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. Ina 
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 
