The racial history of apes can only be traced with any certainty up to the Miocene period. The fact of the contemporary existence of apes and pachydermata has been frequently asserted, but it is still too far from being established to merit further consideration here. Traces of the slender ape (Semnopithecus) have, however, been found in the Miocene of Greece, Wurtemburg, the mountains of Sewalik, and in the region bordering on the Himalayas. The name given to one of these fossil species (Semnopithecus subhimalayanus) seems to establish its locality. The numerous remains of Mesopithecus Pentelici in Attica have, however, given rise to controversy. Gaudry and Beyrich were disposed to assign these specimens exclusively to the slender ape, but Gaudry has since declared that, while the structure of the skull and teeth is that of Semnopithecus, the structure of the limbs is that of a macaca. He regards, therefore, Mesopithecus as an interesting form of apes, and this is expressed by its scientific name.126

The separation of these two species of apes (Semnopithecus and Macacus) must, he considers, have occurred rather late. Pliopithecus, from the fresh-water marl, Sansan, is assigned by Gaudry and others to the gibbon. Lartet and Quenstedt believe, however, that it is nearer to the next neighbour on the south, the magot (Inuus), on account of the five fangs of its last tooth. Köllner thinks the connection with Semnopithecus not improbable.

Dryopithecus Fontanii, of which I have already spoken, seems, as I judge from a cast taken by Fric in Prague, to be of an expressly anthropoid character; but the scantiness of the materials do not allow us to form any precise conclusions as to the zoological position of this extinct animal. The structure of the back teeth, as we have already said, is certainly anthropoidal. Quenstedt, always cautious in his judgments, is of opinion that the ape’s teeth found in the ironstone of the Suabian Alps in the secondary mammal formation, are of a decidedly anthropoid character, and the animals to which they belonged must therefore have been of the same type. Fossil remains of the African stumpy ape (Colobus) have also been found at Steinheim.127 Macacus priscus of the valley of the Arno seems to be allied with the African macaca.128 Owen’s Macacus pliocenus from Essex is closely related to Macacus sinicus. Fossil apes have also been observed in America. Protopithecus was a very large animal, related to Mycetes. Another fossil species, found in South America (Laopithecus), must have been closely related to man. This latter fact is the more remarkable, since it has generally been assumed, and indeed with reason, that there is a marked division between the apes of the Old and New Worlds.

The species now found in tropical America of the silky apes (Hapale), the Sahui (Jacchus), the leaping apes (Callithrix), the bellowing apes (Mycetes), and the rolling apes (Cebus), were already represented in the diluvial period of that continent. It does not appear that any extensive generic diffusion of apes has taken place since that period. It is otherwise with the development of species, which seems, at any rate to a partial extent, to have occurred late. This may be inferred from the physical characteristics of gorillas and chimpanzees, which, with all their differences, have much in common with each other. In the fourth chapter we have described forms of apes lying between the gorilla and the chimpanzee, and it seems possible that these are a reversion to one or the other form. The numerous varieties of form among anthropoids point to a continuance of the process of severance in this family of apes, and little more than an isolating influence is needed to produce the gradual conversion of varieties into constant species.

On account of their external bodily characteristics, of their anatomical structure, and their highly developed intelligence, anthropoids not only stand first among apes, but they take a still higher place, approximating to the human species. In accordance with what I have said in the second and third chapters, I set aside the order of the Quadrumana, and accept the Linnæan order of the Primates, both for men and apes. I would include men as Erecti with anthropoids as Anthropomorpha in a sub-family of the Primarii. In the case of apes (Simiina) I should retain the convenient distinction between those with a narrow and those with a wide nasal aperture (Catarrhina and Platyrrhina). The semi-apes (Prosimii) should constitute a separate order of mammals. The following systematic scheme shows the classification I suggest:—

I. Mammals (Mammalia).
A. Monodelphia, Blainv. (Placentalia, Owen).