THE SELECTING POWER OF ENEMIES 45 
environment, it is impossible to conceive of 
two species occupying the same environment : 
because one cannot conceive of two species 
which, being different in structure, are yet 
similar in function; which is the same as 
having similar relations to the same environ- 
ment. Different structures, different functions, 
different environments—here is an analogy ; 
but in different structures, similar functions, 
and the same environment—there is none. 
Allied species, or a species and an estab- 
lished variety of it, may occupy neighbouring 
environments, but never the same. [If allied 
species do not, distant species probably cannot. 
Thus a species must be alone in its specific 
environment, and never the subject of com- 
petition with other species. On account of the 
absence of competition and because a species 
increases in geometrical progression, the 
species must always completely fill its specific 
environment ; except when it has been unable 
to keep pace with an increase in quantity of 
its specific environment. A species is rare or 
common, according to whether its specific 
environment is restricted or unconfined. As 
will be shown later, it is not possible to 
conceive of a restricted common environment, 
and a surplus of specific environment. 
