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.