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). I. Order: _Primates_, Linnæus. 1. Family: _Primarii_. (1) Sub-family: _Erecti_ (_Homo sapiens_). (2) Sub-family: _Anthropomorpha_, Linnæus. (_a_) _Dasypoga_, _i.e._ Anthropomorpha, without the sessor callosities. (α) Genus: _Troglodytes_, E. Geoffroy. Species: The gorilla (_Troglodytes Gorilla_, Savage and Wyman). The chimpanzee (_Tr. niger_, E. Geoffroy). The other species are not accurately known. (β) Genus: _Pithecus_, E. Geoffroy. Species: Orang-utan (_Pithecus Satyrus_, E. Geoffroy). (_b_) _Tylopoga_, _i.e._ Anthropomorpha, with sessor callosities. (λ) Genus: _Hylobates_, Illig. Species: see p. 45. 2. Family: Apes proper (_Simiina_). (1) Sub-family: _Catarrhina_.