Heredity and Natural Selection. 285 



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

 to evolve it as a true and perfect species one thousand 

 years would be a very moderate period. Let ten thous- 

 and years be taken to represent approximately the period 

 of substantially constant conditions, during which no 

 considerable change would be brought about. Now, if 

 one thousand years may represent the period required 

 for the evolution of this species and of the other species 

 of the genus, ten times that period should, I think, be 

 allowed for the differentiation of that genus, the Afri- 

 can Oircopithecus, and the other genera of the family 

 Simiidae, the differences between the genera being cer- 

 tainly more than tenfold greater than those between the 

 species of the same grenns. 



". . . For the differentiation of the families Simii- 

 dae and Cebidaa so very much more distinct and dif- 

 ferent that 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 primates and other root-forms of 

 placental mammals; for those between primary placen- 

 tal 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. 

 Supposing this primitive stock to have arisen directly 

 from a very lowly original animal indeed (such as a ne- 

 matoid worm, an ascidian, or a jelly-fish), yet it is not 



