48 CONTROLLED NATURAL SELECTION 



distributed over an unlimited space, then the 

 environment would be differentiated; but 

 because, in Nature, space is always limited, 

 this environmental classification only occurs 

 to a limited extent ; when a species is dis- 

 tributed over a wide area, when the environ- 

 mental gradient is very slight, then this 

 establishment of varieties and sub-species may 

 take place locally. 



The above use of such words as SPACE, 

 LOCALLY, AREA, &c. may give the impression 

 that distribution through space is alone 

 alluded to : these words are used with refer- 

 ence to the environmental gradient, whatever 

 be the factors of its composition. 



SPACE is chosen for illustration, for clear- 

 ness' sake. 



Varieties and sub-species may thus become 

 established by environmental differentiation, 

 and, it may be, evoluted into distinct species. 

 Thus, without the help of any outside force, 

 new species may arise within the enclosure of 

 a specific environment, as in Fig. 4. In a 

 similar way, the environment may become 

 sexually differentiated when the sexes are 

 separate, as in Fig. 5. Males often differ widely 

 in structure from females, and occupy an 

 entirely different environment. This sexual 



