CHAPTER V

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.