4S6 SEGREGATION. 



uniform incidence of external forces affects the members of 

 a group of organisms similarly in proportion as they are simi- 

 lar, and differently in proportion as they are different; and 

 thus is ever segregating the like by parting the unlike from 

 them. Whether these separated members are killed off, as 

 mostly happens, or whether, as otherwise happens, they sur- 

 vive and multiply into a distinct variety, in consequence of 

 their fitness to certain partially unlike conditions, matters 

 not to the argument. The one case conforms to the law, that 

 the unlike units of an aggregate are sorted into their kinds 

 and parted when uniformly subject to the same incident 

 forces ; and the other to the converse law, that the like units 

 of an aggregate are parted and separately grouped when sub- 

 ject to different incident forces. And on consulting Mr. 

 Darwin's remarks on divergence of character, it will be seen 

 that the segregations thus caused tend ever to become more 

 definite. 



§ 167. Mental evolution under one of its leading as- 

 pects, we found to consist in the formation of groups of like 

 objects and like relations — a differentiation of the various 

 things originally confounded together in one assemblage, 

 and an integration of each separate order of things into a 

 separate group (§ 153). Here it remains to point out that 

 while unlikeness in the incident forces is the cause of such 

 differentiations, likeness in the incident forces is the cause of 

 such integrations. For what is the process through which 

 classifications are established? At first, in common with 

 the uninitiated, the botanist recognizes only such conven- 

 tional divisions as those which agriculture has established — 

 distinguishes a few vegetables and cereals, and groups the 

 rest together into the one miscellaneous aggregate of wild 

 plants. How do these wild plants become grouped in his 

 mind into orders, genera, and species ? Each plant he exam- 

 ines yields him a certain complex impression. Every now 

 and then he picks up a plant like one before seen; and the 



