THE SELECTING POWER OF ENEMIES 55 



long protracted between individuals as dif- 

 ferent in structure as are two species? If 

 it cannot, then such a struggle can have no 

 effect as regards evolution, because a rela- 

 tively long period of time is required both for 

 the origin and fixation of variations. 



Two caterpillars, A and B, feed upon a 

 certain tree ; the tree becomes rare ; competi- 

 tion between A and B at once begins. In a 

 few years A will displace and render B 

 extinct, or vice versa ; because one must pos- 

 sess certain characters, structures, and habits 

 which give it a very decided advantage in 

 the struggle ; or A will be able to turn out B, 

 for instance, from the young leaves, and B 

 turn out A from the old leaves ; A will be 

 found feeding only on young leaves, B on 

 old there will be splitting of the common 

 environment. 



In neither case can this competition in a 

 common environment be a factor in evolution. 



Before proceeding further it is necessary to 

 review the conclusions that have been made. 



1. The boundaries of the environment of a 

 species are definite. 



2. The competition between species and 

 species leads to either extinction of one or 

 the splitting of the common environment into 



