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. Ina 
similar way, the environment may become 
sexually differentiated when the sexes are 
separate, asin Fig. 5. Males often differ widely 
in structure from females, and occupy an 
entirely different environment. This sexual 
