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. 
