426 THE EVOLUTION OF LIFE. 



cause of increasing multiformity. The lapse of a species into 

 divergent varieties, initiates fresh combinations of forces 

 tending to work further divergences. The new varieties 

 compete with the parent species in new ways ; and so add new 

 elements to its circumstances. They modify somewhat the 

 conditions of other species existing in their habitat, or into 

 whose habitat they have spread; and the modifications 

 wrought in such other species, become additional sources of 

 influence. The Flora and Fauna of every region are united 

 by their entangled relations into a whole, of which no part 

 can be affected without affecting the rest. Hence, each dif- 

 ferentiation in a local assemblage of species, becomes the 

 cause of further differentiations in such assemblage. 



§150. One of the universal principles to which we saw 

 that the re- distribution of matter and motion conforms, is 

 that in any aggregate made up of mixed units, incident 

 forces produce segregation — separate unlike units and unite 

 like units ; and it was shown that the increasing integration 

 and definiteness which characterizes each part of an evolving 

 organic aggregate, as of every other aggregate, results from 

 this (First Principles, § 126). It remains here to be 

 pointed out, that while the actions and reactions going on 

 between organisms and their ever- changing environments, 

 add to the heterogeneity of organic structures, they also 

 give to the heterogeneity this growing distinctness. At 

 first sight the reverse might be inferred. It might be argued 

 that any new set of effects wrought in an organism by some 

 new set of external forces, must tend more or less to obliter- 

 ate the effects previously wrought — must produce confusion 

 or indefiniteness. A little consideration, however, will dissi- 

 pate this impression. 



Doubtless the condition under which alone increasing de- 

 finiteness of structure can be acquired by any part of an or- 

 ganism, either in an individual or in successive generations, is 

 that such part shall be exposed to some set of tolerably-con- 



