhines, and differs from that of all other
animals (Chapter 2.23). Above the mouth-cavity is the double nasal
cavity; they are separated by the palate-wall. But we saw that this
separation is not there from the first, and that originally there is a
common mouth-nasal cavity in the embryo; and this is only divided
afterwards by the hard palate into two--the nasal cavity above and
that of the mouth below (Figure 2.311).

At the back the cavity of the mouth is half closed by the vertical
curtain that we call the soft palate, in the middle of which is the
uvula. A glance into a mirror with the mouth wide open will show its
shape. The uvula is interesting because, besides man, it is only found
in the ape. At each side of the soft palate are the tonsils. Through
the curved opening that we find underneath the soft palate we
penetrate into the gullet or pharynx behind the mouth-cavity. Into
this opens on either side a narrow canal (the Eustachian tube),
through which there is direct communication with the tympanic cavity
of the ear (Figure 2.320 e). The pharynx is continued in a long,
narrow tube, the oesophagus (sr). By this the food passes into the
stomach when masticated and swallowed. Into the gullet also opens,
 above, the trachea (lr), that leads to the lungs. The entrance
to it is covered by the epiglottis, over which the food slides. The
cartilaginous epiglottis is found only in the mammals, and has
developed from the fourth branchial arch of the fishes and amphibia.
The lungs are found, in man and all the mammals, to the  and 
in the pectoral cavity, with the heart between them. At the upper end
of the trachea there is, under the epiglottis, a specially
differentiated part, strengthened by a cartilaginous skeleton, the
larynx. This important organ of human speech also develops from a part
of the alimentary canal. In front of the larynx is the thyroid gland,
which sometimes enlarges and forms goitre.

The oesophagus descends into the pectoral cavity along the vertebral
column, behind the lungs and the heart, pierces the diaphragm, and
enters the visceral cavity. The diaphragm is a membrano-muscular
partition that completely separates the thoracic from the abdominal
cavity in all the mammals (and these alone). This separation is not
found in the beginning; there is at first a common breast-belly
cavity, the coeloma or pleuro-peritoneal cavity. The diaphragm is
formed later on as a muscular horizontal partition between the
thoracic and abdominal cavities. It then completely separates the two
cavities, and is only pierced by several organs that pass from the one
to the other. One of the chief of these organs is the oesophagus.
After this has passed through the diaphragm, it expands into the
gastric sac in which digestion chiefly takes place. The stomach of the
adult man (Figure 2.349) is a long, somewhat oblique sac, expanding on
the  into a blind sac, the fundus of the stomach (b apostrophe),
but narrowing on the , and passing at the pylorus (e) into the
small intestine. At this point there is a valve, the pyloric valve
(d), between the two sections of the canal; it opens only when the
pulpy food passes from the stomach into the intestine. In man and the
higher Vertebrates the stomach itself is the chief organ of digestion,
and is especially occupied with the solution of the food; this is not
the case in many of the lower Vertebrates, which have no stomach, and
discharge its function by a part of the gut farther on. The muscular
wall of the stomach is comparatively thick; it has externally strong
muscles that accomplish the digestive movements, and internally a
large quantity of small glands, the peptic glands, which secrete the
gastric juice.

(FIGURE 2.349. Human stomach and duodenum, longitudinal section. a
cardiac (end of oesophagus), b fundus (blind sac of the  side), c
pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i
entrance of the gall-duct and the pancreatic duct. (From Meyer.)

FIGURE 2.350. Median section of the head of a hare-embryo, one-fourth
of an inch in length. (From Mihalcovics.) The deep mouth-c (hp) is
separated by the membrane of the throat (rh) from the blind cavity of
the head-gut (kd). hz heart, ch chorda, hp the point at which the
hypophysis develops from the mouth-c, vh ventricle of the
cerebrum, v3, third ventricle (intermediate brain), v4 fourth
ventricle (hind brain), ck spinal canal.)

Next to the stomach comes the longest section of the alimentary canal,
the middle gut or small intestine. Its chief function is to absorb the
peptonised fluid mass of food, or the chyle, and it is subdivided into
several sections, of which the first (next to the stomach) is called
the duodenum (Figure 2.349 fgh). It is a short, horseshoe-shaped loop
of the gut. The largest glands of the alimentary canal open into
it--the liver, the chief digestive gland, that secretes the gall, and
the pancreas, which secretes the pancreatic juice. The two glands pour
their secretions, the bile and pancreatic juice, close together into
the duodenum (i). The opening of the gall-duct is of particular
phylogenetic importance, as it is the same in all the Vertebrates, and
indicates the principal point of the hepatic or trunk-gut (Gegenbaur).
The liver, phylogenetically older than the stomach, is a large gland,
rich in blood, in the adult man, immediately under the diaphragm on
the  side, and separated by it from the lungs. The pancreas lies a
little further back and more to the . The remaining part of the
small intestine is so long that it has to coil itself in many folds in
order to find room in the narrow space of the abdominal cavity. It is
divided into the jejunum above and the ileum below. In the last
section of it is the part of the small intestine at which in the
embryo the yelk-sac opens into the gut. This long and thin intestine
then passes into the large intestine, from which it is cut off by a
special valve. Immediately behind this "Bauhin-valve" the first part
of the large intestine forms a wide, pouch-like structure, the caecum.
The atrophied end of the caecum is the famous rudimentary organ, the
vermiform appendix. The large intestine (colon) consists of three
parts--an ascending part on the , a transverse middle part, and a
descending part on the . The latter finally passes through an
S-shaped bend into the last section of the alimentary canal, the
rectum, which opens behind by the anus. Both the large and small
intestines are equipped with numbers of small glands, which secrete
mucous and other fluids.

For the greater part of its length the alimentary canal is attached to
the inner dorsal surface of the abdominal cavity, or to the lower
surface of the vertebral column. The fixing is accomplished by means
of the thin membranous plate that we call the mesentery.

Although the fully-formed alimentary canal is thus a very elaborate
organ, and although in detail it has a quantity of complex structural
features into which we cannot enter here, nevertheless the whole
complicated structure has been historically evolved from the very
simple form of the primitive gut that we find in our
gastraead-ancestors, and that every gastrula brings before us to-day.
We have already pointed out (Chapter 1.9) how the epigastrula of the
mammals (Figure 1.67) can be reduced to the original type of the
bell-gastrula, which is now preserved by the amphioxus alone (Figure
1.35). Like the latter, the human gastrula and that of all other
mammals must be regarded as the ontogenetic reproduction of the
phylogenetic form that we call the Gastraea, in which the whole body
is nothing but a double-walled gastric sac.

We already know from embryology the manner in which the gut develops
in the embryo of man and the other mammals. From the gastrula is first
formed the spherical embryonic vesicle filled with fluid
(gastrocystis, Figure 1.106). In the dorsal wall of this the
sole-shaped embryonic shield is developed, and on the under-side of
this a shallow groove appears in the middle line, the first trace of
the later, secondary alimentary tube. The gut-groove becomes deeper
and deeper, and its edges bend towards each other, and finally form a
tube.

As we have seen, this simple cylindrical gut-tube is at first
completely closed before and behind in man and in the Vertebrates
generally (Figure 1.148); the permanent openings of the alimentary
canal, the mouth and anus, are only formed later on, and from the
outer skin. A mouth-pit appears in the skin in front (Figure 2.350
hp), and this grows towards the blind fore-end of the cavity of the
head-gut (kd), and at length breaks into it. In the same way a shallow
anus-pit is formed in the skin behind, which grows deeper and deeper,
advances towards the blind hinder end of the pelvic gut, and at last
connects with it. There is at first, both before and behind, a thin
partition between the external cutaneous pit and the blind end of the
gut--the throat-membrane in front and the anus-membrane behind; these
disappear when the connection takes place.

Directly in front of the anus-opening the allantois develops from the
hind gut; this is the important embryonic structure that forms into
the placenta in the Placentals (including man). In this more advanced
form the human alimentary canal (and that of all the other mammals) is
a slightly bent, cylindrical tube, with an opening at each end, and
two appendages growing from its lower wall: the anterior one is the
umbilical vesicle or yelk-sac, and the posterior the allantois or
urinary sac (Figure 1.195).

The thin wall of this simple alimentary tube and its ventral
appendages is found, on microscopic examination, to consist of two
strata of cells. The inner stratum, lining the entire cavity, consists
of larger and darker cells, and is the gut-gland layer. The outer
stratum consists of smaller and lighter cells, and is the gut-fibre
layer. The only exception is in the cavities of the mouth and anus,
because these originate from the skin. The inner coat of the
mouth-cavity is not provided by the gut-gland layer, but by the
skin-sense layer; and its muscular substratum is provided, not by the
gut-fibre, but the skin-fibre, layer. It is the same with the wall of
the small anus-cavity.

If it is asked how these constituent layers of the primitive gut-wall
are related to the various tissues and organs that we find afterwards
in the fully-developed system, the answer is very simple. It can be
put in a single sentence. The epithelium of the gut--that is to say,
the internal soft stratum of cells that lines the cavity of the
alimentary canal and all its appendages, and is immediately occupied
with the processes of nutrition--is formed solely from the gut-gland
layer; all other tissues and organs that belong to the alimentary
canal and its appendages originate from the gut-fibre layer. From the
latter is also developed the whole of the outer envelope of the gut
and its appendages; the fibrous connective tissue and the smooth
muscles that compose its muscular layer, the cartilages that support
it (such as the cartilages of the larynx and the trachea), the
blood-vessels and lymph-vessels that absorb the nutritive fluid from
the intestines--in a word, all that there is in the alimentary system
besides the epithelium of the gut. From the same layer we also get the
whole of the mesentery, with all the organs embedded in it--the heart,
the large blood-vessels of the body, etc.

(FIGURE 2.351. Scales or cutaneous teeth of a shark (Centrophorus
calceus). A three-pointed tooth rises obliquely on each of the
quadrangular bony plates that lie in the corium. (From Gegenbaur.))

Let us now leave this original structure of the mammal gut for a
moment, in order to compare it with the alimentary canal of the lower
Vertebrates, and of those Invertebrates that we have recognised as
man's ancestors. We find, first of all, in the lowest Metazoa, the
Gastraeads, that the gut remains permanently in the very simple form
in which we find it transitorily in the palingenetic gastrula of the
other animals; it is thus in the Gastremaria (Pemmatodiscus), the
Physemaria (Prophysema), the simplest Sponges (Olynthus), the
freshwater Polyps (Hydra), and the ascula-embryos of many other
Coelenteria (Figures 2.233 to 2.238). Even in the simplest forms of
the Platodes, the Rhabdocoela (Figure 2.240), the gut is still a
simple straight tube, lined with the entoderm; but with the important
difference that in this case its single opening, the primitive mouth
(m), has formed a muscular gullet (sd) by invagination of the skin.

(FIGURE 2.352. Gut of a human embryo, one-sixth of an inch long,
magnified fifteen times. (From His. Showing: Epiglottis, Tongue,
Hypophysis, Hepatic duct, Tail, Allantoic duct, Tail-gut, Umbilical
cord, Larynx, Rudimentary lungs, Stomach, Pancreas, Bladder, Wolffian
duct, Rudimentary kidneys.))

We have the same simple form in the gut of the lowest Vermalia
(Gastrotricha, Figure 2.242, Nematodes, Sagitta, etc.). But in these a
second important opening of the gut has been formed at the opposite
end to the mouth, the anus (Figure 2.242 a).

We see a great advance in the structure of the vermalian gut in the
remarkable Balanoglossus (Figure 2.245), the sole survivor of the
Enteropneust class. Here we have the first appearance of the division
of the alimentary tube into two sections that characterises the
Chordonia. The fore half, the head-gut (cephalogaster), becomes the
organ of respiration (branchial gut, Figure 2.245 k); the hind half,
the trunk-gut (truncogaster), alone acts as digestive organ (hepatic
gut, d). The differentiation of these two parts of the gut in the
Enteropneust is just the same as in all the Tunicates and Vertebrates.

It is particularly interesting and instructive in this connection to
compare the Enteropneusts with the Ascidia and the Amphioxus (Figures
2.220 and 2.210)--the remarkable animals that form the connecting link
between the Invertebrates and the Vertebrates. In both forms the gut
is of substantially the same construction; the anterior section forms
the respiratory branchial gut, the posterior the digestive hepatic
gut. In both it develops palingenetically from the primitive gut of
the gastrula, and in both the hinder end of the medullary tube covers
the primitive mouth to such an extent that the remarkable medullary
intestinal duct is formed, the passing communication between the
neural and intestinal tubes (canalis neurentericus, Figures 1.83 and
1.85 ne). In the vicinity of the closed primitive mouth, possibly in
its place, the later anus is developed. In the same way the mouth is a
fresh formation in the Amphioxus and the Ascidia. It is the same with
the human mouth and that of the Craniotes generally. The secondary
formation of the mouth in the Chordonia is probably connected with the
development of the gill-cs which are formed in the gut-wall
immediately behind the mouth. In this way the anterior section of the
gut is converted into a respiratory organ. I have already pointed out
that this modification is distinctive of the Vertebrates and
Tunicates. The phylogenetic appearance of the gill-cs indicates
the commencement of a new epoch in the stem-history of the
Vertebrates.

In the further ontogenetic development of the alimentary canal in the
human embryo the appearance of the gill-cs is the most important
process. At a very early stage the gullet-wall joins with the external
body-wall in the head of the human embryo, and this is followed by the
formation of four cs, which lead directly into the gullet from
without, on the  and  sides of the neck, behind the mouth.
These are the gill or gullet cs, and the partitions that separate
them are the gill or gullet-arches (Figure 1.171). These are most
interesting embryonic structures. They show us that all the higher
Vertebrates reproduce in their earlier stages, in harmony with the
biogenetic law, the process that had so important a part in the rise
of the whole Chordonia-stem. This process was the differentiation of
the gut into two sections--an anterior respiratory section, the
branchial gut, that was restricted to breathing, and a posterior
digestive section, the hepatic gut. As we find this highly
characteristic differentiation of the gut into two different sections
in all the Vertebrates and all the Tunicates, we may conclude that it
was also found in their common ancestors, the Prochordonia--especially
as even the Enteropneusts have it. (Cf. Chapters 1.12, 1.14 and 2.20,
and Figures 2.210, 2.220, 2.245.) It is entirely wanting in all the
other Invertebrates.

(FIGURE 2.353. Gut of a dog-embryo (shown in Figure 1.202, from
Bischoff), seen from the ventral side, a gill-arches (four pairs), b
rudiments of pharynx and larynx, c lungs, d stomach, f liver, g walls
of the open yelk-sac (into which the middle gut opens with a wide
aperture), h rectum.

FIGURE 2.354. The same gut seen from the . a lungs, b stomach, c
liver, d yelk-sac, e rectum.)

There is at first only one pair of gill-cs in the Amphioxus, as in
the Ascidia and Enteropneusts; and the Copelata (Figure 2.225) have
only one pair throughout life. But the number presently increases in
the former. In the Craniotes, however, it decreases still further. The
Cyclostomes have six to eight pairs (Figure 2.247); some of the
Selachii six or seven pairs, most of the fishes only four or five
pairs. In the embryo of man, and the higher Vertebrates generally,
where they make an appearance at an early stage, only three or four
pairs are developed. In the fishes they remain throughout life, and
form an exit for the water taken in at the mouth (Figures 2.249 to
2.251). But they are partly lost in the amphibia, and entirely in the
higher Vertebrates. In these nothing is  but a relic of the first
gill-c. This is formed into a part of the organ of hearing; from
it are developed the external meatus, the tympanic cavity, and the
Eustachian tube. We have already considered these remarkable
structures, and need only point here to the interesting fact that our
middle and external ear is a modified inheritance from the fishes. The
branchial arches also, which separate the cs, develop into very
different parts. In the fishes they remain gill-arches, supporting the
respiratory gill-leaves. It is the same with the lowest amphibia, but
in the higher amphibia they undergo various modifications; and in the
three higher classes of Vertebrates (including man) the hyoid bone and
the ossicles of the ear develop from them. (Cf. Chapter 2.25.)

(FIGURE 2.355. Median section of the head of a Petromyzon-larva. (From
Gegenbaur,) h hypobranchial groove (above it in the gullet we see the
internal openings of the seven gill-cs), v velum, o mouth, c
heart, a auditory vesicle, n neural tube, ch chorda.)

From the first gill-arch, from the inner surface of which the muscular
tongue proceeds, we get the first structure of the maxillary
skeleton--the upper and lower jaws, which surround the mouth and
support the teeth. These important parts are wholly wanting in the two
lowest classes of Vertebrates, the Acrania and Cyclostoma. They appear
first in the earliest Selachii (Figures 2.248 to 2.251), and have been
transmitted from this stem-group of the Gnathostomes to the higher
Vertebrates. Hence the original formation of the skeleton of the mouth
can be traced to these primitive fishes, from which we have inherited
it. The teeth are developed from the skin that clothes the jaws. As
the whole mouth cavity originates from the outer integument (Figure
2.350), the teeth also must come from it. As a fact, this is found to
be the case on microscopic examination of the development and finer
structure of the teeth. The scales of the fishes, especially of the
shark type (Figure 2.351), are in the same position as their teeth in
this respect (Figure 2.252). The osseous matter of the tooth (dentine)
develops from the corium; its enamel covering is a secretion of the
epidermis that covers the corium. It is the same with the cutaneous
teeth or placoid scales of the Selachii. At first the whole of the
mouth was armed with these cutaneous teeth in the Selachii and in the
earliest amphibia. Afterwards the formation of them was restricted to
the edges of the jaws.

Hence our human teeth are, in relation to their original source,
modified fish-scales. For the same reason we must regard the salivary
glands, which open into the mouth, as epidermic glands, as they are
formed, not from the glandular layer of the gut like the rest of the
alimentary glands, but from the epidermis, from the horny plate of the
outer germinal layer. Naturally, in harmony with this evolution of the
mouth, the salivary glands belong genetically to one series with the
sudoriferous, sebaceous, and mammary glands.

Thus the human alimentary canal is as simple as the primitive gut of
the gastrula in its original structure. Later it resembles the gut of
the earliest Vermalia (Gastrotricha). It then divides into two
sections, a fore or branchial gut and a hind or hepatic gut, like the
alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus.
The formation of the jaws and the branchial arches changes it into a
real fish-gut (Selachii). But the branchial gut, the one reminiscence
of our fish-ancestors, is afterwards atrophied as such. The parts of
it that remain are converted into entirely different structures.

(FIGURE 2.356. Transverse section of the head of a Petromyzon-larva.
(From Gegenbaur.) Beneath the pharynx (d) we see the hypobranchial
groove; above it the chorda and neural tube. A, B, C stages of
constriction.)

But, although the anterior section of our alimentary canal thus
entirely loses its original character of branchial gut, it retains the
physiological character of respiratory gut. We are now astonished to
find that the permanent respiratory organ of the higher Vertebrates,
the air-breathing lung, is developed from this first part of the
alimentary canal. Our lungs, trachea, and larynx are formed from the
ventral wall of the branchial gut. The whole of the respiratory
apparatus, which occupies the greater part of the pectoral cavity in
the adult man, is at first merely a small pair of vesicles or sacs,
which grow out of the floor of the head-gut immediately behind the
gills (Figures 2.354 C, 1.147 l). These vesicles are found in all the
Vertebrates except the two lowest classes, the Acrania and
Cyclostomes. In the lower Vertebrates they do not develop into lungs,
but into a large air-filled bladder, which occupies a good deal of the
body-cavity and has a quite different purport. It serves, not for
breathing, but to effect swimming movements up and down, and so is a
sort of hydrostatic apparatus--the floating bladder of the fishes
(nectocystis, Chapter 2.21). However, the human lungs, and those of
all air-breathing Vertebrates, develop from the same simple vesicular
appendage of the head-gut that becomes the floating bladder in the
fishes.

At first this bladder has no respiratory function, but merely acts as
hydrostatic apparatus for the purpose of increasing or lessening the
specific gravity of the body. The fishes, which have a fully-developed
floating bladder, can press it together, and thus condense the air it
contains. The air also escapes sometimes from the alimentary canal,
through an air-duct that connects the floating bladder with the
pharynx, and is ejected by the mouth. This lessens the size of the
bladder, and so the fish becomes heavier and sinks. When it wishes to
rise again, the bladder is expanded by relaxing the pressure. In many
of the Crossopterygii the wall of the bladder is covered with bony
plates, as in the Triassic Undina (Figure 2.254).

This hydrostatic apparatus begins in the Dipneusts to change into a
respiratory organ; the blood-vessels in the wall of the bladder now no
longer merely secrete air themselves, but also take in fresh air
through the air-duct. This process reaches its full development in the
Amphibia. In these the floating bladder has turned into lungs, and the
air-passage into a trachea. The lungs of the Amphibia have been
transmitted to the three higher classes of Vertebrates. In the lowest
Amphibia the lungs on either side are still very simple transparent
sacs with thin walls, as in the common water-salamander, the Triton.
It still entirely resembles the floating bladder of the fishes. It is
true that the Amphibia have two lungs,  and . But the
floating bladder is also double in many of the fishes (such as the
early Ganoids), and divides into  and  halves. On the other
hand, the lung is single in Ceratodus (Figure 2.257).

(FIGURE 2.357. Thoracic and abdominal viscera of a human embryo of
twelve weeks, natural size, (From Kolliker.) The head is omitted.
Ventral and pectoral walls are removed. The greater part of the
body-cavity is taken up with the liver, from the middle part of which
the caecum and the vermiform appendix protrude. Above the diaphragm,
in the middle, is the conical heart; to the  and  of it are
the two small lungs.)

In the human embryo and that of all the other Amniotes the lungs
develop from the hind part of the ventral wall of the head-gut (Figure
1.149). Immediately behind the single structure of the thyroid gland a
median groove, the rudiment of the trachea, is detached from the
gullet. From its hinder end a couple of vesicles develop--the simple
tubular rudiments of the  and  lungs. They afterwards
increase considerably in size, fill the greater part of the thoracic
cavity, and take the heart between them. Even in the frogs we find
that the simple sac has developed into a spongy body of peculiar
froth-like tissue. The originally short connection of the pulmonary
sacs with the head-gut extends into a long, thin tube. This is the
wind-pipe (trachea); it opens into the gullet above, and divides below
into two branches which go to the two lungs. In the wall of the
trachea circular cartilages develop, and these keep it open. At its
upper end, underneath its pharyngeal opening, the larynx is
formed--the organ of voice and speech. The larynx is found at various
stages of development in the Amphibia, and comparative anatomists are
in a position to trace the progressive growth of this important organ
from the rudimentary structure of the lower Amphibia up to the
elaborate and delicate vocal apparatus that we have in the larynx of
man and of the birds.

We must refer here to an interesting rudimentary organ of the
respiratory gut, the thyroid gland, the large gland in front of the
larynx, that lies below the "Adam's apple," and is often especially
developed in the male sex. It has a certain function--not yet fully
understood--in the nutrition of the body, and arises in the embryo by
constriction from the lower wall of the pharynx. In many mining
districts the thyroid gland is peculiarly liable to morbid
enlargement, and then forms goitre, a growth that hangs at the front
of the neck. But it is much more interesting phylogenetically. As
Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last
relic of the hypobranchial groove, which we considered in a previous
chapter, and which runs in the middle line of the gill-crate in the
Ascidia and Amphioxus, and conveys food to the stomach. (Cf. Chapter
2.16, Figure 2.246). We still find it in its original character in the
larvae of the Cyclostomes (Figures 2.355 and 2.356).

The second section of the alimentary canal, the trunk or hepatic gut,
undergoes not less important modifications among our vertebrate
ancestors than the first section. In tracing the further development
of this digestive part of the gut, we find that most complex and
elaborate organs originate from a very rudimentary original structure.
For clearness we may divide the digestive gut into three sections: the
fore gut (with oesophagus and stomach), the middle gut (duodenum, with
liver, pancreas, jejunum, and ileum, and the hind gut (colon and
rectum). Here again we find vesicular growths or appendages of the
originally simple gut developing into a variety of organs. Two of
these embryonic structures, the yelk-sac and allantois, are already
known to us. The two large glands that open into the duodenum, the
liver and pancreas, are growths from the middle and most important
part of the trunk-gut.

Immediately behind the vesicular rudiments of the lungs comes the
section of the alimentary canal that forms the stomach (Figures 2.353
d and 2.354 b). This sac-shaped organ, which is chiefly responsible
for the solution and digestion of the food, has not in the lower
Vertebrates the great physiological importance and the complex
character that it has in the higher. In the Acrania and Cyclostomes
and the earlier fishes we can scarcely distinguish a real stomach; it
is represented merely by the short piece from the branchial to the
hepatic gut. In some of the other fishes also the stomach is only a
very simple spindle-shaped enlargement at the beginning of the
digestive section of the gut, running straight from front to back in
the median plane of the body, underneath the vertebral column. In the
mammals its first structure is just as rudimentary as it is
permanently in the preceding. But its various parts soon begin to
develop. As the  side of the spindle-shaped sac grows much more
quickly than the , and as it turns considerably on its axis at
the same time, it soon comes to lie obliquely. The upper end is more
to the , and the lower end more to the . The foremost end
draws up into the longer and narrower canal of the oesophagus.
Underneath this on the  the blind sac (fundus) of the stomach
bulges out, and thus the later form gradually develops (Figures 2.349
and 1.184 e). The original longitudinal axis becomes oblique, sinking
below to the  and rising to the , and approaches nearer and
nearer to a transverse position. In the outer layer of the
stomach-wall the powerful muscles that accomplish the digestive
movements develop from the gut-fibre layer. In the inner layer a
number of small glandular tubes are formed from the gut-gland layer;
these are the peptic glands that secrete the gastric juice. At the
lower end of the gastric sac is developed the valve that separates it
from the duodenum (the pylorus, Figure 2.349 d).

Underneath the stomach there now develops the disproportionately long
stretch of the small intestine. The development of this section is
very simple, and consists essentially in an extremely rapid and
considerable growth lengthways. It is at first very short, quite
straight, and simple. But immediately behind the stomach we find at an
early stage a horseshoe-shaped bend and loop of the gut, in connection
with the severance of the alimentary canal from the yelk-sac and the
development of the first mesentery. The thin delicate membrane that
fastens this loop to the ventral side of the vertebral column, and
fills the inner bend of the horseshoe formation, is the first rudiment
of the mesentery (Figure 1.147 g). We find at an early stage a
considerable growth of the small intestine; it is thus forced to coil
itself in a number of loops. The various sections that we have to
distinguish in it are differentiated in a very simple way--the
duodenum (next to the stomach), the succeeding long jejunum, and the
last section of the small intestine, the ileum.

From the duodenum are developed the two large glands that we have
already mentioned--the liver and pancreas. The liver appears first in
the shape of two small sacs, that are found to the  and 
immediately behind the stomach (Figures 2.353 f, and 2.354 c). In many
of the lower Vertebrates they remain separate for a long time (in the
Myxinoides throughout life), or are only imperfectly joined. In the
higher Vertebrates they soon blend more or less completely to form a
single large organ. The growth of the liver is very brisk at first. In
the human embryo it grows so much in the second month of development
that in the third it occupies by far the greater part of the
body-cavity (Figure 2.357). At first the two halves develop equally;
afterwards the  falls far behind the . In consequence of the
unsymmetrical development and turning of the stomach and other
abdominal viscera, the whole liver is now pushed to the  side.
Although the liver does not afterwards grow so disproportionately, it
is comparatively larger in the embryo at the end of pregnancy than in
the adult. Its weight relatively to that of the whole body is 1 : 36
in the adult, and 1 : 18 in the embryo. Hence it is very important
physiologically during embryonic life; it is chiefly concerned in the
formation of blood, not so much in the secretion of bile.

Immediately behind the liver a second large visceral gland develops
from the duodenum, the pancreas or sweetbread. It is wanting in most
of the lowest classes of Vertebrates, and is first found in the
fishes. This organ is also an outgrowth from the gut.

The last section of the alimentary canal, the large intestine, is at
first in the embryo a very simple, short, and straight tube, which
opens behind by the anus. It remains thus throughout life in the lower
Vertebrates. But it grows considerably in the mammals, coils into
various folds, and divides into two sections, the first and longer of
which is the colon, and the second the rectum. At the beginning of the
colon there is a valve (valvula Bauhini) that separates it from the
small intestine. Immediately behind this there is a sac-like growth,
which enlarges into the caecum (Figure 2.357 v). In the plant-eating
mammals this is very large, but it is very small or completely
atrophied in the flesh-eaters. In man, and most of the apes, only the
first portion of the caecum is wide; the blind end-part of it is very
narrow, and seems later to be merely a useless appendage of the
former. This "vermiform appendage" is very interesting as a
rudimentary organ. The only significance of it in man is that not
infrequently a cherry-stone or some other hard and indigestible matter
penetrates into its narrow cavity, and by setting up inflammation and
suppuration causes the death of otherwise sound men. Teleology has
great difficulty in giving a rational explanation of, and attributing
to a beneficent Providence, this dreaded appendicitis. In our
plant-eating ancestors this rudimentary organ was much larger and had
a useful function.

Finally, we have important appendages of the alimentary tube in the
bladder and urethra, which belong to the alimentary system. These
urinary organs, acting as reservoir and duct for the urine excreted by
the kidneys, originate from the innermost part of the allantoic
pedicle. In the Dipneusts and Amphibia, in which the allantoic sac
first makes its appearance, it remains within the body-cavity, and
functions entirely as bladder. But in all the Amniotes it grows far
outside of the body-cavity of the embryo, and forms the large
embryonic "primitive bladder," from which the placenta develops in the
higher mammals. This is lost at birth. But the long stalk or pedicle
of the allantois remains, and forms with its upper part the middle
vesico-umbilical ligament, a rudimentary organ that goes in the shape
of a solid string from the vertex of the bladder to the navel. The
lowest part of the allantoic pedicle (or the "urachus") remains
hollow, and forms the bladder. At first this opens into the last
section of the gut in man as in the lower Vertebrates; thus there is a
real cloaca, which takes off both urine and excrements. But among the
mammals this cloaca is only permanent in the Monotremes, as it is in
all the birds, reptiles, and amphibia. In all the other mammals
(marsupials and placentals) a transverse partition is afterwards
formed, and this separates the urogenital aperture in front from the
anus-opening behind. (Cf. Chapters 2.22 and 2.29.)

</section>
<section id="15" fatherid="0" title="EVOLUTION OF THE VASCULAR SYSTEM" lv="CHAPTER 2.28." priority="0" level="1">
The use that we have hitherto made of our biogenetic law will give the
reader an idea how far we may trust its guidance in phylogenetic
investigation. This differs considerably in the various systems of
organs; the reason is that heredity and variability have a very
different range in these systems. While some of them faithfully
preserve the original palingenetic development inherited from earlier
animal ancestors, others show little trace of this rigid heredity;
they are rather disposed to follow new and divergent CENOGENETIC lines
of development in consequence of adaptation. The organs of the first
kind represent the CONSERVATIVE element in the multicellular state of
the human frame, while the latter represent the PROGRESSIVE element.
The course of historic development is a result of the correlation of
the two tendencies, and they must be carefully distinguished.

There is perhaps no other system of organs in the human body in which
this is more necessary than in that of which we are now going to
consider the obscure development--the vascular system, or apparatus of
circulation. If we were to draw our conclusions as to the original
features in our earlier animal ancestors solely from the phenomena of
the development of this system in the embryo of man and the other
higher Vertebrates, we should be wholly misled. By a number of
important embryonic adaptations, the chief of which is the formation
of an extensive food-yelk, the original course of the development of
the vascular system has been so much falsified and curtailed in the
higher Vertebrates that little or nothing now remains in their
embryology of some of the principal phylogenetic features. We should
be quite unable to explain these if comparative anatomy and ontogeny
did not come to our assistance.

The vascular system in man and all the Craniotes is an elaborate
apparatus of cavities filled with juices or cell-containing fluids.
These "vessels" (vascula) play an important part in the nutrition of
the body. They partly conduct the nutritive red blood to the various
parts of the body (blood-vessels); partly absorb from the gut the
white chyle formed in digestion (chyle-vessels); and partly collect
the used-up juices and convey them away from the tissues (lymphatic
vessels). With the latter are connected the large cavities of the
body, especially the body-cavity, or coeloma. The lymphatic vessels
conduct both the colourless lymph and the white chyle into the venous
part of the circulation. The lymphatic glands act as producers of new
blood-cells, and with them is associated the spleen. The centre of
movement for the circulation of the fluids is the heart, a strong
muscular sac, which contracts regularly and is equipped with valves
like a pump. This constant and steady circulation of the blood makes
possible the complex metabolism of the higher animals.

But, however important the vascular system may be to the more advanced
and larger and highly-differentiated animals, it is not at all so
indispensable an element of animal life as is commonly supposed. The
older science of medicine regarded the blood as the real source of
life. Even in the still prevalent confused notions of heredity the
blood plays the chief part. People speak generally of full blood, half
blood, etc., and imagine that the hereditary transmission of certain
characters "lies in the blood." The incorrectness of these ideas is
clearly seen from the fact that in the act of generation the blood of
the parents is not directly transmitted to the offspring, nor does the
embryo possess blood in its early stages. We have already seen that
not only the differentiation of the four secondary germinal layers,
but also the first structures of the principal organs in the embryo of
all the Vertebrates, take place long before there is any trace of the
vascular system--the heart and the blood. In accordance with this
ontogenetic fact, we must regard the vascular system as one of the
latest organs from the phylogenetic point of view; just as we have
found the alimentary canal to be one of the earliest. In any case, the
vascular system is much later than the alimentary.

(FIGURE 2.358. Red blood-cells of various Vertebrates (equally
magnified). 1. of man, 2. camel, 3. dove, 4. proteus, 5.
water-salamander (Triton), 6. frog, 7. merlin (Cobitis), 8. lamprey
(Petromyzon). a surface-view, b edge-view. (From Wagner.)

FIGURE 2.359. Vascular tissues or endothelium (vasalium). A capillary
from the mesentery. a vascular cells, b their nuclei.)

The important nutritive fluid that circulates as blood and lymph in
the elaborate canals of our vascular system is not a clear, simple
fluid, but a very complex chemical juice with millions of cells
floating in it. These blood-cells are just as important in the
complicated life of the higher animal body as the circulation of money
is to the commerce of a civilised community. Just as the citizens meet
their needs most conveniently by means of a financial circulation, so
the various tissue-cells, the microscopic citizens of the
multicellular human body, have their food conveyed to them best by the
circulating cells in the blood. These blood cells (haemocytes) are of
two kinds in man and all the other Craniotes--red cells (rhodocytes or
erythrocytes) and colourless or lymph cells (leucocytes). The red
colour of the blood is caused by the great accumulation of the former,
the others circulate among them in much smaller quantity. When the
colourless cells increase at the expense of the red we get anaemia (or
chlorosis).

(FIGURE 2.360. Transverse section of the trunk of a chick-embryo,
forty-five hours old. (From Balfour.) A ectoderm (horny-plate), Mc
medullary tube, ch chorda, C entoderm (gut-gland layer), Pv primitive
segment (episomite), Wd prorenal duct, pp coeloma (secondary
body-cavity). So skin-fibre layer, Sp gut-fibre layer, v blood-vessels
in latter, ao primitive aortas, containing red blood-cells.)

The lymph-cells (leucocytes), commonly called the "white corpuscles"
of the blood, are phylogenetically older and more widely distributed
in the animal world than the red. The great majority of the
Invertebrates that have acquired an independent vascular system have
only colourless lymph-cells in the circulating fluid. There is an
exception in the Nemertines (Figure 2.358) and some groups of
Annelids. When we examine the colourless blood of a cray-fish or a
snail (Figure 2.358) under a high power of the microscope, we find in
each drop numbers of mobile leucocytes, which behave just like
independent Amoebae (Figure 1.17). Like these unicellular Protozoa,
the colourless blood-cells creep slowly about, their unshapely
plasma-body constantly changing its form, and stretching out
finger-like processes first in one direction, then another. Like the
Amoebae, they take particles into their cell-body. On account of this
feature these amoeboid plastids are called "eating cells"
(phagocytes), and on account of their motions "travelling cells"
(planocytes). It has been shown by the discoveries of the last few
decades that these leucocytes are of the greatest physiological and
pathological consequence to the organism. They can absorb either solid
or dissolved particles from the wall of the gut, and convey them to
the blood in the chyle; they can absorb and remove unusable matter
from the tissues. When they pass in large quantities through the fine
pores of the capillaries and accumulate at irritated spots, they cause
inflammation. They can consume and destroy bacteria, the dreaded
vehicles of infectious diseases; but they can also transport these
injurious Monera to fresh regions, and so extend the sphere of
infection. It is probable that the sensitive and travelling leucocytes
of our invertebrate ancestors have powerfully co-operated for millions
of years in the phylogenesis of the advancing animal organisation.

The red blood-cells have a much more restricted sphere of distribution
and activity. But they also are very important in connection with
certain functions of the craniote-organism, especially the exchange of
gases or respiration. The cells of the dark red, carbonised or venous,
blood, which have absorbed carbonic acid from the animal tissues, give
this off in the respiratory organs; they receive instead of it fresh
oxygen, and thus bring about the b red colour that distinguishes
oxydised or arterial blood. The red colouring matter of the blood
(haemoglobin) is regularly distributed in the pores of their
protoplasm. The red cells of most of the Vertebrates are elliptical
flat disks, and enclose a nucleus of the same shape; they differ a
good deal in size (Figure 2.358). The mammals are distinguished from
the other Vertebrates by the circular form of their biconcave red
cells and by the absence of a nucleus (Figure 1.1); only a few genera
still have the elliptic form inherited from the reptiles (Figure 1.2).
In the embryos of the mammals the red cells have a nucleus and the
power of increasing by cleavage (Figure 1.10).

The origin of the blood-cells and vessels in the embryo, and their
relation to the germinal layers and tissues, are among the most
difficult problems of ontogeny--those obscure questions on which the
most divergent opinions are still advanced by the most competent
scientists. In general, it is certain that the greater part of the
cells that compose the vessels and their contents come from the
mesoderm--in fact, from the gut-fibre layer; it was on this account
that Baer gave the name of "vascular layer" to this visceral layer of
the coeloma. But other important observers say that a part of these
cells come from other germinal layers, especially from the gut-gland
layer. It seems to be true that blood-cells may be formed from the
cells of the entoderm before the development of the mesoderm. If we
examine sections of chickens, the earliest and most familiar subjects
of embryology, we find at an early stage the "primitive-aortas" we
have already described (Figure 2.360 ao) in the ventral angle between
the episoma (Pv) and hyposoma (Sp). The thin wall of these first
vessels of the amniote embryo consists of flat cells (endothelia or
vascular epithelia); the fluid within already contains numbers of red
blood-cells; both have been developed from the gut-fibre layer. It is
the same with the vessels of the germinative area (Figure 2.361 v),
which lie on the entodermic membrane of the yelk-sac (c). These
features are seen still more clearly in the transverse section of the
duck-embryo in Figure 1.152. In this we see clearly how a number of
stellate cells proceed from the "vascular layer" and spread in all
directions in the "primary body-cavity"--i.e. in the spaces between
the germinal layers. A part of these travelling cells come together
and line the wall of the larger spaces, and thus form the first
vessels; others enter into the cavity, live in the fluid that fills
it, and multiply by cleavage--the first blood-cells.

But, besides these mesodermic cells of the "vascular layer" proper,
other travelling cells, of which the origin and purport are still
obscure, take part in the formation of blood in the meroblastic
Vertebrates (especially fishes). The chief of these are those that
Ruckert has most aptly denominated "merocytes." These "eating
yelk-cells" are found in large numbers in the food-yelk of the
Selachii, especially in the yelk-wall--the border zone of the germinal
disk in which the embryonic vascular net is first developed. The
nuclei of the merocytes become ten times as large as the ordinary
cell-nucleus, and are distinguished by their strong capacity for
taking colour, or their special richness in chromatin. Their
protoplasmic body resembles the stellate cells of osseous tissue
(astrocytes), and behaves just like a rhizopod (such as Gromia); it
sends out numbers of stellate processes all round, which ramify and
stretch into the surrounding food-yelk. These variable and very mobile
processes, the pseudopodia of the merocytes, serve both for locomotion
and for getting food; as in the real rhizopods, they surround the
solid particles of food (granules and plates of yelk), and accumulate
round their nucleus the food they have received and digested. Hence we
may regard them both as eating-cells (phagocytes) and travelling-cells
(planocytes). Their lively nucleus divides quickly and often
repeatedly, so that a number of new nuclei are formed in a short time;
as each fresh nucleus surrounds itself with a mantle of protoplasm, it
provides a new cell for the construction of the embryo. Their origin
is still much disputed.

(FIGURE 2.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells
underneath the embryonic cavity (B). (From Ruckert.) z two embryonic
cells, k nuclei of the merocytes, which wander about in the yelk and
eat small yelk-plates (d), k smaller, more superficial, lighter
nuclei, k apostrophe a deeper nucleus, in the act of cleavage, k
asterisk chromatin-filled border-nucleus, freed from the surrounding
yelk in order to show the numerous pseudopodia of the protoplasmic
cell-body.)

Half of the twelve stems of the animal world have no blood-vessels.
They make their first appearance in the Vermalia. Their earliest
source is the primary body-cavity, the simple space between the two
primary germinal layers, which is either a relic of the
segmentation-cavity, or is a subsequent formation. Amoeboid
planocytes, which migrate from the entoderm and reach this
fluid-filled primary cavity, live and multiply there, and form the
first colourless blood-cells. We find the vascular system in this very
simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other
lower Vermalia.

The first step in the improvement of this primitive vascular system is
the formation of larger canals or blood-conducting tubes. The spaces
filled with blood, the relics of the primary body-cavity, receive a
special wall. "Blood-vessels" of this kind (in the narrower sense) are
found among the higher worms in various forms, sometimes very simple,
at other times very complex. The form that was probably the incipient
structure of the elaborate vascular system of the Vertebrates (and of
the Articulates) is found in two primordial principal vessels--a
dorsal vessel in the middle line of the dorsal wall of the gut, and a
ventral vessel that runs from front to rear in the middle line of its
ventral wall. From the dorsal vessel is evolved the aorta (or
principal artery), from the ventral vessel the principal or
subintestinal vein. The two vessels are connected in front and behind
by a loop that runs round the gut. The blood contained in the two
tubes is propelled by their peristaltic contractions.

(FIGURE 2.362. Vascular system of an Annelid (Saenuris), foremost
section. d dorsal vessel, v ventral vessel, c transverse connection of
two (enlarged in shape of heart). The arrows indicate the direction of
the flow of blood. (From Gegenbaur.)

The earliest Vermalia in which we first find this independent vascular
system are the Nemertina (Figure 2.244). As a rule, they have three
parallel longitudinal vessels connected by loops, a single dorsal
vessel above the gut and a pair of lateral vessels to the  and
. In some of the Nemertina the blood is already coloured, and the
red colouring matter is real haemoglobin, connected with elliptical
discoid cells, as in the Vertebrates. The further evolution of this
rudimentary vascular system can be gathered from the class of the
Annelids in which we find it at various stages of development. First,
a number of transverse connections are formed between the dorsal and
ventral vessels, which pass round the gut ring-wise (Figure 2.362).
Other vessels grow into the body-wall and ramify in order to convey
blood to it. In addition to the two large vessels of the middle plane
there are often two lateral vessels, one to the  and one to the
; as, for instance, in the leech. There are four of these parallel
longitudinal vessels in the Enteropneusts (Balanoglossus, Figure
2.245). In these important Vermalia the foremost section of the gut
has already been converted into a gill-crate, and the vascular arches
that rise in the wall of this from the ventral to the dorsal vessel
have become branchial vessels.

We have a further important advance in the Tunicates, which we have
recognised as the nearest blood-relatives of our early vertebrate
ancestors. Here we find for the first time a real heart--i.e. a
central organ of circulation, driving the blood into the vessels by
the regular contractions of its muscular wall, it is of a very
rudimentary character, a spindle-shaped tube, passing at both ends
into a principal vessel (Figure 2.221). By its original position
behind the gill-crate, on ventral side of the Tunicates (sometimes
more, sometimes less, forward), the head shows clearly that it has
been formed by the local enlargement of a section of the ventral
vessel. We have already noticed the remarkable alternation of the
direction of the blood stream, the heart driving it first from one
end, then from the other (Chapter 2.16). This is very instructive,
because in most of the worms (even the Enteropneust) the blood in the
dorsal vessel travels from back to front, but in the Vertebrates in
the opposite direction. As the Ascidia-heart alternates steadily from
one direction to the other, it shows us permanently, in a sense, the
phylogenetic transition from the earlier forward direction of the
dorsal current (in the worms) to the new backward direction (in the
Vertebrates).

(FIGURE 2.363. Head of a fish-embryo, with rudimentary vascular
system, from the . dc Cuvier's duct (juncture of the anterior and
posterior principal veins), sv venous sinus (enlarged end of Cuvier's
duct), a auricle, v ventricle, abr trunk of branchial artery, s
gill-cs (arterial arches between), ad aorta, c carotid artery, n
nasal pit. (From Gegenbaur.)

FIGURE 2.364. The five arterial arches of the Craniotes (1 to 5) in
their original disposition, a arterial cone or bulb, a double
apostrophe aorta-trunk, c carotid artery (foremost continuation of the
roots of the aorta). (From Rathke.)

FIGURE 2.365. The five arterial arches of the birds; the lighter parts
of the structure disappear; only the shaded parts remain. Letters as
in Figure 2.364. s subclavian arteries, p pulmonary artery, p
apostrophe branches of same, c apostrophe outer carotid, c double
apostrophe inner carotid. (From Rathke.)

FIGURE 2.366. The five arterial arches of mammals; letters as in
Figure 2.365. v vertebral artery, b Botall's duct (open in the embryo,
closed afterwards). (From Rathke.))

As the new direction became permanent in the earlier Prochordonia,
which gave rise to the Vertebrate stem, the two vessels that proceed
from either end of the tubular heart acquired a fixed function. The
foremost section of the ventral vessel henceforth always conveys blood
from the heart, and so acts as an artery; the hind section of the same
vessel brings the blood from the body to the heart, and so becomes a
vein. In view of their relation to the two sections of the gut, we may
call the latter the intestinal vein and the former the branchial
artery. The blood contained in both vessels, and also in the heart, is
venous or carbonised blood--i.e. rich in carbonic acid; on the other
hand, the blood that passes from the gills into the dorsal vessel is
provided with fresh oxygen--arterial or oxydised blood. The finest
branches of the arteries and veins pass into each other in the tissues
by means of a network of very fine, ventral, hair-like vessels, or
capillaries (Figure 2.359).

When we turn from the Tunicates to the closely-related Amphioxus we
are astonished at first to find an apparent retrogression in the
formation of the vascular system. As we have seen, the Amphioxus has
no real heart; its colourless blood is driven along in its vascular
system by the principal vessel itself, which contracts regularly in
its whole length (cf. Figure 2.210). A dorsal vessel that lies above
the gut (aorta) receives the arterial blood from the gills and drives
it into the body. Returning from here, the venous blood gathers in a
ventral vessel under the gut (intestinal vein), and goes back to the
gills. A number of branchial vascular arches, which effect respiration
and rise in the wall of the branchial gut from belly to back, absorb
oxygen from the water and give off carbonic acid; they connect the
ventral with the dorsal vessel. As the same section of the ventral
vessel, which also forms the heart in the Craniotes, has developed in
the Ascidia into a simple tubular heart, we may regard the absence of
this in the Amphioxus as a result of degeneration, a return in this
case to the earlier form of the vascular system, as we find it in many
of the worms. We may assume that the Acrania that really belong to our
ancestral series did not share this retrogression, but inherited the
one-chambered heart of the Prochordonia, and transmitted it directly
to the earliest Craniotes (cf. the ideal Primitive Vertebrate,
Prospondylus, Figures 1.98 to 1.102).

(FIGURES 2.367 TO 2.370. Metamorphosis of the five arterial arches in
the human embryo (diagram from Rathke). la arterial cone, 1, 2, 3, 4,
5 first to fifth pair of arteries, ad trunk of aorta, aw roots of
aorta. In Figure 2.367 only three, in Figure 2.368 all five, of the
aortic arches are given (the dotted ones only are developed). In
Figure 2.369 the first two pairs have disappeared again. In Figure
2.370 the permanent trunks of the artery are shown; the dotted parts
disappear, s subclavian artery, v vertebral, ax axillary, c carotid (c
apostrophe outer, c double apostrophe inner carotid), p pulmonary.)

The further phylogenetic evolution of the vascular system is revealed
to us by the comparative anatomy of the Craniotes. At the lowest stage
of this group, in the Cyclostomes, we find for the first time the
differentiation of the vasorium into two sections: a system of
blood-vessels proper, which convey the RED blood about the body, and a
system of lymphatic vessels, which absorb the colourless lymph from
the tissues and convey it to the blood. The lymphatics that absorb
from the gut and pour into the blood-stream the milky food-fluid
formed by digestion are distinguished by the special name of
"chyle-vessels." While the chyle is white on account of its high
proportion of fatty particles, the lymph proper is colourless. Both
chyle and lymph contain the colourless amoeboid cells (leucocytes,
Figure 1.12) that we also find distributed in the blood as colourless
blood-cells (or "white corpuscles"); but the blood also contains a
much larger quantity of red cells, and these give its characteristic
colour to the blood of the Craniotes (rhodocytes, Figure 2.358). The
distinction between lymph, chyle, and blood-vessels which is found in
all the Craniotes may be regarded as an outcome of division of labour
between various sections of our originally simple vascular system. In
the Gnathostomes the spleen makes its first appearance, an organ rich
in blood, the chief function of which is the extensive formation of
new colourless and red cells. It is not found in the Acrania and
Cyclostomes, or any of the Invertebrates. It has been transmitted from
the earliest fishes to all the Craniotes.

The heart also, the central organ of circulation in all the Craniotes,
shows an advance in structure in the Cyclostomes. The simple,
spindle-shaped heart-tube, found in the same form in the embryo of all
the Craniotes, is divided into two sections or chambers in the
Cyclostomes, and these are separated by a pair of valves. The hind
section, the auricle, receives the venous blood from the body and
passes it on to the anterior section, the ventricle. From this it is
driven through the trunk of the branchial artery (the foremost section
of the ventral vessel or principal vein) into the gills.

In the Selachii an arterial cone is developed from the foremost end of
the ventricle, as a special division, cut off by valves. It passes
into the enlarged base of the trunk of the branchial artery (Figure
2.363 abr). On each side 5 to 7 arteries proceed from it. These rise
between the gill-cs (s) on the gill-arches, surround the gullet,
and unite above into a common trunk-aorta, the continuation of which
over the gut corresponds to the dorsal vessel of the worms. As the
curved arteries on the gill-arches spread into a network of
respiratory capillaries, they contain venous blood in their lower part
(as arches of the branchial artery) and arterial blood in the upper
part (as arches of the aorta). The junctures of the various aortic
arches on the  and  are called the roots of the aorta. Of an
originally large number of aortic arches there remain at first six,
then (owing to degeneration of the fifth arch) only five, pairs; and
from these five pairs (Figure 2.364) the chief parts of the arterial
system develop in all the higher Vertebrates.

(FIGURE 2.371. Heart of a rabbit-embryo, from behind, a vitelline
veins, b auricles of the heart, c atrium, d ventricle, e arterial
bulb, f base of the three pairs of arterial arches. (From Bischoff.)

FIGURE 2.372. Heart of the same embryo (Figure 2.371), from the front.
v vitelline veins, a auricle, ca auricular canal, l  ventricle, r
 ventricle, ta arterial bulb. (From Bischoff.))

The appearance of the lungs and the atmospheric respiration connected
therewith, which we first meet in the Dipneusts, is the next important
step in vascular evolution. In the Dipneusts the auricle of the heart
is divided by an incomplete partition into two halves. Only the 
auricle now receives the venous blood from the veins of the body. The
 auricle receives the arterial blood from the pulmonary veins. The
two auricles have a common opening into the simple ventricle, where
the two kinds of blood mix, and are driven through the arterial cone
or bulb into the arterial arches. From the last arterial arches the
pulmonary arteries arise (Figure 2.365 p). These force a part of the
mixed blood into the lungs, the other part of it going through the
aorta into the body.

From the Dipneusts upwards we now trace a progressive development of
the vascular system, which ends finally with the loss of branchial
respiration and a complete separation of the two halves of the
circulation. In the Amphibia the partition between the two auricles is
complete. In their earlier stages, as tadpoles (Figure 2.262), they
have still the branchial respiration and the circulation of the
fishes, and their heart contains venous blood alone. Afterwards the
lungs and pulmonary vessels are developed, and henceforth the
ventricle of the heart contains mixed blood. In the reptiles the
ventricle and its arterial cone begin to divide into two halves by a
longitudinal partition, and this partition becomes complete in the
higher reptiles and birds on the one hand, and the stem-forms of the
mammals on the other. Henceforth, the  half of the heart contains
only venous, and the  half only arterial, blood, as we find in all
birds and mammals. The  auricle receives its carbonised or venous
blood from the veins of the body, and the  ventricle drives it
through the pulmonary arteries into the lungs. From here the blood
returns, as oxydised or arterial blood, through the pulmonary veins to
the  auricle, and is forced by the  ventricle into the
arteries of the body. Between the pulmonary arteries and veins is the
capillary system of the small or pulmonary circulation. Between the
body-arteries and veins is the capillary system of the large or
body-circulation. It is only in the two highest classes of
Vertebrates--the birds and mammals--that we find a complete division
of the circulations. Moreover, this complete separation has been
developed quite independently in the two classes, as the dissimilar
formation of the aortas shows of itself. In the birds the RIGHT half
of the fourth arterial arch has become the permanent arch (Figure
2.365). In the mammals this has been developed from the LEFT half of
the same fourth arch (Figure 2.366).

(FIGURE 2.373. Heart and head of a dog-embryo, from the front, a fore
brain, b eyes, c middle brain, d primitive lower jaw, e primitive
upper jaw, f gill-arches, g  auricle, h  auricle, i 
ventricle, k  ventricle. (From Bischoff.)

FIGURE 2.374. Heart of the same dog-embryo, from behind. a
inosculation of the vitelline veins, b  auricle, c  auricle,
d auricle, e auricular canal, f  ventricle, g  ventricle, h
arterial bulb, (From Bischoff)

FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view,
2. back view, 3. opened, and upper half of the atrium removed. a
apostrophe  auricle, a double apostrophe  auricle, v
apostrophe  ventricle, v double apostrophe  ventricle, ao
arterial bulb, c superior vena cava (cd , cs ), s rudiment of
the interventricular wall. (From Kolliker.)

FIGURE 2.376. Heart of a human embryo, six weeks old, front view. r
 ventricle, t  ventricle, s furrow between ventricles, ta
arterial bulb, af furrow on its surface; to  and  are the two
large auricles. (From Ecker.)

FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a
apostrophe  auricle, a double apostrophe  auricle, v
apostrophe  ventricle, v double apostrophe  ventricle, cd
apostrophe  superior vena cava, ci inferior vena cava. (From
Kolliker.))

If we compare the fully-developed arterial system of the various
classes of Craniotes, it shows a good deal of variety, yet it always
proceeds from the same fundamental type. Its development is just the
same in man as in the other mammals; in particular, the modification
of the six pairs of arterial arches is the same in both (Figures 2.367
to 2.370). At first there is only a single pair of arches, which lie
on the inner surface of the first pair of gill-arches. Behind this
there then develop a second and third pair of arches (lying on the
inner side of the second and third gill-arches, Figure 2.367).
Finally, we get a fourth, fifth, and sixth pair. Of the six primitive
arterial arches of the Amniotes three soon pass away (the first,
second, and fifth); of the remaining three, the third gives the
carotids, the fourth the aortas, and the sixth (number 5 in Figures
2.364 and 2.368) the pulmonary arteries.

The human heart also develops in just the same way as that of the
other mammals (Figure 2.378). We have already seen the first rudiments
of its embryology, which in the main corresponds to its phylogeny
(Figures 1.201 and 1.202). We saw that the palingenetic form of the
heart is a spindle-shaped thickening of the gut-fibre layer in the
ventral wall of the head-gut. The structure is then hollowed out,
forms a simple tube, detaches from its place of origin, and henceforth
lies freely in the cardiac cavity. Presently the tube bends into the
shape of an S, and turns spirally on an imaginary axis in such a way
that the hind part comes to lie on the dorsal surface of the fore
part. The united vitelline veins open into the posterior end. From the
anterior end spring the aortic arches.

(FIGURE 2.378. Heart of the adult man, fully developed, front view,
natural position. a  auricle (underneath it the  ventricle),
b  auricle (under it the  ventricle), C superior vena cava, V
pulmonary veins, P pulmonary artery, d Botalli's duct, A aorta. (From
Meyer.))

This first structure of the human heart, enclosing a very simple
cavity, corresponds to the tunicate-heart, and is a reproduction of
that of the Prochordonia, but it now divides into two, and
subsequently into three, compartments; this reminds us for a time of
the heart of the Cyclostomes and fishes. The spiral turning and
bending of the heart increases, and at the same time two transverse
constrictions appear, dividing it externally into three sections
(Figures 2.371 and 2.372). The foremost section, which is turned
towards the ventral side, and from which the aortic arches rise,
reproduces the arterial bulb of the Selachii. The middle section is a
simple ventricle, and the hindmost, the section turned towards the
dorsal side, into which the vitelline veins inosculate, is a simple
auricle (or atrium). The latter forms, like the simple atrium of the
fish-heart, a pair of lateral dilatations, the auricles (Figure 2.371
b); and the constriction between the atrium and ventricle is called
the auricular canal (Figure 2.372 ca). The heart of the human embryo
is now a complete fish-heart.

(FIGURE 2.379. Transverse section of the back of the head of a
chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata,
ph pharyngeal cavity (head-gut), h horny plate, h apostrophe thicker
part of it, from which the auscultory pits afterwards develop, hp
skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp
cardiac plate (the outermost mesodermic wall of the heart), connected
by the ventral mesocardium (uhg) with the gut-fibre layer or visceral
coelom-layer (dfp apostrophe), Ent entoderm, ihh inner (entodermic?)
wall of the heart; the two endothelial cardiac tubes are still
separated by the cenogenetic septum (s) of the Amniotes, g vessels.)

In perfect harmony with its phylogeny, the embryonic development of
the human heart shows a gradual transition from the fish-heart,
through the amphibian and reptile, to the mammal form, The most
important point in the transition is the formation of a longitudinal
partition--incomplete at first, but afterwards complete--which
separates all three divisions of the heart into  (venous) and
 (arterial) halves (cf. Figures 2.373 to 2.378). The atrium is
separated into a  and  half, each of which absorbs the
corresponding auricle; into the  auricle open the body-veins
(upper and lower vena cava, Figures 2.375 c and 2.377 c); the 
auricle receives the pulmonary veins. In the same way a superficial
interventricular furrow is soon seen in the ventricle (Figure 2.376
s). This is the external sign of the internal partition by which the
ventricle is divided into two--a  venous and  arterial
ventricle. Finally a longitudinal partition is formed in the third
section of the primitive fish-like heart, the arterial bulb,
externally indicated by a longitudinal furrow (Figure 2.376 af). The
cavity of the bulb is divided into two lateral halves, the
pulmonary-artery bulb, that opens into the  ventricle, and the
aorta-bulb, that opens into the  ventricle. When all the
partitions are complete, the small (pulmonary) circulation is
distinguished from the large (body) circulation; the motive centre of
the former is the  half, and that of the latter the  half, of
the heart.

The heart of all the Vertebrates belongs originally to the hyposoma of
the head, and we accordingly find it in the embryo of man and all the
other Amniotes  in front on the under-side of the head; just as
in the fishes it remains permanently in front of the gullet. It
afterwards descends into the trunk, with the advance in the
development of the neck and breast, and at last reaches the breast,
between the two lungs. At first it lies symmetrically in the middle
plane of the body, so that its long axis corresponds with that of the
body. In most of the mammals it remains permanently in this position.
But in the apes the axis begins to be oblique, and the apex of the
heart to move towards the  side. The displacement is greatest in
the anthropoid apes--chimpanzee, gorilla, and orang--which resemble
man in this.

As the heart of all Vertebrates is originally, in the light of
phylogeny, only a local enlargement of the middle principal vein, it
is in perfect accord with the biogenetic law that its first structure
in the embryo is a simple spindle-shaped tube in the ventral wall of
the head-gut. A thin membrane, standing vertically in the middle
plane, the mesocardium, connects the ventral wall of the head-gut with
the lower head-wall. As the cardiac tube extends and detaches from the
gut-wall, it divides the mesocardium into an upper (dorsal) and lower
(ventral) plate (usually called the mesocardium anterius and posterius
in man, Figure 2.379 uhg). The mesocardium divides two lateral
cavities, Remak's "neck-cavities" (Figure 2.379 hh). These cavities
afterwards join and form the simple pericardial cavity, and are
therefore called by Kolliker the "primitive pericardial cavities."

(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an
inch long in the neck, magnified forty times; "invented" by Wilhelm
His. Seen from ventral side. mb mouth-fissure, surrounded by the
branchial processes, ab bulbus of aorta, hm middle part of ventricle,
hl  lateral part of same, ho auricle, d diaphragm, vc superior
vena cava, vu umbilical vein, vo vitelline space, lb liver, lg hepatic
duct.)

The double cervical cavity of the Amniotes is very interesting, both
from the anatomical and the evolutionary point of view; it corresponds
to a part of the hyposomites of the head of the lower
Vertebrates--that part of the ventral coelom-pouches which comes next
to Van Wijhe's "visceral cavities" below. Each of the cavities still
communicates freely behind with the two coelom-pouches of the trunk;
and, just as these afterwards coalesce into a simple body-cavity (the
ventral mesentery disappearing), we find the same thing happening in
the head. This simple primary pericardial cavity has been well called
by Gegenbaur the "head-coeloma," and by Hertwig the "pericardial
breast-cavity." As it now encloses the heart, it may also be called
cardiocoel.

The cardiocoel, or head-coelom, is often disproportionately large in
the Amniotes, the simple cardiac tube growing considerably and lying
in several folds. This causes the ventral wall of the amniote embryo,
between the head and the navel, to be pushed outwards as in rupture
(cf. Figure 1.180 h). A transverse fold of the ventral wall, which
receives all the vein-trunks that open into the heart, grows up from
below between the pericardium and the stomach, and forms a transverse
partition, which is the first structure of the primary diaphragm
(Figure 2.380 d). This important muscular partition, which completely
separates the thoracic and abdominal cavities in the mammals alone, is
still very imperfect here; the two cavities still communicate for a
time by two narrow canals. These canals, which belong to the dorsal
part of the head-coelom, and which we may call briefly pleural ducts,
receive the two pulmonary sacs, which develop from the hind end of the
ventral wall of the head-gut; they thus become the two pleural
cavities.

The diaphragm makes its first appearance in the class of the Amphibia
(in the salamanders) as an insignificant muscular transverse fold of
the ventral wall, which rises from the fore end of the transverse
abdominal muscle, and grows between the pericardium and the liver. In
the reptiles (tortoises and crocodiles) a later dorsal part is joined
to this earlier ventral part of the rudimentary diaphragm, a pair of
subvertebral muscles rising from the vertebral column and being added
as "columns" to the transverse partition. But it was probably in the
Permian sauro-mammals that the two originally separate parts were
united, and the diaphragm became a complete partition between the
thoracic and abdominal cavities in the mammals; as it considerably
enlarges the chest-cavity when it contracts, it becomes an important
respiratory muscle. The ontogeny of the diaphragm in man and the other
mammals reproduces this phylogenetic process to-day, in accordance
with the biogenetic law; in all the mammals the diaphragm is formed by
the secondary conjunction of the two originally separate structures,
the earlier ventral part and the later dorsal part.

Sometimes the blending of the two diaphragmatic structures, and
consequently the severance of the one pleural duct from the abdominal
cavity, is not completed in man. This leads to a diaphragmatic rupture
(hernia diaphragmatica). The two cavities then remain in communication
by an open pleural duct, and loops of the intestine may penetrate by
this "rupture opening" into the chest-cavity. This is one of those
fatal mis-growths that show the great part that blind chance has in
organic development.

(FIGURE 2.381. Transverse section of the head of a chick-embryo,
thirty-six hours old. Underneath the medullary tube the two primitive
aortas (pa) can be seen in the head-plates (s) at each side of the
chorda. Underneath the gullet (d) we see the aorta-end of the heart
(ae), hh cervical cavity or head coelom, hk top of heart, ks
head-sheath, amniotic fold, h horny plate. (From Remak.)

(FIGURE 2.382. Transverse section of the cardiac region of the same
chick-embryo (behind the preceding). In the cervical cavity (hh) the
heart (h) is still connected by a mesocard (hg) with the gut-fibre
layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary
auditory vesicle in the horny plate, hp first rise of the amniotic
fold. (From Remak.))

Thus the thoracic cavity of the mammals, with its important contents,
the heart and lungs, belongs originally to the HEAD-PART of the
vertebrate body, and its inclusion in the trunk is secondary. This
instructive and very interesting fact is entirely proved by the
concordant evidence of comparative anatomy and ontogeny. The lungs are
outgrowths of the head-gut; the heart develops from its inner wall.
The pleural sacs that enclose the lungs are dorsal parts of the
head-coelom, originating from the pleuroducts; the pericardium in
which the heart afterwards lies is also double originally, being
formed from ventral halves of the head-coelom, which only combine at a
later stage. When the lung of the air-breathing Vertebrates issues
from the head-cavity and enters the trunk-cavity, it follows the
example of the floating bladder of the fishes, which also originates
from the pharyngeal wall in the shape of a small pouch-like
out-growth, but soon grows so large that, in order to find room, it
has to pass far behind into the trunk-cavity. To put it more
precisely, the lung of the quadrupeds retains this hereditary
growth-process of the fishes; for the hydrostatic floating bladder of
the latter is the air-filled organ from which the air-breathing organ
of the former has been evolved.

There is an interesting cenogenetic phenomenon in the formation of the
heart of the higher Vertebrates that deserves special notice. In its
earliest form the heart is DOUBLE, as recent observation has shown, in
all the Amniotes, and the simple spindle-shaped cardiac tube, which we
took as our starting-point, is only formed at a later stage, when the
two lateral tubes move backwards, touch each other, and at last
combine in the middle line. In man, as in the rabbit, the two
embryonic hearts are still far apart at the stage when there are
already eight primitive segments (Figure 1.134 h). So also the two
coelom-pouches of the head in which they lie are still separated by a
broad space. It is not until the permanent body of the embryo develops
and detaches from the embryonic vesicle that the separate lateral
structures join together, and finally combine in the middle line. As
the median partition between the  and  cardiocoel disappears,
the two cervical cavities freely communicate (Figure 2.381), and form,
on the ventral side of the amniote head, a horseshoe-shaped arch, the
points of which advance backwards into the pleuro-ducts or pleural
cavities, and from there into the two peritoneal sacs of the trunk.
But even after the conjunction of the cervical cavities (Figure 2.381)
the two cardiac tubes remain separate at first; and even after they
have united a delicate partition in the middle of the simple
endothelial tube (Figures 2.379 s and 2.382 h) indicates the original
separation. This CENOGENETIC "primary cardiac septum" presently
disappears, and has no relation to the subsequent permanent partition
between the halves of the heart, which, as a heritage from the
reptiles, has a great PALINGENETIC importance.

Thorough opponents of the biogenetic law have laid great stress on
these and similar cenogenetic phenomena, and endeavoured to urge them
as striking disproofs of the law. As in every other instance, careful,
discriminating, comparative-morphological examination converts these
supposed disproofs of evolution into strong arguments in its favour.
In his excellent work, On the structure of the Heart in the Amphibia
(1886), Carl Rabl has shown how easily these curious cenogenetic facts
can be explained by the secondary adaptation of the embryonic
structure to the great extension of the food-yelk.

The embryology of all the other parts of the vascular system also
gives us abundant and valuable data for the purposes of phylogeny. But
as one needs a thorough knowledge of the intricate structure of the
whole vascular system in man and the other Vertebrates in order to
follow this with profit, we cannot go into it further here. Moreover,
many important features in the ontogeny of the vascular system are
still very obscure and controverted. The characters of the embryonic
circulation of the Amniotes, which we have previously considered
(Chapter 1.15), are late acquisitions and entirely cenogenetic. (Cf.
Chapter 1.15 and Figures 1.198 to 1.202.)


In the Selachii also we find a longitudinal row of segmental canals on
each side, which open outwards into the primitive renal ducts
(nephrotomes, Chapter 1.14). The segmental canals (a pair in each
segment of the middle part of the body) open internally by a ciliated
funnel into the body-cavity. From the posterior group of these organs
a compact primitive kidney is formed, the anterior group taking part
in the construction of the sexual organs.

In the same simple form that remains throughout life in the Myxinoides
and partly in the Selachii we find the primitive kidney first
developing in the embryo of man and the higher Craniotes (Figures
2.386 and 2.387). Of the two parts that compose the comb-shaped
primitive kidney the longitudinal channel, or nephroduct, is always
the first to appear; afterwards the transverse "canals," the excreting
nephridia, are formed in the mesoderm; and after this again the
Malpighian capsules with their arterial coils are associated with
these as coelous outgrowths. The primitive renal duct, which appears
first, is found in all craniote embryos at the early stage in which
the differentiation of the medullary tube takes place in the ectoderm,
the severance of the chorda from the visceral layer in the entoderm,
and the first trace of the coelom-pouches arises between the limiting
layers (Figure 2.385). The nephroduct (ung) is seen on each side,
directly under the horny plate, in the shape of a long, thin,
thread-like string of cells. It presently hollows out and becomes a
canal, running straight from front to back, and clearly showing in the
transverse section of the embryo its original position in the space
between horny plate (h), primitive segments (uw), and lateral plates
(hpl). As the originally very short urinary canals lengthen and
multiply, each of the two primitive kidneys assumes the form of a
half-feathered leaf (Figure 2.387). The lines of the leaf are
represented by the urinary canals (u), and the rib by the outlying
nephroduct (w). At the inner edge of the primitive kidneys the
rudiment of the ventral sexual gland (g) can now be seen as a body of
some size. The hindermost end of the nephroduct opens  behind
into the last section of the rectum, thus making a cloaca of it.
However, this opening of the nephroducts into the intestine must be
regarded as a secondary formation. Originally they open, as the
Cyclostomes clearly show, quite independently of the gut, in the
external skin of the abdomen.

(FIGURE 2.395. Primitive kidneys and germinal glands of a human
embryo, three inches in length (beginning of the sixth week),
magnified fifteen times. k germinal gland, u primitive kidney, z
diaphragmatic ligament of same, w Wolffian duct (opened on the ),
g directing ligament (gubernaculum), a allantoic duct. (From
Kollmann.))

In the Myxinoides the primitive kidneys retain this simple comb-shaped
structure, and a part of it is preserved in the Selachii; but in all
the other Craniotes it is only found for a short time in the embryo,
as an ontogenetic reproduction of the earlier phylogenetic structure.
In these the primitive kidney soon assumes the form (by the rapid
growth, lengthening, increase, and serpentining of the urinary canals)
of a large compact gland, of a long, oval or spindle-shaped character,
which passes through the greater part of the embryonic body-cavity
(Figures 1.183 m, 1.184 m, 2.388 n). It lies near the middle line,
directly under the primitive vertebral column, and reaches from the
cardiac region to the cloaca. The  and  kidneys are parallel
to each other, quite close together, and only separated by the
mesentery--the thin narrow layer that attaches the middle gut to the
under surface of the vertebral column. The passage of each primitive
kidney, the nephroduct, runs towards the back on the lower and outer
side of the gland, and opens in the cloaca, close to the
starting-point of the allantois; it afterwards opens into the
allantois itself.

(FIGURES 2.396 TO 2.398. Urinary and sexual organs of ox-embryos.
Figure 2.396, female embryo one and a half inches long; Figure 2.397,
male embryo, one and a half inches long. Figure 2.398 female embryo
two and a half inches long. w primitive kidney, wg Wolffian duct, m
Mullerian duct, m apostrophe upper end of same (opened at t), i lower
and thicker part of same (rudiment of uterus), g genital cord, h
testicle, (h apostrophe, lower and h double apostrophe, upper
testicular ligament), o ovary, o apostrophe lower ovarian ligament, i
inguinal ligament of primitive kidney, d diaphragmatic ligament of
primitive kidney, nn accessory kidneys, n permanent kidneys, under
them the S-shaped ureters, between these the rectum, v bladder, a
umbilical artery. (From Kolliker.))

The primitive or primordial kidneys of the amniote embryo were
formerly called the "Wolffian bodies," and sometimes "Oken's bodies."
They act for a time as kidneys, absorbing unusable juices from the
embryonic body and conducting them to the cloaca--afterwards to the
allantois. There the primitive urine accumulates, and thus the
allantois acts as bladder or urinary sac in the embryos of man and the
other Amniotes. It has, however, no genetic connection with the
primitive kidneys, but is a pouch-like growth from the anterior wall
of the rectum (Figure 1.147 u). Thus it is a product of the visceral
layer, whereas the primitive kidneys are a product of the middle
layer. Phylogenetically we must suppose that the allantois originated
as a pouch-like growth from the cloaca-wall in consequence of the
expansion caused by the urine accumulated in it and excreted by the
kidneys. It is originally a blind sac of the rectum. The real bladder
of the vertebrate certainly made its first appearance among the
Dipneusts (in Lepidosiren), and has been transmitted from them to the
Amphibia, and from these to the Amniotes. In the embryo of the latter
it protrudes far out of the not yet closed ventral wall. It is true
that many of the fishes also have a "bladder." But this is merely a
local enlargement of the lower section of the nephroducts, and so
totally different in origin and composition from the real bladder. The
two structures can be compared from the physiological point of view,
and so are ANALOGOUS, as they have the same function; but not from the
morphological point of view, and are therefore not HOMOLOGOUS. The
false bladder of the fishes is a mesodermic product of the
nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes
is an entodermic blind sac of the rectum.

In all the Anamnia (the lower amnionless Craniotes, Cyclostomes,
Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower
stage of development to this extent, that the primitive kidneys
(protonephri) act permanently as urinary glands. This is only so as a
passing phase of the early embryonic life in the three higher classes
of Vertebrates, the Amniotes. In these the permanent or after or
secondary (really tertiary) kidneys (renes or metanephri) that are
distinctive of these three classes soon make their appearance. They
represent the third and last generation of the vertebrate kidneys. The
permanent kidneys do not arise (as was long supposed) as independent
glands from the alimentary tube, but from the last section of the
primitive kidneys and the nephroduct. Here a simple tube, the
secondary renal duct, develops, near the point of its entry into the
cloaca; and this tube grows considerably forward. With its blind upper
or anterior end is connected a glandular renal growth, that owes its
origin to a differentiation of the last part of the primitive kidneys.
This rudiment of the permanent kidneys consists of coiled urinary
canals with Malpighian capsules and vascular coils (without ciliated
funnels), of the same structure as the segmental mesonephridia of the
primitive kidneys. The further growth of these metanephridia gives
rise to the compact permanent kidneys, which have the familiar
bean-shape in man and most of the higher mammals, but consist of a
number of separate folds in the lower mammals, birds, and reptiles. As
the permanent kidneys grow rapidly and advance forward, their passage,
the ureter, detaches altogether from its birth-place, the posterior
end of the nephroduct; it passes to the posterior surface of the
allantois. At first in the oldest Amniotes this ureter opens into the
cloaca together with the last section of the nephroduct, but
afterwards separately from this, and finally into the permanent
bladder apart from the rectum altogether. The bladder originates from
the hindmost and lowest part of the allantoic pedicle (urachus), which
enlarges in spindle shape before the entry into the cloaca. The
anterior or upper part of the pedicle, which runs to the navel in the
ventral wall of the embryo, atrophies subsequently, and only a useless
string-like relic of it is  as a rudimentary organ; that is the
single vesico-umbilical ligament. To the  and  of it in the
adult male are a couple of other rudimentary organs, the lateral
vesico-umbilical ligaments. These are the degenerate string-like
relics of the earlier umbilical arteries.

Though in man and all the other Amniotes the primitive kidneys are
thus early replaced by the permanent kidneys, and these alone then act
as urinary organs, all the parts of the former are by no means lost.
The nephroducts become very important physiologically by being
converted into the passages of the sexual glands. In all the
Gnathostomes--or all the Vertebrates from the fishes up to man--a
second similar canal develops beside the nephroduct at an early stage
of embryonic evolution. The latter is usually called the Mullerian
duct, after its discoverer, Johannes Muller, while the former is
called the Wolffian duct. The origin of the Mullerian duct is still
obscure; comparative anatomy and ontogeny seem to indicate that it
originates by differentiation from the Wolffian duct. Perhaps it would
be best to say: "The original primary nephroduct divides by
differentiation (or longitudinal cleavage) into two secondary
nephroducts, the Wolffian and the Mullerian ducts." The latter (Figure
2.387 m) lies just on the inner side of the former (Figure 2.387 w).
Both open behind into the cloaca.

However uncertain the origin of the nephroduct and its two products,
the Mullerian and the Wolffian ducts, may be, its later development is
clear enough. In all the Gnathostomes the Wolffian duct is converted
into the spermaduct, and the Mullerian duct into the oviduct. Only one
of them is retained in each sex; the other either disappears
altogether, or only leaves relics in the shape of rudimentary organs.
In the male sex, in which the two Wolffian ducts become the
spermaducts, we often find traces of the Mullerian ducts, which I have
called "Rathke's canals" (Figure 2.394 c). In the female sex, in which
the two Mullerian ducts form the oviducts, there are relics of the
Wolffian ducts, which are called "the ducts of Gaertner."

(FIGURE 2.399. Female sexual organs of a Monotreme (Ornithorhynchus,
Figure 2.269). o ovaries, t oviducts, u womb, sug urogenital sinus; at
u apostrophe is the outlet of the two wombs, and between them the
bladder (vu). cl cloaca. (From Gegenbaur.)

FIGURES 2.400 AND 2.401. Original position of the sexual glands in the
ventral cavity of the human embryo (three months old).

FIGURE 2.400 male (natural size). h testicles, gh conducting ligament
of the testicles, wg spermaduct, h bladder, uh inferior vena cava, nn
accessory kidneys, n kidneys.

FIGURE 2.401 female, slightly magnified. r round maternal ligament
(underneath it the bladder, over it the ovaries). r apostrophe
kidneys, s accessory kidneys, c caecum, o small reticle, om large
reticle (stomach between the two), l spleen. (From Kolliker.))

We obtain the most interesting information with regard to this
remarkable evolution of the nephroducts and their association with the
sexual glands from the Amphibia (Figures 2.390 to 2.395). The first
structure of the nephroduct and its differentiation into Mullerian and
Wolffian ducts are just the same in both sexes in the Amphibia, as in
the mammal embryos (Figures 2.392 and 2.396). In the female Amphibia
the Mullerian duct develops on either side into a large oviduct
(Figure 2.393 od), while the Wolffian duct acts permanently as ureter
(u). In the male Amphibia the Mullerian duct only remains as a
rudimentary organ without any functional significance, as Rathke's
canal (Figure 2.394 c); the Wolffian duct serves also as ureter, but
at the same time as spermaduct, the sperm-canals (ve) that proceed
from the testicles (t) entering the fore part of the primitive kidneys
and combining there with the urinary canals.

In the mammals these permanent amphibian features are only seen as
brief phases of the earlier period of embryonic development (Figure
2.392). Here the primitive kidneys, which act as excretory organs of
urine throughout life in the amnion-less Vertebrates, are replaced in
the mammals by the permanent kidneys. The real primitive kidneys
disappear for the most part at an early stage of development, and only
small relics of them remain. In the male mammal the epididymis
develops from the uppermost part of the primitive kidney; in the
female a useless rudimentary organ, the epovarium, is formed from the
same part. The atrophied relic of the former is known as the
paradidymis, that of the latter as the parovarium.

(FIGURE 2.402. Urogenital system of a human embryo of three inches in
length, double natural size. h testicles, wg spermaducts, gh
conducting ligament, p processus vaginalis, b bladder, au umbilical
arteries, m mesorchium, d intestine, u ureter, n kidney, nn accessory
kidney. (From Kollman.))

The Mullerian ducts undergo very important changes in the female
mammal. The oviducts proper are developed only from their upper part;
the lower part dilates into a spindle-shaped tube with thick muscular
wall, in which the impregnated ovum develops into the embryo. This is
the womb (uterus). At first the two wombs (Figure 2.399 u) are
completely separate, and open into the cloaca on either side of the
bladder (vu), as is still the case in the lowest living mammals, the
Monotremes. But in the Marsupials a communication is opened between
the two Mullerian ducts, and in the Placentals they combine below with
the rudimentary Wolffian ducts to form a single "genital cord." The
original independence of the two wombs and the vaginal canals formed
from their lower ends are retained in many of the lower Placentals,
but in the higher they gradually blend and form a single organ. The
conjunction proceeds from below (or behind) upwards (or forwards). In
many of the Rodents (such as the rabbit and squirrel) two separate
wombs still open into the simple and single vaginal canal; but in
others, and in the Carnivora, Cetacea, and Ungulates, the lower halves
of the wombs have already fused into a single piece, though the upper
halves (or "horns") are still separate ("two-horned" womb, uteris
bicornis). In the bats and lemurs the "horns" are very short, and the
lower common part is longer. Finally, in the apes and in man the
blending of the two halves is complete, and there is only the one
simple, pear-shaped uterine pouch, into which the oviducts open on
each side. This simple uterus is a late evolutionary product, and is
found ONLY in the ape and man.

(FIGURES 2.403 TO 2.406. Origin of human ova in the female ovary.

FIGURE 2.403. Vertical section of the ovary of a new-born female
infant, a ovarian epithelium, b rudimentary string of ova, c young ova
in the epithelium, d long string of ova with follicle-formation
(Pfluger's tube), e group of young follicles, f isolated young
follicle, g blood-vessels in connective tissue (stroma) of the ovary.
In the strings the young ova are distinguished by their considerable
size from the surrounding follicle-cells. (From Waldeyer.)

FIGURE 2.404. Two young Graafian follicles, isolated. In 1 the
follicle-cells still form a simple, and in 2 a double, stratum round
the young ovum; in 2 they are beginning to form the ovolemma or the
zona pellucida (a).

FIGURES 2.405 AND 2.406. Two older Graafian follicles, in which fluid
is beginning to accumulate inside the eccentrically thickened
epithelial mass of the follicle-cells (Figure 2.405 with little, 2.406
with much, follicle-water). ei the young ovum, with embryonic vesicle
and spot, zp ovolemma or zona pellucida, dp discus proligerus, formed
of an accumulation of follicle-cells, which surround the ovum, ff
follicle-liquid (liquor folliculi), gathered inside the stratified
follicle-epithelium (fe), fk connective-tissue fibrous capsule of the
Graafian follicle (theca folliculi).)

In the male mammals there is the same fusion of the Mullerian and
Wolffian ducts at their lower ends. Here again they form a single
genital cord (Figure 2.397 g), and this opens similarly into the
original urogenital sinus, which develops from the lowest section of
the bladder (v). But while in the male mammal the Wolffian ducts
develop into the permanent spermaducts, there are only rudimentary
relics  of the Mullerian ducts. The most notable of these is the
"male womb" (uterus masculinus), which originates from the lowest
fused part of the ducts, and corresponds to the female uterus. It is a
small, flask-shaped vesicle without any physiological significance,
which opens into the ureter between the two spermaducts and the
prostate folds (vesicula prostatica).

(FIGURE 2.407. A ripe human Graafian follicle. a the mature ovum, b
the surrounding follicle-cells, c the epithelial cells of the
follicle, d the fibrous membrane of the follicle, e its outer
surface.)

The internal sexual organs of the mammals undergo very distinctive
changes of position. At first the germinal glands of both sexes lie
deep inside the ventral cavity, at the inner edge of the primitive
kidneys (Figures 2.386 g and 2.392 k), attached to the vertebral
column by a short mesentery (mesorchium in the male, mesovarium in the
female). But this primary arrangement is retained permanently only in
the Monotremes (and the lower Vertebrates). In all other mammals (both
Marsupials and Placentals) they leave their original cradle and travel
more or less far down (or behind), following the direction of a
ligament that goes from the primitive kidneys to the inguinal region
of the ventral wall. This is the inguinal ligament of the primitive
kidneys, known in the male as the Hunterian ligament (Figure 2.400
gh), and in the female as the "round maternal ligament" (Figure 2.401
r). In woman the ovaries travel more or less towards the small pelvis,
or enter into it altogether. In the male the testicles pass out of the
ventral cavity, and penetrate by the inguinal canal into a sac-shaped
fold of the outer skin. When the  and  folds ("sexual
swellings") join together they form the scrotum. The various mammals
bring before us the successive stages of this displacement. In the
elephant and the whale the testicles descend very little, and remain
underneath the kidneys. In many of the rodents and carnassia they
enter the inguinal canal. In most of the higher mammals they pass
through this into the scrotum. As a rule, the inguinal canal closes
up. When it remains open the testicles may periodically pass into the
scrotum, and withdraw into the ventral cavity again in time of rut (as
in many of the marsupials, rodents, bats, etc.).

The structure of the external sexual organs, the copulative organs
that convey the fecundating sperm from the male to the female organism
in the act of copulation, is also peculiar to the mammals. There are
no organs of this character in most of the other Vertebrates. In those
that live in water (such as the Acrania and Cyclostomes, and most of
the fishes) the ova and sperm-cells are simply ejected into the water,
where their conjunction and fertilisation are  to chance. But in
many of the fishes and amphibia, which are viviparous, there is a
direct conveyance of the male sperm into the female body; and this is
the case with all the Amniotes (reptiles, birds, and mammals). In
these the urinary and sexual organs always open originally into the
last section of the rectum, which thus forms a cloaca (Chapter 2.22).
Among the mammals this arrangement is permanent only in the
Monotremes, which take their name from it (Figure 2.399 cl). In all
the other mammals a frontal partition is developed in the cloaca (in
the human embryo about the beginning of the third month), and this
divides it into two cavities. The anterior cavity receives the
urogenital canal, and is the sole outlet of the urine and the sexual
products; the hind or anus-cavity passes the excrements only.

Even before this partition has been formed in the Marsupials and
Placentals, we see the first trace of the external sexual organs.
First a conical protuberance rises at the anterior border of the
cloaca-outlet--the sexual prominence (phallus, Figure 2.402 A, e, B,
e). At the tip it is swollen in the shape of a club ("acorn" glans).
On its under side there is a furrow, the sexual groove (sulcus
genitalis, f), and on each side of this a fold of skin, the "sexual
pad" (torus genitalis, h l). The sexual protuberance or phallus is the
chief organ of the sexual sense (Chapter 2.25); the sexual nerves
spread on it, and these are the principal organs of the specific
sexual sensation. As erectile bodies (corpora cavernosa) are developed
in the male phallus by peculiar modifications of the blood-vessels, it
becomes capable of erecting periodically on a strong accession of
blood, becoming stiff, so as to penetrate into the female vagina and
thus effect copulation. In the male the phallus becomes the penis; in
the female it becomes the much smaller clitoris; this is only found to
be very large in certain apes (Ateles). A prepuce ("foreskin") is
developed in both sexes as a protecting fold on the anterior surface
of the phallus.

(FIGURE 408. The human ovum after issuing from the Graafian follicle,
surrounded by the clinging cells of the discus proligerus (in two
radiating crowns). z ovolemma (zona pellucida, with radial porous
canals), p cytosoma (protoplasm of the cell-body, darker within,
lighter without), k nucleus of the ovum (embryonic vesicle). (From
Nagel, magnified 250 times.) (Cf. Figures 1.1 and 1.14.)

The external sexual member (phallus) is found at various stages of
development within the mammal class, both in regard to size and shape,
and the differentiation and structure of its various parts; this
applies especially to the terminal part of the phallus, the glans,
both the larger glans penis of the male and the smaller glans
clitoridis of the female. The part of the cloaca from the upper wall
of which it forms belongs to the proctodaeum, the ectodermic
invagination of the rectum (Chapter 2.27); hence its epithelial
covering can develop the same horny growths as the corneous layer of
the epidermis. Thus the glans, which is quite smooth in man and the
higher apes, is covered with spines in many of the lower apes and in
the cat, and in many of the rodents with hairs (marmot) or scales
(guinea-pig) or solid horny warts (beaver). Many of the Ungulates have
a free conical projection on the glans, and in many of the Ruminants
this "phallus-tentacle" grows into a long cone, bent hook-wise at the
base (as in the goat, antelope, gazelle, etc.). The different forms of
the phallus are connected with variations in the structure and
distribution of the sensory corpuscles--i.e. the real organs of the
sexual sense, which develop in certain papillae of the corium of the
phallus, and have been evolved from ordinary tactile corpuscles of the
corium by erotic adaptation (Chapter 2.25).

The formation of the corpora cavernosa, which cause the stiffness of
the phallus and its capability of penetrating the vagina, by certain
special structures of their spongy vascular spaces, also shows a good
deal of variety within the vertebrate stem. This stiffness is
increased in many orders of mammals (especially the carnassia and
rodents) by the ossification of a part of the fibrous body (corpus
fibrosum). This penis-bone (os priapi) is very large in the badger and
dog, and bent like a hook in the marten; it is also very large in some
of the lower apes, and protrudes far out into the glans. It is wanting
in most of the anthropoid apes; it seems to have been lost in their
case (and in man) by atrophy.

The sexual groove on the under side of the phallus receives in the
male the mouth of the urogenital canal, and is changed into a
continuation of this, becoming a closed canal by the juncture of its
parallel edges, the male urethra. In the female this only takes place
in a few cases (some of the lemurs, rodents, and moles); as a rule,
the groove remains open, and the borders of this "vestibule of the
vagina" develop into the smaller labia (nymphae). The large labia of
the female develop from the sexual pads (tori genitales), the two
parallel folds of the skin that are found on each side of the genital
groove. They join together in the male, and form the closed scrotum.
These striking differences between the two sexes cannot yet be
detected in the human embryo of the ninth week. We begin to trace them
in the tenth week of development, and they are accentuated in
proportion as the difference of the sexes develops.

Sometimes the normal juncture of the two sexual pads in the male fails
to take place, and the sexual groove may also remain open
(hypospadia). In these cases the external male genitals resemble the
female, and they are often wrongly regarded as cases of hermaphrodism.
Other malformations of various kinds are not infrequently found in the
human external sexual organs, and some of them have a great
morphological interest. The reverse of hypospadia, in which the penis
is split open below, is seen in epispadia, in which the urethra is
open above. In this case the urogenital canal opens above at the
dorsal root of the penis; in the former case down below. These and
similar obstructions interfere with a man's generative power, and thus
prejudicially affect his whole development. They clearly prove that
our history is not guided by a "kind Providence," but  to the play
of blind chance.

We must carefully distinguish the rarer cases of real hermaphrodism
from the preceding. This is only found when the essential organs of
reproduction, the genital glands of both kinds, are united in one
individual. In these cases either an ovary is developed on the 
and a testicle on the  (or vice versa); or else there are
testicles and ovaries on both sides, some more and others less
developed. As hermaphrodism was probably the original arrangement in
all the Vertebrates, and the division of the sexes only followed by
later differentiation of this, these curious cases offer no
theoretical difficulty. But they are rarely found in man and the
higher mammals. On the other hand, we constantly find the original
hermaphrodism in some of the lower Vertebrates, such as the
Myxinoides, many fishes of the perch-type (serranus), and some of the
Amphibia (ringed snake, toad). In these cases the male often has a
rudimentary ovary at the fore end of the testicle; and the female
sometimes has a rudimentary, inactive testicle. In the carp also and
some other fishes this is found occasionally. We have already seen how
traces of the earlier hemaphrodism can be traced in the passages of
the Amphibia.

Man has faithfully preserved the main features of his stem-history in
the ontogeny of his urinary and sexual organs. We can follow their
development step by step in the human embryo in the same advancing
gradation that is presented to us by the comparison of the urogenital
organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and
then (within the mammal series) in the Monotremes, Marsupials, and the
various Placentals. All the peculiarities of urogenital structure that
distinguish the mammals from the rest of the Vertebrates are found in
man; and in all special structural features he resembles the apes,
particularly the anthropoid apes. In proof of the fact that the
special features of the mammals have been inherited by man, I will, in
conclusion, point out the identical way in which the ova are formed in
the ovary. In all the mammals the mature ova are contained in special
capsules, which are known as the Graafian
follicles, after their discoverer, Roger de Graaf (1677). They were
formerly supposed to be the ova themselves; but Baer discovered the
ova within the follicles (Chapter 1.3). Each follicle (Figure 2.407)
consists of a round fibrous capsule (d), which contains fluid and is
lined with several strata of cells (c). The layer is thickened like a
knob at one point (b); this ovum-capsule encloses the ovum proper (a).
The mammal ovary is originally a very simple oval body (Figure 2.387
g), formed only of connective tissue and blood-vessels, covered with a
layer of cells, the ovarian epithelium or the female germ epithelium.
From this germ epithelium strings of cells grow out into the
connective tissue or "stroma" of the ovary (Figure 2.403 b). Some of
the cells of these strings (or Pfluger's tubes) grow larger and become
ova (primitive ova, c); but the great majority remain small, and form
a protective and nutritive stratum of cells round each ovum--the
"follicle-epithelium" (e).

The follicle-epithelium of the mammal has at first one stratum (Figure
2.404 1), but afterwards several (2). It is true that in all the other
Vertebrates the ova are enclosed in a membrane, or "follicle," that
consists of smaller cells. But it is only in the mammals that fluid
accumulates between the growing follicle-cells, and distends the
follicle into a large round capsule, on the inside wall of which the
ovum lies, at one side (Figures 2.405 and 2.406). There again, as in
the whole of his morphology, man proves indubitably his descent from
the mammals.

In the lower Vertebrates the formation of ova in the germ-epithelium
of the ovary continues throughout life; but in the higher it is
restricted to the earlier stages, or even to the period of embryonic
development. In man it seems to cease in the first year; in the second
year we find no new-formed ova or chains of ova (Pfluger's tubes).
However, the number of ova in the two ovaries is very large in the
young girl; there are calculated to be 72,000 in the sexually-mature
maiden. In the production of the ova men resemble most of the
anthropoid apes.

Generally speaking, the natural history of the human sexual organs is
one of those parts of anthropology that furnish the most convincing
proofs of the animal origin of the human race. Any man who is
acquainted with the facts and impartially weighs them will conclude
from them alone that we have been evolved from the lower Vertebrates.
The larger and the detailed structure, the action, and the
embryological development of the sexual organs are just the same in
man as in the apes. This applies equally to the male and the female,
the internal and the external organs. The differences we find in this
respect between man and the anthropoid apes are much slighter than the
differences between the various species of apes. But all the apes have
certainly a common origin, and have been evolved from a long-extinct
early-Tertiary stem-form, which we must trace to a branch of the
lemurs. If we had this unknown pithecoid stem-form before us, we
should certainly put it in the order of the true apes in the primate
system; but within this order we cannot, for the anatomic and
ontogenetic reasons we have seen, separate man from the group of the
anthropoid apes. Here again, therefore, on the ground of the
pithecometra-principle, comparative anatomy and ontogeny teach with
full confidence the descent of man from the ape.

</section>
<section id="16" fatherid="0" title="RESULTS OF ANTHROPOGENY" lv="CHAPTER 2.30." priority="0" level="1">
Now that we have traversed the wonderful region of human embryology
and are familiar with the principal parts of it, it will be well to
look back on the way we have come, and forward to the further path to
truth to which it has led us. We started from the simplest facts of
ontogeny, or the development of the individual--from observations that
we can repeat and verify by microscopic and anatomic study at any
moment. The first and most important of these facts is that every man,
like every other animal, begins his existence as a simple cell. This
round ovum has the same characteristic form and origin as the ovum of
any other mammal. From it is developed in the same manner in all the
Placentals, by repeated cleavage, a multicellular blastula. This is
converted into a gastrula, and this in turn into a blastocystis (or
embryonic vesicle). The two strata of cells that compose its wall are
the primary germinal layers, the skin-layer (ectoderm), and gut-layer
(entoderm). This two-layered embryonic form is the ontogenetic
reproduction of the extremely important phylogenetic stem-form of all
the Metazoa, which we have called the Gastraea. As the human embryo
passes through the gastrula-form like that of all the other Metazoa,
we can trace its phylogenetic origin to the Gastraea.

As we continued to follow the embryonic development of the two-layered
structure, we saw that first a third, or middle layer (mesoderm),
appears between the two primary layers; when this divides into two, we
have the four secondary germinal layers. These have just the same
composition and genetic significance in man as in all the other
Vertebrates. From the skin-sense layer are developed the epidermis,
the central nervous system, and the chief part of the sense-organs.
The skin-fibre layer forms the corium and the motor organs--the
skeleton and the muscular system. From the gut-fibre layer are
developed the vascular system, the muscular wall of the gut, and the
sexual glands. Finally, the gut-gland layer only forms the epithelium,
or the inner cellular stratum of the mucous membrane of the alimentary
canal and glands (lungs, liver, etc.).

The manner in which these different systems of organs arise from the
secondary germinal layers is essentially the same from the start in
man as in all the other Vertebrates. We saw, in studying the embryonic
development of each organ, that the human embryo follows the special
lines of differentiation and construction that are only found
otherwise in the Vertebrates. Within the limits of this vast stem we
have followed, step by step, the development both of the body as a
whole and of its various parts. This higher development follows in the
human embryo the form that is peculiar to the mammals. Finally, we saw
that, even within the limits of this class, the various phylogenetic
stages that we distinguish in a natural classification of the mammals
correspond to the ontogenetic stages that the human embryo passes
through in the course of its evolution. We were thus in a position to
determine precisely the position of man in this class, and so to
establish his relationship to the different orders of mammals.

The line of argument we followed in this explanation of the
ontogenetic facts was simply a consistent application of the
biogenetic law. In this we have throughout taken strict account of the
distinction between palingenetic and cenogenetic phenomena.
Palingenesis (or "synoptic development") alone enables us to draw
conclusions from the observed embryonic form to the stem-form
preserved by heredity. Such inference becomes more or less precarious
when there has been cenogenesis, or disturbance of development, owing
to fresh adaptations. We cannot understand embryonic development
unless we appreciate this very important distinction. Here we stand at
the very limit that separates the older and the new science or
philosophy of nature. The whole of the results of recent morphological
research compel us irresistibly to recognise the biogenetic law and
its far-reaching consequences. These are, it is true, irreconcilable
with the legends and doctrines of former days, that have been
impressed on us by religious education. But without the biogenetic
law, without the distinction between palingenesis and cenogenesis, and
without the theory of evolution on which we base it, it is quite
impossible to understand the facts of organic development; without
them we cannot cast the faintest gleam of explanation over this
marvellous field of phenomena. But when we recognise the causal
correlation of ontogeny and phylogeny expressed in this law, the
wonderful facts of embryology are susceptible of a very simple
explanation; they are found to be the necessary mechanical effects of
the evolution of the stem, determined by the laws of heredity and
adaptation. The correlative action of these laws under the universal
influence of the struggle for existence, or--as we may say in a word,
with Darwin--"natural selection," is entirely adequate to explain the
whole process of embryology in the light of phylogeny. It is the chief
merit of Darwin that he explained by his theory of selection the
correlation of the laws of heredity and adaptation that Lamarck had
recognised, and pointed out the true way to reach a causal
interpretation of evolution.

The phenomenon that it is most imperative to recognise in this
connection is the inheritance of functional variations. Jean Lamarck
was the first to appreciate its fundamental importance in 1809, and we
may therefore justly give the name of Lamarckism to the theory of
descent he based on it. Hence the radical opponents of the latter have
very properly directed their attacks chiefly against the former. One
of the most distinguished and most narrow-minded of these opponents,
Wilhelm His, affirms very positively that "characteristics acquired in
the life of the individual are not inherited."

The inheritance of acquired characters is denied, not only by thorough
opponents of evolution, but even by scientists who admit it and have
contributed a good deal to its establishment, especially Weismann,
Galton, Ray Lankester, etc. Since 1884 the chief opponent has been
August Weismann, who has rendered the greatest service in the
development of Darwin's theory of selection. In his work on The
Continuity of the Germ-plasm, and in his recent excellent Lectures on
the Theory of Descent (1902), he has with great success advanced the
opinion that "only those characters can be transmitted to subsequent
generations that were contained in rudimentary form in the embryo."
However, this germ-plasm theory, with its attempt to explain heredity,
is merely a "provisional molecular hypothesis"; it is one of those
metaphysical speculations that attribute the evolutionary phenomena
exclusively to internal causes, and regard the influence of the
environment as insignificant. Herbert Spencer, Theodor Eimer, Lester
Ward, Hering, and Zehnder have pointed out the untenable consequences
of this position. I have given my view of it in the tenth edition of
the History of Creation (pages 192 and 203). I hold, with Lamarck and
Darwin, that the hereditary transmission of acquired characters is one
of the most important phenomena in biology, and is proved by thousands
of morphological and physiological experiences. It is an indispensable
foundation of the theory of evolution.

Of the many and weighty arguments for the truth of this conception of
evolution I will for the moment merely point to the invaluable
evidence of dysteleology, the science of rudimentary organs. We cannot
insist too often or too strongly on the great morphological
significance of these remarkable organs, which are completely useless
from the physiological point of view. We find some of these useless
parts, inherited from our lower vertebrate ancestors, in every system
of organs in man and the higher Vertebrates. Thus we find at once on
the skin a scanty and rudimentary coat of hair, only fully developed
on the head, under the shoulders, and at a few other parts of the
body. The short hairs on the greater part of the body are quite
useless and devoid of physiological value; they are the last relic of
the thicker hairy coat of our simian ancestors. The sensory apparatus
presents a series of most remarkable rudimentary organs. We have seen
that the whole of the shell of the external ear, with its cartilages,
muscles, and skin, is in man a useless appendage, and has not the
physiological importance that was formerly ascribed to it. It is the
degenerate remainder of the pointed, freely moving, and more advanced
mammal ear, the muscles of which we still have, but cannot work them.
We found at the inner corner of our eye a small, curious, semi-lunar
fold that is of no use whatever to us, and is only interesting as the
last relic of the nictitating membrane, the third, inner eye-lid that
had a distinct physiological purpose in the ancient sharks, and still
has in many of the Amniotes.

The motor apparatus, in both the skeleton and muscular systems,
provides a number of interesting dysteleological arguments. I need
only recall the projecting tail of the human embryo, with its
rudimentary caudal vertebrae and muscles; this is totally useless in
man, but very interesting as the degenerate relic of the long tail of
our simian ancestors. From these we have also inherited various bony
processes and muscles, which were very useful to them in climbing
trees, but are useless to us. At various points of the skin we have
cutaneous muscles which we never use--remnants of a strongly-developed
cutaneous muscle in our lower mammal ancestors. This "panniculus
carnosus" had the function of contracting and creasing the skin to
chase away the flies, as we see every day in the horse. Another relic
in us of this large cutaneous muscle is the frontal muscle, by which
we knit our forehead and raise our eye-brows; but there is another
considerable relic of it, the large cutaneous muscle in the neck
(platysma myoides), over which we have no voluntary control.

Not only in the systems of animal organs, but also in the vegetal
apparatus, we find a number of rudimentary organs, many of which we
have already noticed. In the alimentary apparatus there are the
thymus-gland and the thyroid gland, the seat of goitre and the relic
of a ciliated groove that the Tunicates and Acrania still have in the
gill-pannier; there is also the vermiform appendix to the caecum. In
the vascular system we have a number of useless cords which represent
relics of atrophied vessels that were once active as blood-canals--the
ductus Botalli between the pulmonary artery and the aorta, the ductus
venosus Arantii between the portal vein and the vena cava, and many
others. The many rudimentary organs in the urinary and sexual
apparatus are particularly interesting. These are generally developed
in one sex and rudimentary in the other. Thus the spermaducts are
formed from the Wolffian ducts in the male, whereas in the female we
have merely rudimentary traces of them in Gaertner's canals. On the
other hand, in the female the oviducts and womb are developed from the
Mullerian ducts, while in the male only the lowest ends of them remain
as the "male womb" (vesicula prostatica). Again, the male has in his
nipples and mammary glands the rudiments of organs that are usually
active only in the female.

A careful anatomic study of the human frame would disclose to us
numbers of other rudimentary organs, and these can only be explained
on the theory of evolution. Robert Wiedersheim has collected a large
number of them in his work on The Human Frame as a Witness to its
Past. They are some of the weightiest proofs of the truth of the
mechanical conception and the strongest disproofs of the teleological
view. If, as the latter demands, man or any other organism had been
designed and fitted for his life-purposes from the start and brought
into being by a creative act, the existence of these rudimentary
organs would be an insoluble enigma; it would be impossible to
understand why the Creator had put this useless burden on his
creatures to walk a path that is in itself by no means easy. But the
theory of evolution gives the simplest possible explanation of them.
It says: The rudimentary organs are parts of the body that have fallen
into disuse in the course of centuries; they had definite functions in
our animal ancestors, but have lost their physiological significance.
On account of fresh adaptations they have become superfluous, but are
transmitted from generation to generation by heredity, and gradually
atrophy.

We have inherited not only these rudimentary parts, but all the organs
of our body, from the mammals--proximately from the apes. The human
body does not contain a single organ that has not been inherited from
the apes. In fact, with the aid of our biogenetic law we can trace the
origin of our various systems of organs much further, down to the
lowest stages of our ancestry. We can say, for instance, that we have
inherited the oldest organs of the body, the external skin and the
internal coat of the alimentary system, from the Gastraeads; the
nervous and muscular systems from the Platodes; the vascular system,
the body-cavity, and the blood from the Vermalia; the chorda and the
branchial gut from the Prochordonia; the articulation of the body from
the Acrania; the primitive skull and the higher sense-organs from the
Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot
from the Amphibia; the palate from the Reptiles; the hairy coat, the
mammary glands, and the external sexual organs from the Pro-mammals.
When we formulated "the law of the ontogenetic connection of
systematically related forms," and determined the relative age of
organs, we saw how it was possible to draw phylogenetic conclusions
from the ontogenetic succession of systems of organs.

With the aid of this important law and of comparative anatomy we were
also enabled to determine "man's place in nature," or, as we put it,
assign to man his position in the classification of the animal
kingdom. In recent zoological classification the animal world is
divided into twelve stems or phyla, and these are broadly sub-divided
into about sixty classes, and these classes into at least 300 orders.
In his whole organisation man is most certainly, in the first place, a
member of one of these stems, the vertebrate stem; secondly, a member
of one particular class in this stem, the Mammals; and thirdly, of one
particular order, the order of Primates. He has all the
characteristics that distinguish the Vertebrates from the other eleven
animal stems, the Mammals from the other sixty classes, and the
Primates from the 300 other orders of the animal kingdom. We may turn
and twist as we like, but we cannot get over this fact of anatomy and
classification. Of late years this fact has given rise to a good deal
of discussion, and especially of controversy as to the particular
anatomic relationship of man to the apes. The most curious opinions
have been advanced on this "ape-question," or "pithecoid-theory." It
is as well, therefore, to go into it once more and distinguish the
essential from the unessential. (Cf. Chapter 2.23.)

We start from the undisputed fact that man is in any case--whether we
accept or reject his special blood-relationship to the apes--a true
mammal; in fact, a placental mammal. This fundamental fact can be
proved so easily at any moment from comparative anatomy that it has
been universally admitted since the separation of the Placentals from
the lower mammals (Marsupials and Monotremes). But for every
consistent subscriber to the theory of evolution it must follow at
once that man descends from a common stem-form with all the other
Placentals, the stem-ancestor of the Placentals, just as we must admit
a common mesozoic ancestor of all the mammals. This is, however, to
settle decisively the great and burning question of man's place in
nature, whether or no we go on to admit a nearer or more distant
relationship to the apes. Whether man is or is not a member of the
ape-order (or, if you prefer, the primate-order.) in the phylogenetic
sense, in any case his direct blood-relationship to the rest of the
mammals, and especially the Placentals, is established. It is possible
that the affinities of the various orders of mammals to each other are
different from what we hypothetically assume to-day. But, in any case,
the common descent of man and all the other mammals from one stem-form
is beyond question. This long-extinct Promammal was probably evolved
from Proreptiles during the Triassic period, and must certainly be
regarded as the monotreme and oviparous ancestor of ALL the mammals.

If we hold firmly to this fundamental and most important thesis, we
shall see the "ape-question" in a very different light from that in
which it is usually regarded. Little reflection is then needed to see
that it is not nearly so important as it is said to be. The origin of
the human race from a series of mammal ancestors, and the historic
evolution of these from an earlier series of lower vertebrate
ancestors, together with all the weighty conclusions that every
thoughtful man deduces therefrom, remain untouched; so far as these
are concerned, it is immaterial whether we regard true "apes" as our
nearest ancestors or not. But as it has become the fashion to lay the
chief stress in the whole question of man's origin on the "descent
from the apes," I am compelled to return to it once more, and recall
the facts of comparative anatomy and ontogeny that give a decisive
answer to this "ape-question."

The shortest way to attain our purpose is that followed by Huxley in
1863 in his able work, which I have already often quoted, Man's Place
in Nature--the way of comparative anatomy and ontogeny. We have to
compare impartially all man's organs with the same organs in the
higher apes, and then to examine if the differences between the two
are greater than the corresponding differences between the higher and
the lower apes. The indubitable and incontestable result of this
comparative-anatomical study, conducted with the greatest care and
impartiality, was the pithecometra-principle, which we have called the
Huxleian law in honour of its formulator--namely, that the differences
in organisation between man and the most advanced apes we know are
much slighter than the corresponding differences in organisation
between the higher and lower apes. We may even give a more precise
formula to this law, by excluding the Platyrrhines or American apes as
distant relatives, and restricting the comparison to the narrower
family-circle of the Catarrhines, the apes of the Old World. Within
the limits of this small group of mammals we found the structural
differences between the lower and higher catarrhine apes--for
instance, the baboon and the gorilla--to be much greater than the
differences between the anthropoid apes and man. If we now turn to
ontogeny, and find, according to our "law of the ontogenetic
connection of systematically related forms," that the embryos of the
anthropoid apes and man retain their resemblance for a longer time
than the embryos of the highest and the lowest apes, we are forced,
whether we like it or no, to recognise our descent from the order of
apes. We can assuredly construct an approximate picture in the
imagination of the form of our early Tertiary ancestors from the
foregoing facts of comparative anatomy; however we may frame this in
detail, it will be the picture of a true ape, and a distinct
catarrhine ape. This has been shown so well by Huxley (1863) that the
recent attacks of Klaatsch, Virchow, and other anthropologists, have
completely failed (cf. Chapter 2.23). All the structural characters
that distinguish the Catarrhines from the Platyrrhines are found in
man. Hence in the genealogy of the mammals we must derive man
immediately from the catarrhine group, and locate the origin of the
human race in the Old World. Only the early root-form from which both
descended was common to them.

It is, therefore, established beyond question for all impartial
scientific inquiry that the human race comes directly from the apes of
the Old World; but, at the same time, I repeat that this is not so
important in connection with the main question of the origin of man as
is commonly supposed. Even if we entirely ignore it, all that we have
learned from the zoological facts of comparative anatomy and ontogeny
as to the placental character of man remains untouched. These prove
beyond all doubt the common descent of man and all the rest of the
mammals. Further, the main question is not in the least affected if it
is said: "It is true that man is a mammal; but he has diverged at the
very root of the class from all the other mammals, and has no closer
relationship to any living group of mammals." The affinity is more or
less close in any case, if we examine the relation of the mammal class
to the sixty other classes of the animal world. Quite certainly the
whole of the mammals, including man, have had a common origin; and it
is equally certain that their common stem-forms were gradually evolved
from a long series of lower Vertebrates.

The resistance to the theory of a descent from the apes is clearly due
in most men to feeling rather than to reason. They shrink from the
notion of such an origin just because they see in the ape organism a
caricature of man, a distorted and unattractive image of themselves,
because it hurts man's aesthetic complacency and self-ennoblement. It
is more flattering to think we have descended from some lofty and
god-like being; and so, from the earliest times, human vanity has been
pleased to believe in our origin from gods or demi-gods. The Church,
with that sophistic reversal of ideas of which it is a master, has
succeeded in representing this ridiculous piece of vanity as
"Christian humility"; and the very men who reject with horror the
notion of an animal origin, and count themselves "children of God,"
love to prate of their "humble sense of servitude." In most of the
sermons that have poured out from pulpit and altar against the
doctrine of evolution human vanity and conceit have been a conspicuous
element; and, although we have inherited this very characteristic
weakness from the apes, we must admit that we have developed it to a
higher degree, which is entirely repudiated by sound and normal
intelligence. We are greatly amused at all the childish follies that
the ridiculous pride of ancestry has maintained from the Middle Ages
to our own time; yet there is a large amount of this empty feeling in
most men. Just as most people much prefer to trace their family back
to some degenerate baron or some famous prince rather than to an
unknown peasant, so most men would rather have as parent of the race a
sinful and fallen Adam than an advancing, and vigorous ape. It is a
matter of taste, and to that extent we cannot quarrel over these
genealogical tendencies. Personally, the notion of ascent is more
congenial to me than that of descent. It seems to me a finer thing to
be the advanced offspring of a simian ancestor, that has developed
progressively from the lower mammals in the struggle for life, than
the degenerate descendant of a god-like being, made from a clod, and
fallen for his sins, and an Eve created from one of his ribs. Speaking
of the rib, I may add to what I have said about the development of the
skeleton, that the number of ribs is just the same in man and woman.
In both of them the ribs are formed from the middle germinal layer,
and are, from the phylogenetic point of view, lower or ventral
vertebral arches.

But it is said: "That is all very well, as far as the human body is
concerned; on the facts quoted it is impossible to doubt that it has
really and gradually been evolved from the long ancestral series of
the Vertebrates. But it is quite another thing as regards man's mind,
or soul; this cannot possibly have been developed from the
vertebrate-soul."* (* The English reader will recognise here the
curious position of Dr. Wallace and of the late Dr.
Mivart.--Translator.) Let us see if we cannot meet this grave
stricture from the well-known facts of comparative anatomy,
physiology, and embryology. It will be best to begin with a
comparative study of the souls of various groups of Vertebrates. Here
we find such an enormous variety of vertebrate souls that, at first
sight, it seems quite impossible to trace them all to a common
"Primitive Vertebrate." Think of the tiny Amphioxus, with no real
brain but a simple medullary tube, and its whole psychic life at the
very lowest stage among the Vertebrates. The following group of the
Cyclostomes are still very limited, though they have a brain. When we
pass on to the fishes, we find their intelligence remaining at a very
low level. We do not see any material advance in mental development
until we go on to the Amphibia and Reptiles. There is still greater
advance when we come to the Mammals, though even here the minds of the
Monotremes and of the stupid Marsupials remain at a low stage. But
when we rise from these to the Placentals we find within this one vast
group such a number of important stages of differentiation and
progress that the psychic differences between the least intelligent
(such as the sloths and armadillos) and the most intelligent
Placentals (such as the dogs and apes) are much greater than the
psychic differences between the lowest Placentals and the Marsupials
or Monotremes. Most certainly the differences are far greater than the
differences in mental power between the dog, the ape, and man. Yet all
these animals are genetically-related members of a single natural
class.

We see this to a still more astonishing extent in the comparative
psychology of another class of animals, that is especially interesting
for many reasons--the insect class. It is well known that we find in
many insects a degree of intelligence that is found in man alone among
the Vertebrates. Everybody knows of the famous communities and states
of bees and ants, and of the very remarkable social arrangements in
them, such as we find among the more advanced races of men, but among
no other group of animals. I need only mention the social organisation
and government of the monarchic bees and the republican ants, and
their division into different conditions--queen, drone-nobles,
workers, educators, soldiers, etc. One of the most remarkable
phenomena in this very interesting province is the cattle-keeping of
the ants, which rear plant-lice as milch-cows and regularly extract
their honeyed juice. Still more remarkable is the slave-holding of the
large red ants, which steal the young of the small black ants and
bring them up as slaves. It has long been known that these political
and social arrangements of the ants are due to the deliberate
cooperation of the countless citizens, and that they understand each
other. A number of recent observers, especially Fritz Muller, Sir J.
Lubbock (Lord Avebury), and August Forel, have put the astonishing
degree of intelligence of these tiny Articulates beyond question.

Now, compare with these the mental life of many of the lower,
especially the parasitic insects, as Darwin did. There is, for
instance, the cochineal insect (Coccus), which, in its adult state,
has a motionless, shield-shaped body, attached to the leaves of
plants. Its feet are atrophied. Its snout is sunk in the tissue of the
plants of which it absorbs the sap. The whole psychic life of these
inert female parasites consists in the pleasure they experience from
sucking the sap of the plant and in sexual intercourse with the males.
It is the same with the maggot-like females of the fan-fly
(Strepsitera), which spend their lives parasitically and immovably,
without wings or feet, in the abdomen of wasps. There is no question
here of higher psychic action. If we compare these sluggish parasites
with the intelligent and active ants, we must admit that the psychic
differences between them are much greater than the psychic differences
between the lowest and highest mammals, between the Monotremes,
Marsupials, and armadillos on the one hand, and the dog, ape, or man
on the other. Yet all these insects belong to the same class of
Articulates, just as all the mammals belong to one and the same class.
And just as every consistent evolutionist must admit a common
stem-form for all these insects, so he must also for all the mammals.

If we now turn from the comparative study of psychic life in different
animals to the question of the organs of this function, we receive the
answer that in all the higher animals they are always bound up with
certain groups of cells, the ganglionic cells or neurona that compose
the nervous system. All scientists without exception are agreed that
the central nervous system is the organ of psychic life in the animal,
and it is possible to prove this experimentally at any moment. When we
partially or wholly destroy the central nervous system, we extinguish
in the same proportion, partially or wholly, the "soul" or psychic
activity of the animal. We have, therefore, to examine the features of
the psychic organ in man. The reader already knows the incontestable
answer to this question. Man's psychic organ is, in structure and
origin, just the same organ as in all the other Vertebrates. It
originates in the shape of a simple medullary tube from the outer
membrane of the embryo--the skin-sense layer. The simple cerebral
vesicle that is formed by the expansion of the head-part of this
medullary tube divides by transverse constrictions into five, and
these pass through more or less the same stages of construction in the
human embryo as in the rest of the mammals. As these are undoubtedly
of a common origin, their brain and spinal cord must also have a
common origin.

Physiology teaches us further, on the ground of observation and
experiment, that the relation of the "soul" to its organ, the brain
and spinal cord, is just the same in man as in the other mammals. The
one cannot act at all without the other; it is just as much bound up
with it as muscular movement is with the muscles. It can only develop
in connection with it. If we are evolutionists at all, and grant the
causal connection of ontogenesis and phylogenesis, we are forced to
admit this thesis: The human soul or psyche, as a function of the
medullary tube, has developed along with it; and just as brain and
spinal cord now develop from the simple medullary tube in every human
individual, so the human mind or the psychic life of the whole human
race has been gradually evolved from the lower vertebrate soul. Just
as to-day the intricate structure of the brain proceeds step by step
from the same rudiment in every human individual--the same five
cerebral vesicles--as in all the other Craniotes; so the human soul
has been gradually developed in the course of millions of years from a
long series of craniote-souls. Finally, just as to-day in every human
embryo the various parts of the brain differentiate after the special
type of the ape-brain, so the human psyche has proceeded historically
from the ape-soul.

It is true that this Monistic conception is rejected with horror by
most men, and the Dualistic idea, which denies the inseparable
connection of brain and mind, and regards body and soul as two totally
different things, is still popular. But how can we reconcile this view
with the known facts of evolution? It meets with difficulties equally
great and insuperable in embryology and in phylogeny. If we suppose
with the majority of men that the soul is an independent entity, which
has nothing to do with the body originally, but merely inhabits it for
a time, and gives expression to its experiences through the brain just
as the pianist does through his instrument, we must assign a point in
human embryology at which the soul enters into the brain; and at death
again we must assign a moment at which it abandons the body. As,
further, each human individual has inherited certain personal features
from each parent, we must suppose that in the act of conception pieces
were detached from their souls and transferred to the embryo. A piece
of the paternal soul goes with-the spermatozoon, and a piece of the
mother's soul remains in the ovum. At the moment of conception, when
portions of the two nuclei of the copulating cells join together to
form the nucleus of the stem-cell, the accompanying fragments of the
immaterial souls must also be supposed to coalesce.

On this Dualistic view the phenomena of psychic development are
totally incomprehensible. Everybody knows that the new-born child has
no consciousness, no knowledge of itself and the surrounding world.
Every parent who has impartially followed the mental development of
his children will find it impossible to deny that it is a case of
biological evolutionary processes. Just as all other functions of the
body develop in connection with their organs, so the soul does in
connection with the brain. This gradual unfolding of the soul of the
child is, in fact, so wonderful and glorious a phenomenon that every
mother or father who has eyes to observe is never tired of
contemplating it. It is only our manuals of psychology that know
nothing of this development; we are almost tempted to think sometimes
that their authors can never have had children themselves. The human
soul, as described in most of our psychological works, is merely the
soul of a learned philosopher, who has read a good many books, but
knows nothing of evolution, and never even reflects that his own soul
has had a development.

When these Dualistic philosophers are consistent they must assign a
moment in the phylogeny of the human soul at which it was first
"introduced" into man's vertebrate body. Hence, at the time when the
human body was evolved from the anthropoid body of the ape (probably
in the Tertiary period), a specific human psychic element--or, as
people love to say, "a spark of divinity"--must have been suddenly
infused or breathed into the anthropoid brain, and been associated
with the ape-soul already present in it. I need not insist on the
enormous theoretical difficulties of this idea. I will only point out
that this "spark of divinity," which is supposed to distinguish the
soul of man from that of the other animals, must be itself capable of
development, and has, as a matter of fact, progressively developed in
the course of human history. As a rule, reason is taken to be this
"spark of divinity," and is supposed to be an exclusive possession of
humanity. But comparative psychology shows us that it is quite
impossible to set up this barrier between man and the brute. Either we
take the word "reason" in the wider sense, and then it is found in the
higher mammals (ape, dog, elephant, horse) just as well as in most
men; or else in the narrower sense, and then it is lacking in most men
just as much as in the majority of animals. On the whole, we may still
say of man's reason what Goethe's Mephistopheles said:--

        Life somewhat better might content him
        But for the gleam of heavenly light that Thou hast given him.
        He calls it reason; thence his power's increased
        To be still beastlier than any beast.

If, then, we must reject these popular and, in some respects,
agreeable Dualistic theories as untenable, because inconsistent with
the genetic facts, there remains only the opposite or Monistic
conception, according to which the human soul is, like any other
animal soul, a function of the central nervous system, and develops in
inseparable connection therewith. We see this ontogenetically in every
child. The biogenetic law compels us to affirm it phylogenetically.
Just as in every human embryo the skin-sense layer gives rise to the
medullary tube, from the anterior end of which the five cerebral
vesicles of the Craniotes are developed, and from these the mammal
brain (first with the characters of the lower, then with those of the
higher mammals); and as the whole of this ontogenetic process is only
a brief, hereditary reproduction of the same process in the
phylogenesis of the Vertebrates; so the wonderful spiritual life of
the human race through many thousands of years has been evolved step
by step from the lowly psychic life of the lower Vertebrates, and the
development of every child-soul is only a brief repetition of that
long and complex phylogenetic process. From all these facts sound
reason must conclude that the still prevalent belief in the
immortality of the soul is an untenable superstition. I have shown its
inconsistency with modern science in the eleventh chapter of The
Riddle of the Universe.

Here it may also be well to point out the great importance of
anthropogeny, in the light of the biogenetic law, for the purposes of
philosophy. The speculative philosophers who take cognizance of these
ontogenetic facts, and explain them (in accordance with the law)
phylogenetically, will advance the great questions of philosophy far
more than the most distinguished thinkers of all ages have yet
succeeded in doing. Most certainly every clear and consistent thinker
must derive from the facts of comparative anatomy and ontogeny we have
adduced a number of suggestive ideas that cannot fail to have an
influence on the progress of philosophy. Nor can it be doubted that
the candid statement and impartial appreciation of these facts will
lead to the decisive triumph of the philosophic tendency that we call
"Monistic" or "Mechanical," as opposed to the "Dualistic" or
"Teleological," on which most of the ancient, medieval, and modern
systems of philosophy are based. The Monistic or Mechanical philosophy
affirms that all the phenomena of human life and of the rest of nature
are ruled by fixed and unalterable laws; that there is everywhere a
necessary causal connection of phenomena; and that, therefore, the
whole knowable universe is a harmonious unity, a monon. It says,
further, that all phenomena are due solely to mechanical or efficient
causes, not to final causes. It does not admit free-will in the
ordinary sense of the word. In the light of the Monistic philosophy
the phenomena that we are wont to regard as the freest and most
independent, the expressions of the human will, are subject just as
much to rigid laws as any other natural phenomenon. As a matter of
fact, impartial and thorough examination of our "free" volitions shows
that they are never really free, but always determined by antecedent
factors that can be traced to either heredity or adaptation. We
cannot, therefore, admit the conventional distinction between nature
and spirit. There is spirit everywhere in nature, and we know of no
spirit outside of nature. Hence, also, the common antithesis of
natural science and mental or moral science is untenable. Every
science, as such, is both natural and mental. That is a firm principle
of Monism, which, on its religious side, we may also denominate
Pantheism. Man is not above, but in, nature.

It is true that the opponents of evolution love to misrepresent the
Monistic philosophy based on it as "Materialism," and confuse the
philosophic tendency of this name with a wholly unconnected and
despicable moral materialism. Strictly speaking, it would be just as
proper to call our system Spiritualism as Materialism. The real
Materialistic philosophy affirms that the phenomena of life are, like
all other phenomena, effects or products of matter. The opposite
extreme, the Spiritualistic philosophy, says, on the contrary, that
matter is a product of energy, and that all material forms are
produced by free and independent forces. Thus, according to one-sided
Materialism, the matter is antecedent to the living force; according
to the equally one-sided view of the Spiritist, it is the reverse.
Both views are Dualistic, and, in my opinion, both are false. For us
the antithesis disappears in the Monistic philosophy, which knows
neither matter without force nor force without matter. It is only
necessary to reflect for some time over the question from the strictly
scientific point of view to see that it is impossible to form a clear
idea of either hypothesis. As Goethe said, "Matter can never exist or
act without spirit, nor spirit without matter."

The human "spirit" or "soul" is merely a force or form of energy,
inseparably bound up with the material sub-stratum of the body. The
thinking force of the mind is just as much connected with the
structural elements of the brain as the motor force of the muscles
with their structural elements. Our mental powers are functions of the
brain as much as any other force is a function of a material body. We
know of no matter that is devoid of force, and no forces that are not
bound up with matter. When the forces enter into the phenomenon as
movements we call them living or active forces; when they are in a
state of rest or equilibrium we call them latent or potential. This
applies equally to inorganic and organic bodies. The magnet that
attracts iron filings, the powder that explodes, the steam that drives
the locomotive, are living inorganics; they act by living force as
much as the sensitive Mimosa does when it contracts its leaves at
touch, or the venerable Amphioxus that buries itself in the sand of
the sea, or man when he thinks. Only in the latter cases the
combinations of the different forces that appear as "movement" in the
phenomenon are much more intricate and difficult to analyse than in
the former.

Our study has led us to the conclusion that in the whole evolution of
man, in his embryology and in his phylogeny, there are no living
forces at work other than those of the rest of organic and inorganic
nature. All the forces that are operative in it could be reduced in
the ultimate analysis to growth, the fundamental evolutionary function
that brings about the forms of both the organic and the inorganic. But
growth itself depends on the attraction and repulsion of homogeneous
and heterogeneous particles. Seventy-five years ago Carl Ernst von
Baer summed up the general result of his classic studies of animal
development in the sentence: "The evolution of the individual is the
history of the growth of individuality in every respect." And if we go
deeper to the root of this law of growth, we find that in the long run
it can always be reduced to that attraction and repulsion of animated
atoms which Empedocles called the "love and hatred" of the elements.

Thus the evolution of man is directed by the same "eternal, iron laws"
as the development of any other body. These laws always lead us back
to the same simple principles, the elementary principles of physics
and chemistry. The various phenomena of nature only differ in the
degree of complexity in which the different forces work together. Each
single process of adaptation and heredity in the stem-history of our
ancestors is in itself a very complex physiological phenomenon. Far
more intricate are the processes of human embryology; in these are
condensed and comprised thousands of the phylogenetic processes.

In my General Morphology, which appeared in 1866, I made the first
attempt to apply the theory of evolution, as reformed by Darwin, to
the whole province of biology, and especially to provide with its
assistance a mechanical foundation for the science of organic forms.
The intimate relations that exist between all parts of organic
science, especially the direct causal nexus between the two sections
of evolution--ontogeny and phylogeny--were explained in that work for
the first time by transformism, and were interpreted philosophically
in the light of the theory of descent. The anthropological part of the
General Morphology (Book 7) contains the first attempt to determine
the series of man's ancestors (volume 2 page 428). However imperfect
this attempt was, it provided a starting-point for further
investigation. In the thirty-seven years that have since elapsed the
biological horizon has been enormously widened; our empirical
acquisitions in paleontology, comparative anatomy, and ontogeny have
grown to an astonishing extent, thanks to the united efforts of a
number of able workers and the employment of better methods. Many
important biological questions that then appeared to be obscure
enigmas seem to be entirely settled. Darwinism arose like the dawn of
a new day of clear Monistic science after the dark night of mystic
dogmatism, and we can say now, proudly and gladly, that there is
daylight in our field of inquiry.

Philosophers and others, who are equally ignorant of the empirical
sources of our evidence and the phylogenetic methods of utilising it,
have even lately claimed that in the matter of constructing our
genealogical tree nothing more has been done than the discovery of a
"gallery of ancestors," such as we find in the mansions of the
nobility. This would be quite true if the genealogy given in the
second part of this work were merely the juxtaposition of a series of
animal forms, of which we gathered the genetic connection from their
external physiognomic resemblances. As we have sufficiently proved
already, it is for us a question of a totally different thing--of the
morphological and historical proof of the phylogenetic connection of
these ancestors on the basis of their identity in internal structure
and embryonic development; and I think I have sufficiently shown in
the first part of this work how far this is calculated to reveal to us
their inner nature and its historical development. I see the essence
of its significance precisely in the proof of historical connection. I
am one of those scientists who believe in a real "natural history,"
and who think as much of an historical knowledge of the past as of an
exact investigation of the present. The incalculable value of the
historical consciousness cannot be sufficiently emphasised at a time
when historical research is ignored and neglected, and when an "exact"
school, as dogmatic as it is narrow, would substitute for it physical
experiments and mathematical formulae. Historical knowledge cannot be
replaced by any other branch of science.

It is clear that the prejudices that stand in the way of a general
recognition of this "natural anthropogeny" are still very great;
otherwise the long struggle of philosophic systems would have ended in
favour of Monism. But we may confidently expect that a more general
acquaintance with the genetic facts will gradually destroy these
prejudices, and lead to the triumph of the natural conception of
"man's place in nature." When we hear it said, in face of this
expectation, that this would lead to retrogression in the intellectual
and moral development of mankind, I cannot refrain from saying that,
in my opinion, it will be just the reverse; that it will promote to an
enormous extent the advance of the human mind. All progress in our
knowledge of truth means an advance in the higher cultivation of the
human intelligence; and all progress in its application to practical
life implies a corresponding improvement of morality. The worst
enemies of the human race--ignorance and superstition--can only be
vanquished by truth and reason. In any case, I hope and desire to have
convinced the reader of these chapters that the true scientific
comprehension of the human frame can only be attained in the way that
we recognise to be the sole sound and effective one in organic science
generally--namely, the way of Evolution.

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