426 



THE EVOLUTION OF LIFE. 



cause of increasing multiformity. The lapse of a species info 

 divergent varieties, iaitiates fresh combinations of forces 

 tendiag to work further divergences. The new varieties 

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

 elements to its circiunstances. They modify somewhat th? 

 conditions of other species existing lq their habitat, or intc 

 whoso 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 tlie 

 cause of further differentiations in such assemblage. 



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

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

 that in any aggregate made up of mixed imits, mcident 

 forces produce segregation — separate unUke units and unite 

 Uke 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 efl'i.cts previously wrought — must produce confusioa 

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

 pate this impression. 



Doubtless the condition imder 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, ta 

 i.hat such part shall be exposed to some set of tolerably-con- 



