26 ORGANIC EVOLUTION 



individuals of a single species, fitted as they are for life 

 under the same conditions, enter into the most constant and 

 the most severe rivalry. We may state this fact in another 

 form by saying that the struggle for existence is most severe 

 between near relatives. Now see what is the effect of this. 

 We have a group of individuals belonging to the same 

 species. Between them the competition is more severe than 

 is the rivalry between themselves and any other forms. If 

 now there arise among them individuals that diverge, so as 

 to fit them to occupy a place slightly different from that 

 occupied by the parent stock, this will allow the divergent 

 forms to withdraw a little from the place where competition 

 is most severe, and so will give them a better chance for sur- 

 vival. We see the tendency is constantly toward divergence, 

 since divergence lessens the severity of the competition 

 for life. Variations which arise, if they enable their possess- 

 ors slightly to change their habit of life, will tend to be 

 preserved, even though the place to which the divergent 

 individuals migrate is, in itself, no better than the one they 

 leave. This, we see, may materially affect the result of the 

 process of evolution, causing forms to survive which other- 

 wise would not be chosen. 



Evolution, so far as it is dependent upon natural selec- 

 tion, is more rapid while the environment is changing than 

 it is under stable environmental conditions. By the con- 

 tinued action of natural selection animals and plants become 

 so well adjusted to their environment that while this remains 

 unchanged they undergo comparatively little modification ; 

 but when the environment is changing the plants and ani- 

 mals must change with it, if they are to be well adapted to 



