THE SELECTING POWER OF ENEMIES 57 



selection of common environments is, re 

 evolution and the origin of species, of much 

 less importance than the selection of specific 

 environments. 



Specific environments can often be recog- 

 nised by their being usually restricted. The 

 environment which restricts a species is a 

 part of its specific environment. This will 

 be found to be in the vast majority of cases 

 either food supply or enemies. Food supply 

 and enemies are closely connected ; if one is im- 

 portant, both must be. A is the food supply 

 of B : B is the enemy of A. The greatest 

 disturbances which man has made in Nature 

 have been due to alterations in one of these 

 two factors. The two most efficient ways of 

 either exterminating a species or controlling 

 it are the destruction of its food supply or the 

 increase of its enemies. In Nature's balance 

 the weights are these two factors. 



Isolation is probably the greatest deter- 

 mining factor in the preservation of species. 

 It ensures protection from fresh enemies. 

 Migration the greatest determining factor in 

 the destruction of species. It introduces new 

 enemies. Species with many enemies are the 

 least variable. Protection from enemies causes 

 species to vary, as in domesticated animals. 



