86 EVOLUTION OP LIVING OKGANISMS. 



appear only on those parts which rjre displayed in 

 courtship. 



Much scepticism has been shown concerning the effi- 

 cacy of sexual selection as a factor in evolution, chiefly 

 on the ground that the theory seems to presume an 

 aesthetic taste and power of choice in the female. But 

 this criticism is due, at all events to a great extent, to a 

 misunderstanding of the metaphorical language in which 

 it is convenient, if not necessary, to describe such facts. 

 Strictly speaking, "choice" and "taste" are but words 

 to express the fact that the female is more stimulated by 

 one kind of form, colour, scent or sound than by another. 

 In physiological language it is all a matter of stimulus 

 and response ; those males will succeed best which most 

 effectually stimulate the females. Doubtless in many 

 cases, here as elsewhere, the facts are but incompletely 

 known ; but in others the evidence is convincing enough. 

 We will here mention only one instance, that of the 

 spiders of the family Attidse, so thoroughly studied by 

 Mr. and Mrs. Peckham. These very competent observers 

 show clearly, that when the sexes are differentiated the 

 males are the more brilliantly coloured ; that the young 

 male resembles the adult female, and that it is the male 

 that has departed from the ordinary ancestral colouring ; 

 that the male colours are visible and displayed during 

 courtship ; that the females pay atteution to them and 

 exercise a " choice " ; and lastly, that the more brilliant 

 males may be selected again and again. Moreover, it 

 must be remembered that sexual selection is known to 

 occur in the pairing of the human species, and that its 

 selective value has been statistically estimated. 



Wallace has pointed out that natural selection must 

 often compel the female to keep to a more modest and 

 protective type of coloration than the male ; for, until 

 she has laid her eggs or reared her young, a female may 

 be more essential for the propagation of the race than a 

 particular male ; and there can be no doubt that pro- 



