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THE EVOLUTION OF MAN<br>
Volume I<br>
<br>
<hr noshade size="1" align="center" width="10%">
<br>
C<font size="-2">HAPTER</font> I<br>
<br>
<b>THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION</b></center>
<br>
<p class="one">The field of natural phenomena into which I would
introduce my readers in the following chapters has a quite peculiar
place in the broad realm of scientific inquiry. There is no object
of investigation that touches man more closely, and the knowledge
of which should be more acceptable to him, than his own frame. But
among all the various branches of the natural history of mankind,
or <i>anthropology,</i> the story of his development by natural
means must excite the most lively interest. It gives us the key of
the great world-riddles at which the human mind has been working
for thousands of years. The problem of the nature of man, or the
question of man’s place in nature, and the cognate inquiries
as to the past, the earliest history, the present situation, and
the future of humanity—all these most important questions are
directly and intimately connected with that branch of study which
we call the science of the evolution of man, or, in one word,
“Anthropogeny” (the genesis of man). Yet it is an
astonishing fact that the science of the evolution of man does not
even yet form part of the scheme of general education. In fact,
educated people even in our day are for the most part quite
ignorant of the important truths and remarkable phenomena which
anthropogeny teaches us.</p>
<p>As an illustration of this curious state of things, it may be
pointed out that most of what are considered to be
“educated” people do not know that every human being is
developed from an egg, or ovum, and that this egg is one simple
cell, like any other plant or animal egg. They are equally ignorant
that in the course of the development of this tiny, round egg-cell
there is first formed a body that is totally different from the
human frame, and has not the remotest resemblance to it. Most of
them have never seen such a human embryo in the earlier period of
its development, and do not know that it is quite indistinguishable
from other animal embryos. At first the embryo is no more than a
round cluster of cells, then it becomes a simple hollow sphere, the
wall of which is composed of a layer of cells. Later it approaches
very closely, at one period, to the anatomic structure of the
lancelet, afterwards to that of a fish, and again to the typical
build of the amphibia and mammals. As it continues to develop, a
form appears which is like those we find at the lowest stage of
mammal-life (such as the duck-bills), then a form that resembles
the marsupials, and only at a late stage a form that has a
resemblance to the ape; until at last the definite human form
emerges and closes the series of transformations. These suggestive
facts are, as I said, still almost unknown to the general
public—so completely unknown that, if one casually mentions
them, they are called in question or denied outright as
fairy-tales. Everybody knows that the butterfly emerges from the
pupa, and the pupa from a quite different thing called a larva, and
the larva from the butterfly’s egg. But few besides medical
men are aware that <i>man</i>, in the course of his individual
formation, passes through a series of transformations which are not
less surprising and wonderful than the familiar metamorphoses of
the butterfly.</p>
<p>The mere description of these remarkable changes through which
man passes during his embryonic life should arouse considerable
interest. But the mind will experience a far keener satisfaction
when</p>
<br>
<hr>
<p class="page"><a name="page 2">[ 2 ]</a></p>
<p> </p>
<p class="one">we trace these curious facts to their causes, and
when we learn to behold in them natural phenomena which are of the
highest importance throughout the whole field of human knowledge.
They throw light first of all on the “natural history of
creation,” then on psychology, or “the science of the
soul,” and through this on the whole of philosophy. And as
the general results of every branch of inquiry are summed up in
philosophy, all the sciences come in turn to be touched and
influenced more or less by the study of the evolution of man.</p>
<p>But when I say that I propose to present here the most important
features of these phenomena and trace them to their causes, I take
the term, and I interpret my task, in a very much wider sense than
is usual. The lectures which have been delivered on this subject in
the universities during the last half-century are almost
exclusively adapted to medical men. Certainly, the medical man has
the greatest interest in studying the origin of the human body,
with which he is daily occupied. But I must not give here this
special description of the embryonic processes such as it has
hitherto been given, as most of my readers have not studied
anatomy, and are not likely to be entrusted with the care of the
adult organism. I must content myself with giving some parts of the
subject only in general outline, and must not enter upon all the
marvellous, but very intricate and not easily described, details
that are found in the story of the development of the human frame.
To understand these fully a knowledge of anatomy is needed. I will
endeavour to be as plain as possible in dealing with this branch of
science. Indeed, a sufficient general idea of the course of the
embryonic development of man can be obtained without going too
closely into the anatomic details. I trust we may be able to arouse
the same interest in this delicate field of inquiry as has been
excited already in other branches of science; though we shall meet
more obstacles here than elsewhere.</p>
<p>The story of the evolution of man, as it has hitherto been
expounded to medical students, has usually been confined to
embryology—more correctly, <i>ontogeny</i>—or the
science of the development of the individual human organism. But
this is really only the first part of our task, the first half of
the story of the evolution of man in that wider sense in which we
understand it here. We must add as the second half—as another
and not less important and interesting branch of the science of the
evolution of the human stem—<i>phylogeny</i>: this may be
described as the science of the evolution of the various animal
forms from which the human organism has been developed in the
course of countless ages. Everybody now knows of the great
scientific activity that was occasioned by the publication of
Darwin’s <i>Origin of Species</i> in 1859. The chief direct
consequence of this publication was to provoke a fresh inquiry into
the origin of the human race, and this has proved beyond question
our gradual evolution from the lower species. We give the name of
“Phylogeny” to the science which describes this ascent
of man from the lower ranks of the animal world. The chief source
that it draws upon for facts is “Ontogeny,” or
embryology, the science of the development of the individual
organism. Moreover, it derives a good deal of support from <i>
paleontology,</i> or the science of fossil remains, and even more
from comparative anatomy, or <i>morphology.</i></p>
<p>These two branches of our science—on the one side ontogeny
or embryology, and on the other phylogeny, or the science of
race-evolution—are most vitally connected. The one cannot be
understood without the other. It is only when the two branches
fully co-operate and supplement each other that
“Biogeny” (or the science of the genesis of life in the
widest sense) attains to the rank of a philosophic science. The
connection between them is not external and superficial, but
profound, intrinsic, and causal. This is a discovery made by recent
research, and it is most clearly and correctly expressed in the
comprehensive law which I have called “the fundamental law of
organic evolution,” or “the fundamental law of
biogeny.” This general law, to which we shall find ourselves
constantly recurring, and on the recognition of which depends
one’s whole insight into the story of evolution, may be
briefly expressed in the phrase: “The history of the
fœtus is a recapitulation of the history of the race”;
or, in other words, “Ontogeny is a recapitulation of
phylogeny.” It may be more fully stated as follows: The
series of forms through which the individual organism passes during
its development from the ovum to the complete bodily structure is a
brief, condensed repetition</p>
<br>
<hr>
<p class="page"><a name="page 3">[ 3 ]</a></p>
<p> </p>
<p class="one">of the long series of forms which the animal
ancestors of the said organism, or the ancestral forms of the
species, have passed through from the earliest period of organic
life down to the present day.</p>
<p>The causal character of the relation which connects embryology
with stem-history is due to the action of heredity and adaptation.
When we have rightly understood these, and recognised their great
importance in the formation of organisms, we can go a step further
and say: Phylogenesis is the mechanical cause of
ontogenesis.<sup>1</sup> In other words, the development of the
stem, or race, is, in accordance with the laws of heredity and
adaptation, the cause of all the changes which appear in a
condensed form in the evolution of the fœtus.</p>
<p>The chain of manifold animal forms which represent the ancestry
of each higher organism, or even of man, according to the theory of
descent, always form a connected whole. We may designate this
uninterrupted series of forms with the letters of the alphabet: A,
B, C, D, E, etc., to Z. In apparent contradiction to what I have
said, the story of the development of the individual, or the
ontogeny of most organisms, only offers to the observer a part of
these forms; so that the defective series of embryonic forms would
run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M,
N, etc. Here, then, as a rule, several of the evolutionary forms of
the original series have fallen out. Moreover, we often
find—to continue with our illustration from the
alphabet—one or other of the original letters of the
ancestral series represented by corresponding letters from a
different alphabet. Thus, instead of the Roman B and D, we often
have the Greek Β and Δ. In this case the text of the
biogenetic law has been corrupted, just as it had been abbreviated
in the preceding case. But, in spite of all this, the series of
ancestral forms remains the same, and we are in a position to
discover its original complexion.</p>
<p>In reality, there is always a certain parallel between the two
evolutionary series. But it is obscured from the fact that in the
embryonic succession much is wanting that certainly existed in the
earlier ancestral succession. If the parallel of the two series
were complete, and if this great fundamental law affirming the
causal connection between ontogeny and phylogeny in the proper
sense of the word were directly demonstrable, we should only have
to determine, by means of the microscope and the dissecting knife,
the series of forms through which the fertilised ovum passes in its
development; we should then have before us a complete picture of
the remarkable series of forms which our animal ancestors have
successively assumed from the dawn of organic life down to the
appearance of man. But such a repetition of the ancestral history
by the individual in its embryonic life is very rarely complete. We
do not often find our full alphabet. In most cases the
correspondence is very imperfect, being greatly distorted and
falsified by causes which we will consider later. We are thus, for
the most part, unable to determine in detail, from the study of its
embryology, all the different shapes which an organism’s
ancestors have assumed; we usually—and especially in the case
of the human fœtus—encounter many gaps. It is true that
we can fill up most of these gaps satisfactorily with the help of
comparative anatomy, but we cannot do so from direct embryological
observation. Hence it is important that we find a large number of
lower animal forms to be still represented in the course of
man’s embryonic development. In these cases we may draw our
conclusions with the utmost security as to the nature of the
ancestral form from the features of the form which the embryo
momentarily assumes.</p>
<p>To give a few examples, we can infer from the fact that the
human ovum is a simple cell that the first ancestor of our species
was a tiny unicellular being, something like the amœba. In
the same way, we know, from the fact that the human fœtus
consists, at the first, of two simple cell-layers (the <i>
gastrula</i>), that the <i>gastræa</i>, a form with two such
layers, was certainly in the line of our ancestry. A later human
embryonic form (the <i>chordula</i>) points just as clearly to a
worm-like ancestor (the <i>prochordonia</i>), the nearest living
relation of which is found among the actual ascidiæ. To this
succeeds a most important embryonic stage (<i>acrania</i>), in
which our headless fœtus</p>
<br>
<p class="fnote">1. The term “genesis,” which occurs
throughout, means, of course, “birth” or origin. From
this we get: Biogeny = the origin of life (<i>bios</i>);
Anthropogeny = the origin of man (<i>anthropos</i>); Ontogeny = the
origin of the individual (<i>on</i>); Phylogeny = the origin of the
species (<i>phulon</i>); and so on. In each case the term may refer
to the process itself, or to the science describing the
process.—Translator.</p>
<br>
<hr>
<p class="page"><a name="page 4">[ 4 ]</a></p>
<p> </p>
<p class="one">presents, in the main, the structure of the
lancelet. But we can only indirectly and approximately, with the
aid of comparative anatomy and ontogeny, conjecture what lower
forms enter into the chain of our ancestry between the
gastræa and the chordula, and between this and the lancelet.
In the course of the historical development many intermediate
structures have gradually fallen out, which must certainly have
been represented in our ancestry. But, in spite of these many, and
sometimes very appreciable, gaps, there is no contradiction between
the two successions. In fact, it is the chief purpose of this work
to prove the real harmony and the original parallelism of the two.
I hope to show, on a substantial basis of facts, that we can draw
most important conclusions as to our genealogical tree from the
actual and easily-demonstrable series of embryonic changes. We
shall then be in a position to form a general idea of the wealth of
animal forms which have figured in the direct line of our ancestry
in the lengthy history of organic life.</p>
<p>In this evolutionary appreciation of the facts of embryology we
must, of course, take particular care to distinguish sharply and
clearly between the primitive, palingenetic (or ancestral)
evolutionary processes and those due to cenogenesis.<sup>1</sup> By
<i>palingenetic</i> processes, or embryonic <i>recapitulations,</i>
we understand all those phenomena in the development of the
individual which are transmitted from one generation to another by
heredity, and which, on that account, allow us to draw direct
inferences as to corresponding structures in the development of the
species. On the other hand, we give the name of <i>cenogenetic</i>
processes, or embryonic <i>variations,</i> to all those phenomena
in the fœtal development that cannot be traced to inheritance
from earlier species, but are due to the adaptation of the
fœtus, or the infant-form, to certain conditions of its
embryonic development. These cenogenetic phenomena are foreign or
later additions; they allow us to draw no direct inference whatever
as to corresponding processes in our ancestral history, but rather
hinder us from doing so.</p>
<p>This careful discrimination between the primary or palingenetic
processes and the secondary or cenogenetic is of great importance
for the purposes of the scientific history of a species, which has
to draw conclusions from the available facts of embryology,
comparative anatomy, and paleontology, as to the processes in the
formation of the species in the remote past. It is of the same
importance to the student of evolution as the careful distinction
between genuine and spurious texts in the works of an ancient
writer, or the purging of the real text from interpolations and
alterations, is for the student of philology. It is true that this
distinction has not yet been fully appreciated by many scientists.
For my part, I regard it as the first condition for forming any
just idea of the evolutionary process, and I believe that we must,
in accordance with it, divide embryology into two
sections—palingenesis, or the science of recapitulated forms;
and cenogenesis, or the science of supervening structures.</p>
<p>To give at once a few examples from the science of man’s
origin in illustration of this important distinction, I may
instance the following processes in the embryology of man, and of
all the higher vertebrates, as <i>palingenetic</i>: the formation
of the two primary germinal layers and of the primitive gut, the
undivided structure of the dorsal nerve-tube, the appearance of a
simple axial rod between the medullary tube and the gut, the
temporary formation of the gill-clefts and arches, the primitive
kidneys, and so on.<sup>2</sup> All these, and many other important
structures, have clearly been transmitted by a steady heredity from
the early ancestors of the mammal, and are, therefore, direct
indications of the presence of similar structures in the history of
the stem. On the other hand, this is certainly not the case with
the following embryonic forms, which we must describe as
cenogenetic processes: the formation of the yelk-sac, the
allantois, the placenta, the amnion, the serolemma, and the
chorion—or, generally speaking, the various fœtal
membranes and the corresponding changes in the blood vessels.
Further instances are: the dual structure of the heart cavity, the
temporary division of the plates of the primitive vertebræ
and</p>
<br>
<p class="fnote">1. Palingenesis = new birth, or re-incarnation
(<i>palin</i> = again, <i>genesis</i> or <i>genea</i> =
development); hence its application to the phenomena which are
recapitulated by heredity from earlier ancestral forms. Cenogenesis
= foreign or negligible development (<i>kenos</i> and <i>
genea</i>); hence, those phenomena which come later in the story of
life to disturb the inherited structure, by a fresh adaptation to
environment.—Translator.<br>
2. All these, and the following structures, will be fully described
in later chapters.—Translator.</p>
<br>
<br>
<hr>
<p class="page"><a name="page 5">[ 5 ]</a></p>
<p> </p>
<p class="one">lateral plates, the secondary closing of the ventral
and intestinal walls, the formation of the navel, and so on. All
these and many other phenomena are certainly not traceable to
similar structures in any earlier and completely-developed
ancestral form, but have arisen simply by adaptation to the
peculiar conditions of embryonic life (within the fœtal
membranes). In view of these facts, we may now give the following
more precise expression to our chief law of biogeny: The evolution
of the fœtus (or <i>ontogenesis</i>) is a condensed and
abbreviated recapitulation of the evolution of the stem (or <i>
phylogenesis</i>); and this recapitulation is the more complete in
proportion as the original development (or <i>palingenesis</i>) is
preserved by a constant heredity; on the other hand, it becomes
less complete in proportion as a varying adaptation to new
conditions increases the disturbing factors in the development (or
cenogenesis).</p>
<p>The cenogenetic alterations or distortions of the original
palingenetic course of development take the form, as a rule, of a
gradual displacement of the phenomena, which is slowly effected by
adaptation to the changed conditions of embryonic existence during
the course of thousands of years. This displacement may take place
as regards either the position or the time of a phenomenon.</p>
<p>The great importance and strict regularity of the
time-variations in embryology have been carefully studied recently
by Ernest Mehnert, in his <i>Biomechanik</i> (Jena, 1898). He
contends that our biogenetic law has not been impaired by the
attacks of its opponents, and goes on to say: “Scarcely any
piece of knowledge has contributed so much to the advance of
embryology as this; its formulation is one of the most signal
services to general biology. It was not until this law passed into
the flesh and blood of investigators, and they had accustomed
themselves to see a reminiscence of ancestral history in embryonic
structures, that we witnessed the great progress which
embryological research has made in the last two decades.” The
best proof of the correctness of this opinion is that now the most
fruitful work is done in all branches of embryology with the aid of
this biogenetic law, and that it enables students to attain every
year thousands of brilliant results that they would never have
reached without it.</p>
<p>It is only when one appreciates the cenogenetic processes in
relation to the palingenetic, and when one takes careful account of
the changes which the latter may suffer from the former, that the
radical importance of the biogenetic law is recognised, and it is
felt to be the most illuminating principle in the science of
evolution. In this task of discrimination it is the silver thread
in relation to which we can arrange all the phenomena of this realm
of marvels—the “Ariadne thread,” which alone
enables us to find our way through this labyrinth of forms. Hence
the brothers Sarasin, the zoologists, could say with perfect
justice, in their study of the evolution of the <i>Ichthyophis,</i>
that “the great biogenetic law is just as important for the
zoologist in tracing long-extinct processes as spectrum analyses is
for the astronomer.”</p>
<p>Even at an earlier period, when a correct acquaintance with the
evolution of the human and animal frame was only just being
obtained—and that is scarcely eighty years ago!—the
greatest astonishment was felt at the remarkable similarity
observed between the embryonic forms, or stages of fœtal
development, in very different animals; attention was called even
then to their close resemblance to certain fully-developed animal
forms belonging to some of the lower groups. The older scientists
(Oken, Treviranus, and others) knew perfectly well that these lower
forms in a sense illustrated and fixed, in the hierarchy of the
animal world, a temporary stage in the evolution of higher forms.
The famous anatomist Meckel spoke in 1821 of a “similarity
between the development of the embryo and the series of
animals.” Baer raised the question in 1828 how far, within
the vertebrate type, the embryonic forms of the higher animals
assume the permanent shapes of members of lower groups. But it was
impossible fully to understand and appreciate this remarkable
resemblance at that time. We owe our capacity to do this to the
theory of descent; it is this that puts in their true light the
action of <i>heredity</i> on the one hand and <i>adaptation</i> on
the other. It explains to us the vital importance of their constant
reciprocal action in the production of organic forms. Darwin was
the first to teach us the great part that was played in this by the
ceaseless struggle for existence between living things, and to show
how, under the influence of this (by natural selection), new
species were produced and maintained solely by the interaction of
heredity and</p>
<br>
<hr>
<p class="page"><a name="page 6">[ 6 ]</a></p>
<p> </p>
<p class="one">adaptation. It was thus Darwinism that first opened
our eyes to a true comprehension of the supremely important
relations between the two parts of the science of organic
evolution—Ontogeny and Phylogeny.</p>
<p>Heredity and adaptation are, in fact, the two constructive
physiological functions of living things; unless we understand
these properly we can make no headway in the study of evolution.
Hence, until the time of Darwin no one had a clear idea of the real
nature and causes of embryonic development. It was impossible to
explain the curious series of forms through which the human embryo
passed; it was quite unintelligible why this strange succession of
animal-like forms appeared in the series at all. It had previously
been generally assumed that the man was found complete in all his
parts in the ovum, and that the development consisted only in an
unfolding of the various parts, a simple process of growth. This is
by no means the case. On the contrary, the whole process of the
development of the individual presents to the observer a connected
succession of different animal-forms; and these forms display a
great variety of external and internal structure. But <i>why</i>
each individual human being should pass through this series of
forms in the course of his embryonic development it was quite
impossible to say until Lamarck and Darwin established the theory
of descent. Through this theory we have at last detected the real
causes, the <i>efficient causes,</i> of the individual development;
we have learned that these <i>mechanical</i> causes suffice of
themselves to effect the formation of the organism, and that there
is no need of the <i>final</i> causes which were formerly assumed.
It is true that in the academic philosophies of our time these
final causes still figure very prominently; in the new philosophy
of nature we can entirely replace them by efficient causes. We
shall see, in the course of our inquiry, how the most wonderful and
hitherto insoluble enigmas in the human and animal frame have
proved amenable to a mechanical explanation, by causes acting
without prevision, through Darwin’s reform of the science of
evolution. We have everywhere been able to substitute unconscious
causes, acting from necessity, for conscious, purposive
causes.<sup>1</sup></p>
<p>If the new science of evolution had done no more than this,
every thoughtful man would have to admit that it had accomplished
an immense advance in knowledge. It means that in the whole of
philosophy that tendency which we call monistic, in opposition to
the dualistic, which has hitherto prevailed, must be
accepted.<sup>2</sup> At this point the science of human evolution
has a direct and profound bearing on the foundations of philosophy.
Modern anthropology has, by its astounding discoveries during the
second half of the nineteenth century, compelled us to take a
completely monistic view of life. Our bodily structure and its
life, our embryonic development and our evolution as a species,
teach us that the same laws of nature rule in the life of man as in
the rest of the universe. For this reason, if for no others, it is
desirable, nay, indispensable, that every man who wishes to form a
serious and philosophic view of life, and, above all, the expert
philosopher, should acquaint himself with the chief facts of this
branch of science.</p>
<p>The facts of embryology have so great and obvious a significance
in this connection that even in recent years dualist and
teleological philosophers have tried to rid themselves of them by
simply denying them. This was done, for instance, as regards the
fact that man is developed from an egg, and that this egg or ovum
is a simple cell, as in the case of other animals. When I had
explained this pregnant fact and its significance in my <i>History
of Creation,</i> it was described in many of the theological
journals as a dishonest invention of my own. The fact that the
embryos of man and the dog are, at a certain stage of their
development, almost indistinguishable was also denied. When we
examine the human embryo in the third or fourth week of its
development, we find it to be quite different in shape and
structure from the full-grown human being, but almost identical
with that of the ape, the dog, the rabbit, and</p>
<p class="fnote">1. The monistic or mechanical philosophy of nature
holds that only unconscious, necessary, efficient causes are at
work in the whole field of nature, in organic life as well as in
inorganic changes. On the other hand, the dualist or vitalist
philosophy of nature affirms that unconscious forces are only at
work in the inorganic world, and that we find conscious, purposive,
or final causes in organic nature.<br>
2. Monism is neither purely materialistic nor purely
spiritualistic, but a reconciliation of these two principles, since
it regards the whole of nature as one, and sees only efficient
causes at work in it. Dualism, on the contrary, holds that nature
and spirit, matter and force, the world and God, inorganic and
organic nature, are separate and independent existences. Cf. <i>The
Riddle of the Universe,</i> chap. xii.</p>
<br>
<br>
<hr>
<p class="page"><a name="page 7">[ 7 ]</a></p>
<p> </p>
<p class="one">other mammals, at the same stage of ontogeny. We
find a bean-shaped body of very simple construction, with a tail
below and a pair of fins at the sides, something like those of a
fish, but very different from the limbs of man and the mammals.
Nearly the whole front half of the body is taken up by a shapeless
head without face, at the sides of which we find gill-clefts and
arches as in the fish. At this stage of its development the human
embryo does not differ in any essential detail from that of the
ape, dog, horse, ox, etc., at a corresponding period. This
important fact can easily be verified at any moment by a comparison
of the embryos of man, the dog, rabbit, etc. Nevertheless, the
theologians and dualist philosophers pronounced it to be a
materialistic invention; even scientists, to whom the facts should
be known, have sought to deny them.</p>
<p>There could not be a clearer proof of the profound importance of
these embryological facts in favour of the monistic philosophy than
is afforded by these efforts of its opponents to get rid of them by
silence or denial. The truth is that these facts are most
inconvenient for them, and are quite irreconcilable with their
views. We must be all the more pressing on our side to put them in
their proper light. I fully agree with Huxley when he says, in his
<i>Man’s Place in Nature</i>: “Though these facts are
ignored by several well-known popular leaders, they are easy to
prove, and are accepted by all scientific men; on the other hand,
their importance is so great that those who have once mastered them
will, in my opinion, find few other biological discoveries to
astonish them.”</p>
<p>We shall make it our chief task to study the evolution of
man’s bodily frame and its various organs in their external
form and internal structures. But I may observe at once that this
is accompanied step by step with a study of the evolution of their
functions. These two branches of inquiry are inseparably united in
the whole of anthropology, just as in zoology (of which the former
is only a section) or general biology. Everywhere the peculiar form
of the organism and its structures, internal and external, is
directly related to the special physiological functions which the
organism or organ has to execute. This intimate connection of
structure and function, or of the instrument and the work done by
it, is seen in the science of evolution and all its parts. Hence
the story of the evolution of structures, which is our immediate
concern, is also the history of the development of functions; and
this holds good of the human organism as of any other.</p>
<p>At the same time, I must admit that our knowledge of the
evolution of functions is very far from being as complete as our
acquaintance with the evolution of structures. One might say, in
fact, that the whole science of evolution has almost confined
itself to the study of structures; the evolution of <i>
functions</i> hardly exists even in name. That is the fault of the
physiologists, who have as yet concerned themselves very little
about evolution. It is only in recent times that physiologists like
W. Engelmann, W. Preyer, M. Verworn, and a few others, have
attacked the evolution of functions.</p>
<p>It will be the task of some future physiologist to engage in the
study of the evolution of functions with the same zeal and success
as has been done for the evolution of structures in morphogeny (the
science of the genesis of forms). Let me illustrate the close
connection of the two by a couple of examples. The heart in the
human embryo has at first a very simple construction, such as we
find in permanent form among the ascidiæ and other low
organisms; with this is associated a very simple system of
circulation of the blood. Now, when we find that with the
full-grown heart there comes a totally different and much more
intricate circulation, our inquiry into the development of the
heart becomes at once, not only an anatomical, but also a
physiological, study. Thus it is clear that the ontogeny of the
heart can only be understood in the light of its phylogeny (or
development in the past), both as regards function and structure.
The same holds true of all the other organs and their functions.
For instance, the science of the evolution of the alimentary canal,
the lungs, or the sexual organs, gives us at the same time, through
the exact comparative investigation of structure-development, most
important information with regard to the evolution of the functions
of these organs.</p>
<p>This significant connection is very clearly seen in the
evolution of the nervous system. This system is in the economy of
the human body the medium of sensation, will, and even thought, the
highest of the psychic functions; in a word, of</p>
<br>
<hr>
<p class="page"><a name="page 8">[ 8 ]</a></p>
<p> </p>
<p class="one">all the various functions which constitute the
proper object of psychology. Modern anatomy and physiology have
proved that these psychic functions are immediately dependent on
the fine structure and the composition of the central nervous
system, or the internal texture of the brain and spinal cord. In
these we find the elaborate cell-machinery, of which the psychic or
soul-life is the physiological function. It is so intricate that
most men still look upon the mind as something supernatural that
cannot be explained on mechanical principles.</p>
<p>But embryological research into the gradual appearance and the
formation of this important system of organs yields the most
astounding and significant results. The first sketch of a central
nervous system in the human embryo presents the same very simple
type as in the other vertebrates. A spinal tube is formed in the
external skin of the back, and from this first comes a simple
spinal cord without brain, such as we find to be the permanent
psychic organ in the lowest type of vertebrate, the amphioxus. Not
until a later stage is a brain formed at the anterior end of this
cord, and then it is a brain of the most rudimentary kind, such as
we find permanently among the lower fishes. This simple brain
develops step by step, successively assuming forms which correspond
to those of the amphibia, the reptiles, the duck-bills, and the
lemurs. Only in the last stage does it reach the highly organised
form which distinguishes the apes from the other vertebrates, and
which attains its full development in man.</p>
<p>Comparative physiology discovers a precisely similar growth. The
function of the brain, the psychic activity, rises step by step
with the advancing development of its structure.</p>
<p>Thus we are enabled, by this story of the evolution of the
nervous system, to understand at length <i>the natural development
of the human mind</i> and its gradual unfolding. It is only with
the aid of embryology that we can grasp how these highest and most
striking faculties of the animal organism have been historically
evolved. In other words, a knowledge of the evolution of the spinal
cord and brain in the human embryo leads us directly to a
comprehension of the historic development (or phylogeny) of the
human mind, that highest of all faculties, which we regard as
something so marvellous and supernatural in the adult man. This is
certainly one of the greatest and most pregnant results of
evolutionary science. Happily our embryological knowledge of
man’s central nervous system is now so adequate, and agrees
so thoroughly with the complementary results of comparative anatomy
and physiology, that we are thus enabled to obtain a clear insight
into one of the highest problems of philosophy, the phylogeny of
the soul, or the ancestral history of the mind of man. Our chief
support in this comes from the embryological study of it, or the
ontogeny of the soul. This important section of psychology owes its
origin especially to W. Preyer, in his interesting works, such as
<i>The Mind of the Child. The Biography of a Baby</i> (1900), of
Milicent Washburn Shinn, also deserves mention. [See also
Preyer’s <i>Mental Development in the Child</i>
(translation), and Sully’s <i>Studies of Childhood</i> and
<i>Children’s Ways.</i>]</p>
<p>In this way we follow the only path along which we may hope to
reach the solution of this difficult problem.</p>
<p>Thirty-six years have now elapsed since, in my <i>General
Morphology,</i> I established phylogeny as an independent science
and showed its intimate causal connection with ontogeny; thirty
years have passed since I gave in my gastræa-theory the proof
of the justice of this, and completed it with the theory of
germinal layers. When we look back on this period we may ask, What
has been accomplished during it by the fundamental law of biogeny?
If we are impartial, we must reply that it has proved its fertility
in hundreds of sound results, and that by its aid we have acquired
a vast fund of knowledge which we should never have obtained
without it.</p>
<p>There has been no dearth of attacks—often violent
attacks—on my conception of an intimate causal connection
between ontogenesis and phylogenesis; but no other satisfactory
explanation of these important phenomena has yet been offered to
us. I say this especially with regard to Wilhelm His’s theory
of a “mechanical evolution,” which questions the truth
of phylogeny generally, and would explain the complicated embryonic
processes without going beyond by simple physical
changes—such as the bending and folding of leaves by
electricity, the origin of cavities through unequal strain of the
tissues, the formation of processes by uneven growth, and so on.
But the fact is that these embryological phenomena themselves
demand explanation in turn, and this can only be found, as a rule,
in the corresponding changes in the long ancestral series, or in
the physiological functions of heredity and adaptation.</p>
<br>
<hr noshade align="left" size="1" width="20%">
<p class="ref"><a href="Title.html">Title and Contents</a><br>
<a href="glossary.html">Glossary</a><br>
<a href="chap2.html">Chapter II</a><br>
<a href="Title.html#Illustrations">Figs. 1–209</a><br>
<a href="title2.html#Illustrations">Figs. 210–408</a></p>
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