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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.


CHAPTER 1.2. THE OLDER EMBRYOLOGY.

It is in many ways useful, on entering upon the study of any science,
to cast a glance at its historical development. The saying that
"everything is best understood in its growth" has a distinct
application to science. While we follow its gradual development we get
a clearer insight into its aims and objects. Moreover, we shall see
that the present condition of the science of human evolution, with all
its characteristics, can only be rightly understood when we examine
its historical growth. This task will, however, not detain us long.
The study of man's evolution is one of the latest branches of natural
science, whether you consider the embryological or the phylogenetic
section of it.

Apart from the few germs of our science which we find in classical
antiquity, and which we shall notice presently, we may say that it
takes its definite rise, as a science, in the year 1759, when one of
the greatest German scientists, Caspar Friedrich Wolff, published his
Theoria generationis. That was the foundation-stone of the science of
animal embryology. It was not until fifty years later, in 1809, that
Jean Lamarck published his Philosophie Zoologique - the first effort to
provide a base for the theory of evolution; and it was another
half-century before Darwin's work appeared (in 1859), which we may
regard as the first scientific attainment of this aim. But before we
go further into this solid establishment of evolution, we must cast a
brief glance at that famous philosopher and scientist of antiquity,
who stood alone in this, as in many other branches of science, for
more than 2000 years: the "father of Natural History," Aristotle.

The extant scientific works of Aristotle deal with many different
sides of biological research; the most comprehensive of them is his
famous History of Animals. But not less interesting is the smaller
work, On the Generation of Animals (Peri zoon geneseos). This work
treats especially of embryonic development, and it is of great
interest as being the earliest of its kind and the only one that has
come down to us in any completeness from classical antiquity.

Aristotle studied embryological questions in various classes of
animals, and among the lower groups he learned many most remarkable
facts which we only rediscovered between 1830 and 1860. It is certain,
for instance, that he was acquainted with the very peculiar mode of
propagation of the cuttlefishes, or cephalopods, in which a yelk-sac
hangs out of the mouth of the foetus. He knew, also, that embryos come
from the eggs of the bee even when they have not been fertilised. This
"parthenogenesis" (or virgin-birth) of the bees has only been
established in our time by the distinguished zoologist of Munich,
Siebold. He discovered that male bees come from the unfertilised, and
female bees only from the fertilised, eggs. Aristotle further states
that some kinds of fishes (of the genus serranus) are hermaphrodites,
each individual having both male and female organs and being able to
fertilise itself; this, also, has been recently confirmed. He knew
that the embryo of many fishes of the shark family is attached to the
mother's body by a sort of placenta, or nutritive organ very rich in
blood; apart from these, such an arrangement is only found among the
higher mammals and man. This placenta of the shark was looked upon as
legendary for a long time, until Johannes Muller proved it to be a
fact in 1839. Thus a number of remarkable discoveries were found in
Aristotle's embryological work, proving a very good acquaintance of
the great scientist - possibly helped by his predecessors - with the
facts of ontogeny, and a great advance upon succeeding generations in
this respect.

In the case of most of these discoveries he did not merely describe
the fact, but added a number of observations on its significance. Some
of these theoretical remarks are of particular interest, because they
show a correct appreciation of the nature of the embryonic processes.
He conceives the development of the individual as a new formation, in
the course of which the various parts of the body take shape
successively. When the human or animal frame is developed in the
mother's body, or separately in an egg, the heart - which he regards as
the starting-point and centre of the organism - must appear first. Once
the heart is formed the other organs arise, the internal ones before
the external, the upper (those above the diaphragm) before the lower
(or those beneath the diaphragm). The brain is formed at an early
stage, and the eyes grow out of it. These observations are quite
correct. And, if we try to form some idea from these data of
Aristotle's general conception of the embryonic process, we find a dim
prevision of the theory which Wolff showed 2000 years afterwards to be
the correct view. It is significant, for instance, that Aristotle
denied the eternity of the individual in any respect. He said that the
species or genus, the group of similar individuals, might be eternal,
but the individual itself is temporary. It comes into being in the act
of procreation, and passes away at death.

During the 2000 years after Aristotle no progress whatever was made in
general zoology, or in embryology in particular. People were content
to read, copy, translate, and comment on Aristotle. Scarcely a single
independent effort at research was made in the whole of the period.
During the Middle Ages the spread of strong religious beliefs put
formidable obstacles in the way of independent scientific
investigation. There was no question of resuming the advance of
biology. Even when human anatomy began to stir itself once more in the
sixteenth century, and independent research was resumed into the
structure of the developed body, anatomists did not dare to extend
their inquiries to the unformed body, the embryo, and its development.
There were many reasons for the prevailing horror of such studies. It
is natural enough, when we remember that a Bull of Boniface VIII
excommunicated every man who ventured to dissect a human corpse. If
the dissection of a developed body were a crime to be thus punished,
how much more dreadful must it have seemed to deal with the embryonic
body still enclosed in the womb, which the Creator himself had
decently veiled from the curiosity of the scientist! The Christian
Church, then putting many thousands to death for unbelief, had a
shrewd presentiment of the menace that science contained against its



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