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
authority. It was powerful enough to see that its rival did not grow
too quickly.
It was not until the Reformation broke the power of the Church, and a
refreshing breath of the spirit dissolved the icy chains that bound
science, that anatomy and embryology, and all the other branches of
research, could begin to advance once more. However, embryology lagged
far behind anatomy. The first works on embryology appear at the
beginning of the sixteenth century. The Italian anatomist, Fabricius
ab Aquapendente, a professor at Padua, opened the advance. In his two
books (De formato foetu, 1600, and De formatione foetus, 1604) he
published the older illustrations and descriptions of the embryos of
man and other mammals, and of the hen. Similar imperfect illustrations
were given by Spigelius (De formato foetu, 1631), and by Needham
(1667) and his more famous compatriot, Harvey (1652), who discovered
the circulation of the blood in the animal body and formulated the
important principle, Omne vivum ex vivo (all life comes from
pre-existing life). The Dutch scientist, Swammerdam, published in his
Bible of Nature the earliest observations on the embryology of the
frog and the division of its egg-yelk. But the most important
embryological studies in the sixteenth century were those of the
famous Italian, Marcello Malpighi, of Bologna, who led the way both in
zoology and botany. His treatises, De formatione pulli and De ovo
incubato (1687), contain the first consistent description of the
development of the chick in the fertilised egg.
Here I ought to say a word about the important part played by the
chick in the growth of our science. The development of the chick, like
that of the young of all other birds, agrees in all its main features
with that of the other chief vertebrates, and even of man. The three
highest classes of vertebrates--mammals, birds, and reptiles (lizards,
serpents, tortoises, etc.)--have from the beginning of their embryonic
development so striking a resemblance in all the chief points of
structure, and especially in their first forms, that for a long time
it is impossible to distinguish between them. We have known now for
some time that we need only examine the embryo of a bird, which is the
easiest to get at, in order to learn the typical mode of development
of a mammal (and therefore of man). As soon as scientists began to
study the human embryo, or the mammal-embryo generally, in its earlier
stages about the middle and end of the seventeenth century, this
important fact was very quickly discovered. It is both theoretically
and practically of great value. As regards the THEORY of evolution, we
can draw the most weighty inferences from this similarity between the
embryos of widely different classes of animals. But for the practical
purposes of embryological research the discovery is invaluable,
because we can fill up the gaps in our imperfect knowledge of the
embryology of the mammals from the more thoroughly studied embryology
of the bird. Hens' eggs are easily to be had in any quantity, and the
development of the chick may be followed step by step in artificial
incubation. The development of the mammal is much more difficult to
follow, because here the embryo is not detached and enclosed in a
large egg, but the tiny ovum remains in the womb until the growth is
completed. Hence, it is very difficult to keep up sustained
observation of the various stages in any great extent, quite apart
from such extrinsic considerations as the cost, the technical
difficulties, and many other obstacles which we encounter when we
would make an extensive study of the fertilised mammal. The chicken
has, therefore, always been the chief object of study in this
connection. The excellent incubators we now have enable us to observe
it in any quantity and at any stage of development, and so follow the
whole course of its formation step by step.
By the end of the seventeenth century Malpighi had advanced as far as
it was possible to do with the imperfect microscope of his time in the
embryological study of the chick. Further progress was arrested until
the instrument and the technical methods should be improved. The
vertebrate embryos are so small and delicate in their earlier stages
that you cannot go very far into the study of them without a good
microscope and other technical aid. But this substantial improvement
of the microscope and the other apparatus did not take place until the
beginning of the nineteenth century.
Embryology made scarcely any advance in the first half of the
eighteenth century, when the systematic natural history of plants and
animals received so great an impulse through the publication of
Linne's famous Systema Naturae. Not until 1759 did the genius arise
who was to give it an entirely new character, Caspar Friedrich Wolff.
Until then embryology had been occupied almost exclusively in
unfortunate and misleading efforts to build up theories on the
imperfect empirical material then available.
The theory which then prevailed, and remained in favour throughout
nearly the whole of the eighteenth century, was commonly called at
that time "the evolution theory"; it is better to describe it as "the
preformation theory."* (* This theory is usually known as the
"evolution theory" in Germany, in contradistinction to the "epigenesis
theory." But as it is the latter that is called the "evolution theory"
in England, France, and Italy, and "evolution" and "epigenesis" are
taken to be synonymous, it seems better to call the first the
"pre-formation theory.") Its chief point is this: There is no new
formation of structures in the embryonic development of any organism,
animal or plant, or even of man; there is only a growth, or unfolding,
of parts which have been constructed or pre-formed from all eternity,
though on a very small scale and closely packed together. Hence, every
living germ contains all the organs and parts of the body, in the form
and arrangement they will present later, already within it, and thus
the whole embryological process is merely an evolution in the literal
sense of the word, or an unfolding, of parts that were pre-formed and
folded up in it. So, for instance, we find in the hen's egg not merely
a simple cell, that divides and subdivides and forms germinal layers,
and at last, after all kinds of variation and cleavage and
reconstruction, brings forth the body of the chick; but there is in
every egg from the first a complete chicken, with all its parts made
and neatly packed. These parts are so small or so transparent that the
microscope cannot detect them. In the hatching, these parts merely
grow larger, and spread out in the normal way.
When this theory is consistently developed it becomes a "scatulation
theory."* (* "Packing theory" would be the literal translation.
Scatula is the Latin for a case or box.--Translator.) According to its
teaching, there was made in the beginning one couple or one individual
of each species of animal or plant; but this one individual contained
the germs of all the other individuals of the same species who should
ever come to life. As the age of the earth was generally believed at
that time to be fixed by the Bible at 5000 or 6000 years, it seemed
possible to calculate how many individuals of each species had lived
in the period, and so had been packed inside the first being that was
created. The theory was consistently extended to man, and it was
affirmed that our common parent Eve had had stored in her ovary the
germs of all the children of men.
The theory at first took the form of a belief that it was the FEMALES
who were thus encased in the first being. One couple of each species
was created, but the female contained in her ovary all the future
individuals of the species, of either sex. However, this had to be
altered when the Dutch microscopist, Leeuwenhoek, discovered the male
spermatozoa in 1690, and showed that an immense number of these
extremely fine and mobile thread-like beings exist in the male sperm
(this will be explained in Chapter 2.7). This astonishing discovery
was further advanced when it was proved that these living bodies,
swimming about in the seminal fluid, were real animalcules, and, in
fact, were the pre-formed germs of the future generation. When the
male and female procreative elements came together at conception,
these thread-like spermatozoa ("seed-animals") were supposed to
penetrate into the fertile body of the ovum and begin to develop
there, as the plant seed does in the fruitful earth. Hence, every
spermatozoon was regarded as a homunculus, a tiny complete man; all
the parts were believed to be pre-formed in it, and merely grew larger
when it reached its proper medium in the female ovum. This theory,
also, was consistently developed in the sense that in each of these
thread-like bodies the whole of its posterity was supposed to be
present in the minutest form. Adam's sexual glands were thought to
have contained the germs of the whole of humanity.
This "theory of male scatulation" found itself at once in keen
opposition to the prevailing "female" theory. The two rival theories
at once opened a very lively campaign, and the physiologists of the
eighteenth century were divided into two great camps--the
Animalculists and the Ovulists--which fought vigorously. The
animalculists held that the spermatozoa were the true germs, and
appealed to the lively movements and the structure of these bodies.
The opposing party of the Ovulists, who clung to the older "evolution
theory," affirmed that the ovum is the real germ, and that the
spermatozoa merely stimulate it at conception to begin its growth; all
the future generations are stored in the ovum. This view was held by
the great majority of the biologists of the eighteenth century, in
spite of the fact that Wolff proved it in 1759 to be without
foundation. It owed its prestige chiefly to the circumstance that the
most weighty authorities in the biology and philosophy of the day
decided in favour of it, especially Haller, Bonnet, and Leibnitz.
Albrecht Haller, professor at Gottingen, who is often called the
father of physiology, was a man of wide and varied learning, but he
does not occupy a very high position in regard to insight into natural
phenomena. He made a vigorous defence of the "evolutionary theory" in
his famous work, Elementa physiologiae, affirming: "There is no such
thing as formation (nulla est epigenesis). No part of the animal frame
is made before another; all were made together." He thus denied that
there was any evolution in the proper sense of the word, and even went
so far as to say that the beard existed in the new-born child and the
antlers in the hornless fawn; all the parts were there in advance, and
were merely hidden from the eye of man for the time being. Haller even
calculated the number of human beings that God must have created on
the sixth day and stored away in Eve's ovary. He put the number at
200,000 millions, assuming the age of the world to be 6000 years, the
average age of a human being to be thirty years, and the population of
the world at that time to be 1000 millions. And the famous Haller
maintained all this nonsense, in spite of its ridiculous consequences,
even after Wolff had discovered the real course of embryonic
development and established it by direct observation!
Among the philosophers of the time the distinguished Leibnitz was the
chief defender of the "preformation theory," and by his authority and
literary prestige won many adherents to it. Supported by his system of
monads, according to which body and soul are united in inseparable
association and by their union form the individual, or the "monad,"
Leibnitz consistently extended the "scatulation theory" to the soul,
and held that this was no more evolved than the body. He says, for
instance, in his Theodicee: "I mean that these souls, which one day
are to be the souls of men, are present in the seed, like those of
other species; in such wise that they existed in our ancestors as far
back as Adam, or from the beginning of the world, in the forms of
organised bodies."
The theory seemed to receive considerable support from the
observations of one of its most zealous supporters, Bonnet. In 1745 he
discovered, in the plant-louse, a case of parthenogenesis, or
virgin-birth, an interesting form of reproduction that has lately been
found by Siebold and others among various classes of the articulata,
especially crustacea and insects. Among these and other animals of
certain lower species the female may reproduce for several generations
without having been fertilised by the male. These ova that do not need
fertilisation are called "false ova," pseudova or spores. Bonnet saw
that a female plant-louse, which he had kept in cloistral isolation,
and rigidly removed from contact with males, had on the eleventh day
(after forming a new skin for the fourth time) a living daughter, and
during the next twenty days ninety-four other daughters; and that all
of them went on to reproduce in the same way without any contact with
males. It seemed as if this furnished an irrefutable proof of the
truth of the scatulation theory, as it was held by the Ovulists; it is
not surprising to find that the theory then secured general
acceptance.
This was the condition of things when suddenly, in 1759, Caspar
Friedrich Wolff appeared, and dealt a fatal blow at the whole
preformation theory with his new theory of epigenesis. Wolff, the son
of a Berlin tailor, was born in 1733, and went through his scientific
and medical studies, first at Berlin under the famous anatomist
Meckel, and afterwards at Halle. Here he secured his doctorate in his
twenty-sixth year, and in his academic dissertation (November 28th,
1759), the Theoria generationis, expounded the new theory of a real
development on a basis of epigenesis. This treatise is, in spite of
its smallness and its obscure phraseology, one of the most valuable in
the whole range of biological literature. It is equally distinguished
for the mass of new and careful observations it contains, and the
far-reaching and pregnant ideas which the author everywhere extracts
from his observations and builds into a luminous and accurate theory
of generation. Nevertheless, it met with no success at the time.
Although scientific studies were then assiduously cultivated owing to
the impulse given by Linne--although botanists and zoologists were no
longer counted by dozens, but by hundreds, hardly any notice was taken
of Wolff's theory. Even when he established the truth of epigenesis by
the most rigorous observations, and demolished the airy structure of
the preformation theory, the "exact" scientist Haller proved one of
the most strenuous supporters of the old theory, and rejected Wolff's
correct view with a dictatorial "There is no such thing as evolution."
He even went on to say that religion was menaced by the new theory! It
is not surprising that the whole of the physiologists of the second
half of the eighteenth century submitted to the ruling of this
physiological pontiff, and attacked the theory of epigenesis as a
dangerous innovation. It was not until more than fifty years
afterwards that Wolff's work was appreciated. Only when Meckel
translated into German in 1812 another valuable work of Wolff's on The
Formation of the Alimentary Canal (written in 1768), and called
attention to its great importance, did people begin to think of him
once more; yet this obscure writer had evinced a profounder insight
into the nature of the living organism than any other scientist of the
eighteenth century.
Wolff's idea led to an appreciable advance over the whole field of
biology. There is such a vast number of new and important observations
and pregnant thoughts in his writings that we have only gradually
learned to appreciate them rightly in the course of the nineteenth
century. He opened up the true path for research in many directions.
In the first place, his theory of epigenesis gave us our first real
insight into the nature of embryonic development. He showed
convincingly that the development of every organism consists of a
series of NEW FORMATIONS, and that there is no trace whatever of the
complete form either in the ovum or the spermatozoon. On the contrary,
these are quite simple bodies, with a very different purport. The
embryo which is developed from them is also quite different, in its
internal arrangement and outer configuration, from the complete
organism. There is no trace whatever of preformation or in-folding of
organs. To-day we can scarcely call epigenesis a THEORY, because we
are convinced it is a fact, and can demonstrate it at any moment with
the aid of the microscope.
Wolff furnished the conclusive empirical proof of his theory in his
classic dissertation on The Formation of the Alimentary Canal (1768).
In its complete state the alimentary canal of the hen is a long and
complex tube, with which the lungs, liver, salivary glands, and many
other small glands, are connected. Wolff showed that in the early
stages of the embryonic chick there is no trace whatever of this
complicated tube with all its dependencies, but instead of it only a
flat, leaf-shaped body; that, in fact, the whole embryo has at first
the appearance of a flat, oval-shaped leaf. When we remember how
difficult the exact observation of so fine and delicate a structure as
the early leaf-shaped body of the chick must have been with the poor
microscopes then in use, we must admire the rare faculty for
observation which enabled Wolff to make the most important discoveries
in this most difficult part of embryology. By this laborious research
he reached the correct opinion that the embryonic body of all the
higher animals, such as the birds, is for some time merely a flat,
thin, leaf-shaped disk--consisting at first of one layer, but
afterwards of several. The lowest of these layers is the alimentary
canal, and Wolff followed its development from its commencement to its
completion. He showed how this leaf-shaped structure first turns into
a groove, then the margins of this groove fold together and form a
closed canal, and at length the two external openings of the tube (the
mouth and anus) appear.
Moreover, the important fact that the other systems of organs are
developed in the same way, from tubes formed out of simple layers, did
not escape Wolff. The nerveless system, muscular system, and vascular
(blood-vessel) system, with all the organs appertaining thereto, are,
like the alimentary system, developed out of simple leaf-shaped
structures. Hence, Wolff came to the view by 1768 which Pander
developed in the Theory of Germinal Layers fifty years afterwards. His
principles are not literally correct; but he comes as near to the
truth in them as was possible at that time, and could be expected of
him.
Our admiration of this gifted genius increases when we find that he
was also the precursor of Goethe in regard to the metamorphosis of
plants and of the famous cellular theory. Wolff had, as Huxley showed,
a clear presentiment of this cardinal theory, since he recognised
small microscopic globules as the elementary parts out of which the
germinal layers arose.
Finally, I must invite special attention to the MECHANICAL character
of the profound philosophic reflections which Wolff always added to
his remarkable observations. He was a great monistic philosopher, in
the best meaning of the word. It is unfortunate that his philosophic
discoveries were ignored as completely as his observations for more
than half a century. We must be all the more careful to emphasise the
fact of their clear monistic tendency.