52 CONTROLLED NATURAL SELECTION 



in this way, there will be evolution. Highly 

 evolved species will be found in highly com- 

 plex and often restricted environments ; such 

 species will be found to vary very little. 

 High specialisation demands great fixity of 

 structure. The local addition of a factor 

 will produce local evolution and the estab- 

 lishment of a sub-species. 



Reasons have been given why one cannot 

 conceive of a species being displaced from its 

 specific environment by the variety of another : 

 for similar reasons, one cannot conceive of a 

 variation of a species being able either to 

 occupy an environment rendered vacant by 

 the extinction of a species, or to discover an 

 entirely new environment. Evolution equals 

 environmental differentiation, the better ap- 

 preciation by organic matter of the diversities 

 of its environment. The question of com- 

 petition yet remains to be considered. As 

 has already been shown, this can only occur 

 within a common environment, and then 

 only when the common environment is not 

 sufficient to accommodate the two or more 

 species which share it. Suppose (see Fig. 6) 

 S x and S 2 are two species, E l and E 2 

 are their specific environments, and E 3 a 

 common environment, where competition is 



