514 THE EVOLUTION OF LIFE. 



new elements to its circumstances. They modify somewhat 

 the conditions of other species existing in their habitat, or 

 in the habitat they have invaded; 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. 



§ 156. 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 bring 

 together like units; and it was shown that the increasing in- 

 .tegration and definiteness which characterizes each part of 

 an evolving organic aggregate, as of every other aggregate, 

 results from this {First Principles, § 166). It remains here 

 to say that while the actions and reactions between organ- 

 isms and their changing environments, add to the hetero- 

 geneity 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 obliterate the 

 effects previously wrought — must produce confusion or in- 

 definiteness. A little consideration, however, will dissipate 

 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- 

 stant forces; and doubtless, continual change of circum- 

 stances interferes with this. But the interference can never 

 be considerable. For the pre-existing structure of an organ- 

 ism prevents it from living under any new conditions except 



