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. 



