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 anthropology, 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.
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 MAN, 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.
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 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.
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.
The story of the evolution of man, as it has hitherto been expounded
to medical students, has usually been confined to embryology--more
correctly, ontogeny--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--phylogeny: 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 Origin of Species
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 paleontology, or the
science of fossil remains, and even more from comparative anatomy, or
morphology.
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 foetus 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 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.
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.* (* The term
"genesis," which occurs throughout, means, of course, "birth" or
origin. From this we get: Biogeny = the origin of life (bios);
Anthropogeny = the origin of man (anthropos); Ontogeny = the origin of
the individual (on); Phylogeny = the origin of the species (phulon);
and so on. In each case the term may refer to the process itself, or
to the science describing the process.--Translator.) 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 foetus.
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
Beta and Delta. 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.
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 foetus--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.
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 amoeba. In the same way, we
know, from the fact that the human foetus consists, at the first, of
two simple cell-layers (the gastrula), that the gastraea, a form with
two such layers, was certainly in the line of our ancestry. A later
human embryonic form (the chordula) points just as clearly to a
worm-like ancestor (the prochordonia), the nearest living relation of
which is found among the actual ascidiae. To this succeeds a most
important embryonic stage (acrania), in which our headless foetus
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 gastraea 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.
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.* (* Palingenesis = new birth,
or re-incarnation (palin = again, genesis or genea = development);
hence its application to the phenomena which are recapitulated by
heredity from earlier ancestral forms. Cenogenesis = foreign or
negligible development (kenos and genea); hence, those phenomena which
come later in the story of life to disturb the inherited structure, by
a fresh adaptation to environment.--Translator.) By palingenetic
processes, or embryonic recapitulations, 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 cenogenetic processes, or embryonic variations, to
all those phenomena in the foetal development that cannot be traced to
inheritance from earlier species, but are due to the adaptation of the
foetus, 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.
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.
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 palingenetic: 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.* (* All
these, and the following structures, will be fully described in later
chapters.--Translator.) 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 foetal 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
vertebrae and 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 foetal 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 foetus (or ontogenesis) is a condensed
and abbreviated recapitulation of the evolution of the stem (or
phylogenesis); and this recapitulation is the more complete in
proportion as the original development (or palingenesis) 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).
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.
The great importance and strict regularity of the time-variations in
embryology have been carefully studied recently by Ernest Mehnert, in
his Biomechanik (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.
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 Ichthyophis,
that "the great biogenetic law is just as important for the zoologist
in tracing long-extinct processes as spectrum analyses is for the
astronomer."
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 foetal 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
heredity on the one hand and adaptation 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 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.
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 WHY 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 efficient causes, of the individual
development; we have learned that these mechanical causes suffice of
themselves to effect the formation of the organism, and that there is
no need of the final 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.* (* 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.)
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.* (* 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. The
Riddle of the Universe chapter 12.) 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.
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 History of Creation, 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 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.
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 "Man's Place in Nature":
"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."
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.
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 FUNCTIONS 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.
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 ascidiae 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.
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 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.
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.
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.
Thus we are enabled, by this story of the evolution of the nervous
system, to understand at length THE NATURAL DEVELOPMENT OF THE HUMAN
MIND 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 The
Mind of the Child. The Biography of a Baby (1900), of Milicent
Washburn Shinn, also deserves mention. [See also Preyer's Mental
Development in the Child (translation), and Sully's Studies of
Childhood and Children's Ways.]
In this way we follow the only path along which we may hope to reach
the solution of this difficult problem.
Thirty-six years have now elapsed since, in my General Morphology, I
established phylogeny as an independent science and showed its
intimate causal connection with ontogeny; thirty years have passed
since I gave in my gastraea-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.
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.