SEXUAL COLOUR 79 
that this is so and that males in this way 
become altered in character. Such a male 
character must be either beneficial or detri- 
mental to the species as a whole; it cannot 
be neutral. If detrimental, the species must 
either become extinct, or Natural Selection 
will remove the offending character ; if bene- 
ficial, then Natural Selection would have 
fixed the male’s useful variation without the 
assistance of female selection. Characters, 
confined to the male, can be fixed by Natural 
Selection alone, and thus selection on the part 
of the female is not necessary to explain 
their origin. In order that females, by selec- 
tion, may alter the characters of males, two 
monosexual variations (one in the male, the 
other in the female) must arise simultaneously, 
and further, these must be related. For in- 
’ stance, a bright colour must arise as a varia- 
tion in the male, and at the same time there 
must be born in the female as a variation, 
an instinct which compels her to prefer this 
bright-coloured male. To conceive of this is 
difficult. 
On the other hand, Natural Selection would 
require only one variation to arise. If a 
beneficial character arises, it will become 
fixed, although the variation be confined to 
