Now, if one thousand years may represent the period required for the evolution of the species _S.

nasalis_, and of the other species of the genus Semnopithecus; ten times that period should, I think, be allowed for the differentiation of that genus, the African Cercopithecus and the other genera of the family Simiidae--the differences between the genera being certainly more than tenfold greater than those between the species of the same genus.

Again we may perhaps interpose a period of ten thousand years' comparative repose.
For the differentiation of the families Simiidae and Cebidae--so very much more distinct and different than any two genera of either family--a period ten times greater should, I believe, be allowed than that required for the evolution of the subordinate groups.

A similarly increasing ratio should be granted for the successive developments of the difference between the Lemuroid and the higher forms of primates; for those between the original primate and other root-forms of placental mammals; for those between primary placental and implacental mammals, and perhaps also for the divergence of the most ancient stock of these and of the monotremes, for in all these cases modifications of structure appear to increase in complexity in at least that ratio.

Finally, a vast period must be granted for the development of the lowest mammalian type from the primitive stock of the whole vertebrate sub-kingdom.