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 



