The wholly reactionary system that this led to has been well described by Robert Drill (1902) in his Virchow as a Reactionary. How little qualified the great pathologist was to appreciate the scientific bases of the pithecoid theory is clear from the absurd statement he made, in the opening speech of the Vienna Congress of Anthropologists, in 1894, that man might just as well be claimed to descend from a sheep or an elephant as from an ape. Any competent zoologist can see from this the little knowledge Virchow had of systematic zoology and comparative anatomy. However, he retained his authority as president of the German Anthropological Society, which remained impervious to Darwinian ideas. Even such vigorous controversialists as Carl Vogt, and such scientific partisans of the ape-man of Neanderthal as Schaafhausen, could make no impression. Virchow's authority was equally great for twenty years in the Berlin Press, both Liberal and Conservative. The Kreutzzeitung and the Evangelische Kirchenzeitung were delighted that "the learned progressist was conservative in the best sense of the word as regards evolution." The ultramontane Germania rejoiced that the powerful representative of pure science had, "with a few strokes of his cudgel, reduced to impotence" the absurd ape-theory and its chief protagonist, Ernst Haeckel. The National-Zeitung could not sufficiently thank the free-thinking, popular leader for having lifted from us for ever the oppressive mountain of the theory of simian descent. The editor of the Volks-Zeitung, Bernstein, who has done so much for the spread of knowledge in his excellent popular manuals of science, obstinately refused to admit articles that ventured to support the erroneous ape-theory "refuted" by Virchow.
It would take up too much space to attempt to give even a general survey of the remarkable and enormous literature of the subject that has accumulated in the last three decades in the shape of thousands of learned treatises and popular articles. The greater part of these works have been written under the influence of conventional religious prejudice, and without the necessary acquaintance with the subject, that can only be obtained by a thorough training in biology. The most curious feature of them is that most of the authors restrict their genealogical interests to the most manlike apes, and do not deal with their origin, or with the deeper roots of our common ancestral tree. They do not see the wood for the trees. Yet it is far easier and safer to penetrate the great mysteries of our animal origin, if we look at the subject from the higher standpoint of vertebrate phylogeny and go deeper into the earlier records of the evolutionary history of the vertebrates.
Since the great Lamarck established the idea of the vertebrate at the beginning of the nineteenth century (1801), and his Parisian colleague, Cuvier, shortly afterwards recognised the vertebrates as one of his four chief animal groups, the natural unity of this advanced section of the animal world has not been contested. In all the vertebrates, from the lowest fishes and amphibians up to the apes and man, we have the same type of structure, the same characteristic disposition and relations of the chief organs; and they differ materially from the corresponding features in all other animals. The mysterious affinities of the vertebrates induced Goethe, 140 years ago, long before Cuvier, to make prolonged and laborious studies in their comparative anatomy at Jena and Weimar. Just as he had, in his Metamorphosis of Plants, established the unity of organisation by means of the leaf as the common primitive organ, he, in the metamorphosis of the vertebrates, found this common element in the vertebral theory of the skull. And when Cuvier established comparative anatomy as an independent science, this branch of biology was developed to such an extent by the classic research of Johannes Müller, Carl Gegenbaur, Richard Owen, Thomas Huxley, and many other morphologists, that Darwinism found its most powerful weapons in this arsenal. The striking differences of external form and internal structure that we find in the fishes, amphibians, reptiles, birds, and mammals, are due to adaptation to the various uses of their organs and their environments. On the other hand, the astonishing agreement in their typical character, that persists in spite of their differences, is due to inheritance from common ancestors.
The evidence thus afforded by comparative anatomy is so cogent that anyone who goes impartially and attentively through a collection of skeletons can convince himself at once of the morphological unity of the vertebrate stem. The evolutionary evidence of comparative ontogeny, or embryology, is less easy to grasp and less accessible, but not less important. It came to light at a much later date, and its extreme value was only made clear, by means of the biogenetic law, some forty years ago. It shows that every vertebrate, like every other animal, develops from a single cell, but that the course of its embryonic development is peculiar, and characterised by embryonic forms that are not found in the invertebrates. We find in them especially the chordula, or chorda-larva, a very simple worm-shaped embryonic form, without limbs, head, or higher sense-organs; the body consists merely of six very simple primitive organs. From these are developed steadily the hundreds of different bones, muscles, and other organs that we afterwards distinguish in the mature vertebrate. The remarkable and very complex course of this embryonic development is essentially the same in man and the ape, and in the amphibians and fishes. We see in it, in accordance with the biogenetic law, a new and important witness to the common descent of all vertebrates from a single primitive form, the chordæa.
But, important as these arguments of comparative embryology are, one needs many years' study in the unfamiliar and difficult province of embryology before one can realise their evolutionary force. There are, in fact, not a few embryologists (especially of the modern school of experimental embryology) who do not succeed in doing so. It is otherwise with the palpable proofs that we take from a remote science, paleontology. The remarkable fossil remains and impressions of extinct animals and plants give us directly the historical evidence we need to understand the successive appearance and disappearance of the various species and groups. Geology has firmly established the chronological order of the sedimentary rocks, which have been successively formed of mud at the floor of the ocean, and has deduced their age from the thickness of the strata, and determined the relative date of their formation. The vast period during which organic life has been developing on the earth runs to many million years. The number is variously estimated at less than a hundred or at several hundred million years.[7] If we take the smaller number of 200 million years, we find them distributed amongst the five chief periods of the earth's organic development in such a way that the earlier or archeozoic period absorbs nearly one half. As the sedimentary rocks of this period, chiefly gneisses and crystalline schists, are in a metamorphosed condition, the fossil remains in them are unrecognisable. In the next succeeding strata of the paleozoic period we find the earliest remains of fossilised vertebrates, Silurian primitive fishes (selachii) and ganoids. These are followed, in the Devonian system, by the first dipneust fishes (a transitional form from the fishes to the amphibia). In the next, the Carboniferous system, we find the first terrestrial or four-footed vertebrates—amphibians of the order of the stegocephala. A little later, in the Permian rocks, the earliest amniotes, lowly, lizard-like reptiles (tocosauria), make their appearance; the warm-blooded birds and mammals are still wanting. We have the first traces of the mammals in the Triassic, the earliest sedimentary rocks of the mesozoic age; these are of the monotreme sub-class (pantotheria and allotheria). They are succeeded by the first marsupials (prodidelphia) in the Jurassic, the ancestral forms of the placentals (mallotheria), in the Cretaceous. See [p. 115.]
But the richest development of the mammal class takes place in the next or Tertiary age. In the course of its four periods—the eocene, oligocene, miocene, and pliocene—the mammal species increase steadily in number, variety, and complexity, down to the present time. From the lowest common ancestral group of the placentals proceed four divergent branches, the legions of the carnassia, rodents, ungulates, and primates. The primate legion surpasses all the rest. In this Linné long ago included the lemurs, apes, and man. The historical order in which the various stages of vertebrate development make their successive appearance corresponds entirely to the morphological order of their advance in organisation, as we have learned it from the study of comparative anatomy and embryology.
These paleontological facts are among the most important proofs of the descent of man from a long series of higher and lower vertebrates. There is no other explanation possible except evolution for the chronological succession of these classes, which is in perfect harmony with the morphological and systematic distribution. The anti-evolutionists have not even attempted to give any other explanation. The fishes, dipneusts, amphibians, reptiles, monotremes, marsupials, placentals, lemurs, apes, anthropoid apes, and ape-men (pithecanthropi), are inseparable links of a long ancestral chain, of which the last and most perfect link is man. (Cf. the tables [pp. 116-118].)
One of the paleontological facts I have quoted, namely, the late appearance of the mammal class in geology—is particularly important. This most advanced group of the vertebrates comes on the stage in the Triassic period, in the second and shorter half of the organic history of the earth. It is represented only by low and small forms in the whole of the mesozoic age, during the domination of the reptiles. Throughout this long period, which is estimated by some geologists at 8-11, by others at 20 or more, million years, the dominant reptile class developed its many remarkable and curious forms; there were swimming marine reptiles (halisauria), flying reptiles (pterosauria), and colossal land reptiles (dinosauria). It was much later, in the Tertiary period, that the mammal class attained the wealth of large and advanced placental forms that secured its predominance over this more recent period.