88 CONTROLLED NATURAL SELECTION 
Finally, it will be well to classify secondary 
sexual characters in the light of this new 
theory. 
Mono-sEXUAL CHARACTERS : 
1. Characters that areadaptations to a mono- 
sexual environment ; for instance, associated 
with a food supply confined to one sex, with 
copulation, with the rearing of young, with 
the recognition or finding of one sex by the 
other, &c. 
2. Characters whose purpose is to control 
Natural Selection within the pair, value marks; 
for instance, attracting colour, sound, scent, 
&c. confined to the male. 
It must be clearly understood that the second 
cause of mono-sexual characters is, like the 
first, an environmental character. It is the 
result of selection by enemies, which selec- 
tion has been shown to be one of the most 
important factors of a species’ specific en- 
vironment. For this reason it should not, 
theoretically, be given a major heading in 
the classification of the causes of mono-sexual 
characters. There is, however, a side issue 
which to some extent justifies this arrange- 
ment ; for the species behaves towards enemies 
in a particular manner. 
It adjusts this selection by enemies so that, 
