THE HISTORY OF OUR SPECIES
Origin of Man--Mythical History of Creation--Moses and Linné--The
Creation of Permanent Species--The Catastrophic Theory:
Cuvier--Transformism: Goethe--Theory of Descent: Lamarck--Theory
of Selection: Darwin--Evolution (Phylogeny)--Ancestral
Trees--General Morphology--Natural History of Creation--Systematic
Phylogeny--Fundamental Law of Biogeny--Anthropogeny--Descent of Man
from the Ape--Pithecoid Theory--The Fossil Pithecanthropus of Dubois
The youngest of the great branches of the living tree of biology is
the science we call biological evolution, or _phylogeny_. It came into
existence much later, and under much more difficult circumstances, than
its natural sister, embryonic evolution or _ontogeny_. The object of
the latter was to attain a knowledge of the mysterious processes by
which the individual organism, plant or animal, developed from the egg.
Phylogeny has to answer the much more obscure and difficult question:
"What is the origin of the different organic species of plants and
animals?"
Ontogeny (embryology and metamorphism) could follow the empirical
method of direct observation in the solution of its not remote problem;
it needed but to follow, day by day and hour by hour, the visible
changes which the foetus experiences during a brief period in the
course of its development from the ovum. Much more difficult was
the remote problem of phylogeny; for the slow processes of gradual
construction, which effect the rise of new species of animals and
plants, go on imperceptibly during thousands and even millions of
years. Their direct observation is possible only within very narrow
limits; the vast majority of these historical processes can only be
known by direct inference--by critical reflection, and by a comparative
use of empirical sciences which belong to very different fields of
thought, palæontology, ontogeny, and morphology. To this we must
add the immense opposition which was everywhere made to biological
evolution on account of the close connection between questions of
organic creation and supernatural myths and religious dogmas. For these
reasons it can easily be understood how it is that the scientific
existence of a true theory of origins was only secured, amid fierce
controversy, in the course of the last forty years.
Every serious attempt that was made before the beginning of the
nineteenth century to solve the problem of the origin of species
lost its way in the mythological labyrinth of the supernatural
stories of creation. The efforts of a few distinguished thinkers to
emancipate themselves from this tyranny and attain to a naturalistic
interpretation proved unavailing. A great variety of creation myths
arose in connection with their religion in all the ancient civilized
nations. During the Middle Ages triumphant Christendom naturally
arrogated to itself the sole right of pronouncing on the question; and,
the Bible being the basis of the structure of the Christian religion,
the whole story of creation was taken from the book of Genesis. Even
Carl Linné, the famous Swedish scientist, started from that basis
when, in 1735, in his classical _Systema Naturae_, he made the first
attempt at a systematic arrangement, nomenclature, and classification
of the innumerable objects in nature. As the best practical aid in that
attempt he introduced the well-known double or binary nomenclature; to
each kind of animals and plants he gave a particular specific name,
and added to it the wider-reaching name of the genus. A _genus_ served
to unite the nearest related _species_; thus, for instance, Linné
grouped under the genus "dog" (_canis_), as different species, the
house-dog (_canis familiaris_), the jackal (_canis aureus_), the wolf
(_canis lupus_) the fox (_canis vulpes_), etc. This binary nomenclature
immediately proved of such great practical assistance that it was
universally accepted, and is still always followed in zoological and
botanical classification.
But the theoretical dogma which Linné himself connected with his
practical idea of species was fraught with the gravest peril to
science. The first question which forced itself on the mind of the
thoughtful scientist was the question as to the nature of the concept
of species, its contents, and its range. And the creator of the idea
answered this fundamental question by a naïve appeal to the dominant
Mosaic legend of creation: "_Species tot sunt diversae, quot diversas
formas ab initio creavit infinitum ens_"--(There are just so many
distinct species as there were distinct types created in the beginning
by the Infinite). This theosophic dogma cut short all attempt at a
natural explanation of the origin of species. Linné was acquainted only
with the plant and animal worlds that exist to-day; he had no suspicion
of the much more numerous extinct species which had peopled the earth
with their varying forms in the earlier period of its development.
It was not until the beginning of the nineteenth century that we were
introduced to these fossil animals by Cuvier. In his famous work on
the fossil bones of the four-footed vertebrates he gave (1812) the
first correct description and true interpretation of many of these
fossil remains. He showed, too, that a series of very different animal
populations have succeeded each other in the various stages of the
earth's history. Since Cuvier held firmly to Linné's idea of the
absolute permanency of species, he thought their origin could only be
explained by the supposition that a series of great cataclysms and new
creations had marked the history of the globe; he imagined that all
living creatures were destroyed at the commencement of each of these
terrestrial revolutions, and an entirely new population was created
at its close. Although this "catastrophic theory" of Cuvier's led to
the most absurd consequences, and was nothing more than a bald faith
in miracles, it obtained almost universal recognition, and reigned
triumphant until the coming of Darwin.
It is easy to understand that these prevalent ideas of the absolute
unchangeability and supernatural creation of organic species could not
satisfy the more penetrating thinkers. We find several eminent minds
already, in the second half of the last century, busy with the attempt
to find a natural explanation of the "problem of creation." Pre-eminent
among them was the great German poet and philosopher, Wolfgang Goethe,
who, by his long and assiduous study of morphology, obtained, more than
a hundred years ago, a clear insight into the intimate connection of
all organic forms, and a firm conviction of a common natural origin.
In his famed _Metamorphosis of Plants_ (1790) he derived all the
different species of plants from one primitive type, and all their
different organs from one primitive organ--the leaf. In his vertebral
theory of the skull he endeavored to prove that the skulls of the
vertebrates--including man--were all alike made up of certain groups
of bones, arranged in a definite structure, and that these bones are
nothing else than transformed vertebræ. It was his penetrating study
of comparative osteology that led Goethe to a firm conviction of the
unity of the animal organization; he had recognized that the human
skeleton is framed on the same fundamental type as that of all other
vertebrates--"built on a primitive plan that only deviates more or less
to one side or other in its very constant features, and still develops
and refashions itself daily." This remodelling, or transformation,
is brought about, according to Goethe, by the constant interaction
of two powerful constructive forces--a centripetal force within the
organism, the "tendency to specification," and a centrifugal force
without, the tendency to variation, or the "idea of metamorphosis";
the former corresponds to what we now call heredity, the latter to
the modern idea of adaptation. How deeply Goethe had penetrated into
their character by these philosophic studies of the "construction and
reconstruction of organic natures," and how far, therefore, he must be
considered the most important precursor of Darwin and Lamarck,[12] may
be gathered from the interesting passages from his works which I have
collected in the fourth chapter of my _Natural History of Creation_.
These evolutionary ideas of Goethe, however, like analogous ideas of
Kant, Owen, Treviranus, and other philosophers of the commencement of
the century (which we have quoted in the above work), did not amount to
more than certain general conclusions. They had not that great lever
which the "natural history of creation" needed for its firm foundation
on a criticism of the dogma of fixed species; this lever was first
supplied by Lamarck.
The first thorough attempt at a scientific establishment of transformism
was made at the beginning of the nineteenth century by the great
French scientist Jean Lamarck, the chief opponent of his colleague,
Cuvier, at Paris. He had already, in 1802, in his _Observations on
Living Organisms_, expressed the new ideas as to the mutability and
formation of species, which he thoroughly established in 1809 in the
two volumes of his profound work, _Philosophie Zoologique_. In this
work he first gave expression to the correct idea, in opposition to
the prevalent dogma of fixed species, that the organic "species" is
an _artificial abstraction_, a concept of only relative value, like
the wider-ranging concepts of genus, family, order, and class. He
went on to affirm that all species are changeable, and have arisen
from older species in the course of very long periods of time. The
common parent forms from which they have descended were originally
very simple and lowly organisms. The first and oldest of them arose
by abiogenesis. While the type is preserved by _heredity_ in the
succession of generations, _adaptation_, on the other hand, effects
a constant modification of the species by change of habits and the
exercise of the various organs. Even our human organism has arisen in
the same natural manner, by gradual transformation, from a group of
pithecoid mammals. For all these phenomena--indeed, for all phenomena
both in nature and in the mind--Lamarck takes exclusively mechanical,
physical, and chemical activities to be the true efficient causes. His
magnificent _Philosophie Zoologique_ contains all the elements of a
purely monistic system of nature on the basis of evolution. I have
fully treated these achievements of Lamarck in the fourth chapter of my
_Anthropogeny_, and in the fourth chapter of the _Natural History of
Creation_.
Science had now to wait until this great effort to give a scientific
foundation to the theory of evolution should shatter the dominant myth
of a "specific creation, and open out the path of natural" development.
In this respect Lamarck was not more successful in resisting the
conservative authority of his great opponent, Cuvier, than was his
colleague and sympathizer, Geoffrey St. Hilaire, twenty years later.
The famous controversies which he had with Cuvier in the Parisian
Academy in 1830 ended with the complete triumph of the latter. I have
elsewhere fully described these conflicts, in which Goethe took so
lively an interest. The great expansion which the study of biology
experienced at that time, the abundance of interesting discoveries
in comparative anatomy and physiology, the establishment of the
cellular theory, and the progress of ontogeny, gave zoologists and
botanists so overwhelming a flood of welcome material to deal with
that the difficult and obscure question of the origin of species was
easily forgotten for a time. People rested content with the old dogma
of creation. Even when Charles Lyell refuted Cuvier's extraordinary
"catastrophic theory" in his _Principles of Geology_, in 1830, and
vindicated a natural, continuous evolution for the inorganic structure
of our planet, his simple principle of continuity found no one to
apply it to the inorganic world. The rudiments of a natural phylogeny
which were buried in Lamarck's works were as completely forgotten as
the germ of a natural ontogeny which Caspar Friedrich Wolff had given
fifty years earlier in his _Theory of Generation_. In both cases a full
half-century elapsed before the great idea of a natural development
won a fitting recognition. Only when Darwin (in 1859) approached the
solution of the problem from a different side altogether, and made
a happy use of the rich treasures of empirical knowledge which had
accumulated in the mean time, did men begin to think once more of
Lamarck as his great precursor.
The unparalleled success of Charles Darwin is well known. It shows him
to-day, at the close of the century, to have been, if not the greatest,
at least the most effective of its distinguished scientists. No other
of the many great thinkers of our time has achieved so magnificent, so
thorough, and so far-reaching a success with a single classical work as
Darwin did in 1859 with his famous _Origin of Species_. It is true that
the reform of comparative anatomy and physiology by Johannes Müller
had inaugurated a new and fertile epoch for the whole of biology, that
the establishment of the cellular theory by Schleiden and Schwann, the
reform of ontogeny by Baer, and the formulation of the law of substance
by Robert Mayer and Helmholtz were scientific facts of the first
importance; but no one of them has had so profound an influence on the
whole structure of human knowledge as Darwin's theory of the natural
origin of species. For it at once gave us the solution of the mystic
"problem of creation," the great "question of all questions"--the
problem of the true character and origin of man himself.
If we compare the two great founders of transformism, we find in
Lamarck a preponderant inclination to _deduction_, and to forming a
completely monistic scheme of nature; in Darwin we have a predominant
application of _induction_, and a prudent concern to establish the
different parts of the theory of selection as firmly as possible on a
basis of observation and experiment. While the French scientist far
outran the then limits of empirical knowledge, and rather sketched the
programme of future investigation, the English empiricist was mainly
preoccupied about securing a unifying principle of interpretation for
a mass of empirical knowledge which had hitherto accumulated without
being understood. We can thus understand how it was that the success
of Darwin was just as overwhelming as that of Lamarck was evanescent.
Darwin, however, had not only the signal merit of bringing all the
results of the various biological sciences to a common focus in the
principle of descent, and thus giving them a harmonious interpretation,
but he also discovered, in the principle of selection, that direct
cause of transformation which Lamarck had missed. In applying, as a
practical breeder, the experience of artificial selection to organisms
in a state of nature, and in recognizing in the "struggle for life" the
selective principle of natural selection, Darwin created his momentous
"theory of selection," which is what we properly call Darwinism.
One of the most pressing of the many important tasks which Darwin
proposed to modern biology was the reform of the zoological and
botanical system. Since the innumerable species of animals and plants
were not created by a supernatural miracle, but evolved by natural
processes, their ancestral tree is their "natural system." The first
attempt to frame a system in this sense was made by myself in 1866,
in my _General Morphology of Organisms_. The first volume of this
work ("General Anatomy") dealt with the "mechanical science of the
developed forms"; the second volume ("General Evolution") was occupied
with the science of the "developing forms." The systematic introduction
to the latter formed a "genealogical survey of the natural system
of organisms." Until that time the term "evolution" had been taken
to mean exclusively, both in zoology and botany, the development of
individual organisms--embryology, or metamorphic science. I established
the opposite view, that this history of the embryo (ontogeny) must be
completed by a second, equally valuable, and closely connected branch
of thought--the history of the race (phylogeny). Both these branches
of evolutionary science are, in my opinion, in the closest causal
connection; this arises from the reciprocal action of the laws of
heredity and adaptation; it has a precise and comprehensive expression
in my "fundamental law of biogeny."
As the new views I had put forward in my _General Morphology_ met with
very little notice, and still less acceptance, from my scientific
colleagues, in spite of their severely scientific setting, I thought
I would make the most important of them accessible to a wider circle
of informed readers by a smaller work, written in a more popular
style. This was done in 1868, in _The Natural History of Creation_ (a
series of popular scientific lectures on evolution in general, and the
systems of Darwin, Goethe, and Lamarck in particular). If the success
of my _General Morphology_ was far below my reasonable anticipation,
that of _The Natural History of Creation_ went far beyond it. In a
period of thirty years nine editions and twelve different translations
of it have appeared. In spite of its great defects, the book has
contributed much to the popularization of the main ideas of modern
evolution. Still, I could only give the barest outlines in it of my
chief object, the phylogenetic construction of a natural system. I
have, therefore, given the complete proof, which is wanting in the
earlier work, of the phylogenetic system in a subsequent larger work,
my _Systematic Phylogeny_ (outlines of a natural system of organisms
on the basis of their specific development). The first volume of
it deals with the protists and plants (1894), the second with the
invertebrate animals (1896), the third with the vertebrates (1895). The
ancestral tree of both the smaller and the larger groups is carried
on in this work as far as my knowledge of the three great "ancestral
documents"--palæontology, ontogeny, and morphology--qualified me to
extend it.
I had already, in my _General Morphology_ (at the end of the fifth
book), described the close causative connection which exists, in
my opinion, between the two branches of organic evolution as one
of the most important ideas of transformism, and I had framed a
precise formula for it in a number of "theses on the causal nexus of
biontic and phyletic development": "_Ontogenesis is a brief and rapid
recapitulation of phylogenesis_, determined by the physiological
functions of heredity (generation) and adaptation (maintenance)."
Darwin himself had emphasized the great significance of his theory
for the elucidation of embryology in 1859, and Fritz Müller had
endeavored to prove it as regards the Crustacea in the able little
work, _Facts and Arguments for Darwin_ (1864). My own task has been
to prove the universal application and the fundamental importance of
the biogenetic law in a series of works, especially in the _Biology
of the Calcispongiae_ (1872), and in _Studies on the Gastraea Theory_
(1873-1884). The theory of the homology of the germinal layers and of
the relations of _palingenesis_ to _cenogenesis_ which I have exposed
in them has been confirmed subsequently by a number of works of other
zoologists. That theory makes it possible to follow nature's law of
unity in the innumerable variations of animal embryology; it gives us
for their ancestral history a common derivation from a simple primitive
stem form.
The far-seeing founder of the theory of descent, Lamarck, clearly
recognized in 1809 that it was of universal application; that even man
himself, the most highly developed of the mammals, is derived from the
same stem as all the other mammals; and that this in its turn belongs
to the same older branch of the ancestral tree as the rest of the
vertebrates. He had even indicated the agencies by which it might be
possible to explain man's descent from the apes as the nearest related
mammals. Darwin, who was, naturally, of the same conviction, purposely
avoided this least acceptable consequence of his theory in his chief
work in 1859, and put it forward for the first time in his _Descent of
Man_ in 1871. In the mean time (1863) Huxley had very ably discussed
this most important consequence of evolution in his famous _Place of
Man in Nature_. With the aid of comparative anatomy and ontogeny,
and the support of the facts of palæontology, Huxley proved that the
"descent of man from the ape" is a necessary consequence of Darwinism,
and that no other scientific explanation of the origin of the human
race is possible. Of the same opinion was Karl Gegenbaur, the most
distinguished representative of comparative anatomy, who lifted his
science to a higher level by a consistent and ingenious application of
the theory of descent.
As a further consequence of the "pithecoid theory" (the theory of the
descent of man from the ape) there now arose the difficult task of
investigating, not only the nearest related mammal ancestors of man
in the Tertiary epoch, but also the long series of the older animal
ancestors which had lived in earlier periods of the earth's history and
been developed in the course of countless millions of years. I had made
a start with the hypothetical solution of this great historic problem
in my _General Morphology_; a further development of it appeared in
1874 in my _Anthropogeny_ (first section, Origin of the Individual;
second section, Origin of the Race). The fourth, enlarged, edition of
this work (1891) contains that theory of the development of man which
approaches nearest, in my own opinion, to the still remote truth, in
the light of our present knowledge of the documentary evidence. I was
especially preoccupied in its composition to use the three empirical
"documents"--palæontology, ontogeny, and morphology (or comparative
anatomy)--as evenly and harmoniously as possible. It is true that my
hypotheses were in many cases supplemented and corrected in detail by
later phylogenetic research; yet I am convinced that the ancestral tree
of human origin which I have sketched therein is substantially correct.
For the historical succession of vertebrate fossils corresponds
completely with the morphological evolutionary scale which is revealed
to us by comparative anatomy and ontogeny. After the Silurian fishes
come the _dipnoi_ of the Devonian period--the Carboniferous amphibia,
the Permian reptilia, and the Mesozoic mammals. Of these, again, the
lowest forms, the monotremes, appear first in the Triassic period,
the marsupials in the Jurassic, and then the oldest placentals in
the Cretaceous. Of the placentals, in turn, the first to appear in
the oldest Tertiary period (the Eocene) are the lowest primates,
the prosimiæ, which are followed by the simiæ in the Miocene. Of the
catarrhinæ, the cynopitheci precede the anthropomorpha; from one branch
of the latter, during the Pliocene period, arises the ape-man without
speech (the _pithecanthropus alalus_); and from him descends, finally,
speaking man.
The chain of our earlier invertebrate ancestors is much more difficult
to investigate and much less safe than this tree of our vertebrate
predecessors; we have no fossilized relics of their soft, boneless
structures, so palæontology can give us no assistance in this case.
The evidence of comparative anatomy and ontogeny, therefore, becomes
all the more important. Since the human embryo passes through the
same _chordula_-stage as the germs of all other vertebrates, since
it evolves, similarly, out of two germinal layers of a _gastrula_,
we infer, in virtue of the biogenetic law, the early existence of
corresponding ancestral forms--vermalia, gastræada, etc. Most important
of all is the fact that the human embryo, like that of all other
animals, arises originally from a single cell; for this "stem-cell"
(_cytula_)--the impregnated egg cell--points indubitably to a
corresponding unicellular ancestor, a primitive, Laurentian protozoon.
For the purpose of our monistic philosophy, however, it is a matter of
comparative indifference how the succession of our animal predecessors
may be confirmed in detail. Sufficient for us, as an incontestable
historical fact, is the important thesis that man descends immediately
from the ape, and secondarily from a long series of lower vertebrates.
I have laid stress on the logical proof of this "pithecometra-thesis"
in the seventh book of the _General Morphology_: "The thesis that
man has been evolved from lower vertebrates, and immediately from the
_simiae_, is a special inference which results with absolute necessity
from the general inductive law of the theory of descent."
For the definitive proof and establishment of this fundamental
pithecometra-thesis the palæontological discoveries of the last
thirty years are of the greatest importance; in particular, the
astonishing discoveries of a number of extinct mammals of the Tertiary
period have enabled us to draw up clearly in its main outlines the
evolutionary history of this most important class of animals, from
the lowest oviparous monotremes up to man. The four chief groups
of the placentals, the heterogeneous legions of the carnassia, the
rodentia, the ungulata, and the primates, seem to be separated by
profound gulfs, when we confine our attention to their representatives
of to-day. But these gulfs are completely bridged, and the sharp
distinctions of the four legions are entirely lost, when we compare
their extinct predecessors of the Tertiary period, and when we go
back into the Eocene twilight of history, in the oldest part of the
Tertiary period--at least three million years ago. There we find the
great sub-class of the placentals, which to-day comprises more than
two thousand five hundred species, represented by only a small number
of little, insignificant "proplacentals"; and in these _prochoriata_
the characters of the four divergent legions are so intermingled and
toned down that we cannot in reason do other than consider them as the
precursors of those features. The oldest carnassia (the _ictopsales_),
the oldest rodentia (the _esthonychales_), the oldest ungulata (the
_condylarthrales_) and the oldest primates (the _lemuravales_), all
have the same fundamental skeletal structure, and the same typical
dentition of the primitive placentals, consisting of forty-four teeth
(three incisors, one canine, four premolars, and three molars in each
half of the jaw); all are characterized by the small size and the
imperfect structure of the brain (especially of its chief part, the
cortex, which does not become a true "organ of thought" until later on
in the Miocene and Pliocene representatives); they have all short legs
and five-toed, flat-soled feet (_plantigrada_). In many cases among
these oldest placentals of the Eocene period it was very difficult
to say at first whether they should be classed with the carnassia,
rodentia, ungulata, or primates; so very closely, even to confusion,
do these four groups of the placentals, which diverge so widely
afterwards, approach each other at that time. Their common origin from
a single ancestral group follows incontestably. These _prochoriata_
lived in the preceding Cretaceous period (more than three million years
ago), and were probably developed in the Jurassic period from a group
of insectivorous marsupials (_amphitheria_) by the formation of a
primitive _placenta diffusa_, a placenta of the simplest type.
But the most important of all the recent palaeontological discoveries
which have served to elucidate the origin of the placentals relate
to our own stem, the legion of primates. Formerly fossil remains of
the primates were very scarce. Even Cuvier, the great founder of
palaeontology, maintained until his last day (1832) that there were no
fossilized primates; he had himself, it is true, described the skull
of an Eocene prosimiæ (_adapis_), but he had wrongly classed it with
the ungulata. However, during the last twenty years a fair number of
well-preserved fossilized skeletons of prosimiæ and simiæ have been
discovered; in them we find all the chief intermediate members which
complete the connecting chain of ancestors from the oldest prosimiæ to
man.
The most famous and most interesting of these discoveries is the
fossil ape-man of Java, the much-talked-of _pithecanthropus erectus_,
found by a Dutch military doctor, Eugen Dubois, in 1894. It is in
truth the much-sought "missing link," supposed to be wanting in
the chain of primates, which stretches unbroken from the lowest
catarrhinæ to the highest-developed man. I have dealt exhaustively
with the significance of this discovery in the paper which I read on
August 26, 1898, at the Fourth International Zoological Congress at
Cambridge.[13] The palæontologist, who knows the conditions of the
formation and preservation of fossils, will think the discovery of the
pithecanthropus an unusually lucky accident. The apes, being arboreal,
seldom came into the circumstances (unless they happened to fall into
the water) which would secure the preservation and petrifaction of
their skeleton. Thus, by the discovery of this fossil man-monkey of
Java the descent of man from the ape has become just as clear and
certain from the palæontological side as it was previously from the
evidence of comparative anatomy and ontogeny. We now have all the
principal documents which tell the history of our race.