VI.] SPECIES AND TIME. 157 



opinion, this form might have been '' sensibly changed " in 

 the course of two or three centuries. According to this, 

 for evolving it as a true and perfect species one thousand 

 years would be a very moderate period. Let ten thousand 

 years be taken to represent approximately the period of 

 substantially constant conditions during which no con- 

 siderable change would be brought about. Now, if one 

 thousand years may represent the period required for the 

 evolution of the species aS'. 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 Cercoj)ithecus and the other genera of the 

 family Simiidse — the differences between the genera being 

 certainly more than tenfold greater than those between the 

 species of the same genus. Again we may perhaps inter- 

 pose a period of ten thousand years' comparative repose. 



For the differentiation of the families Simiidse and 

 Cebidse — 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 differences between the Lemuroid 

 and the higher forms of primates ; for those between the 

 original primate and other root-forins of placental mam- 

 mals ; for those between primary placental and im^^lacental 

 mammals, and perhaps also for the divergence of the most 

 ancient stock of these and of the monotremes ; since 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 mam- 

 malian type from the primitive stock of the whole verte- 



