160 PROBLEMS OF EVOLUTION 



bird. The female has, as a rule, but little share in the splendour 

 of plumage, and hardly an echo of the song. 



Mammals have comparatively little to boast of in the way of 

 colour, very probably because hair is far less suited to its display 

 than feathers. Still, the male sex has some special embellish- 

 ments. With monkeys, as with men, if there is a well-developed 

 beard, it is found only in the male. When there is any differ- 

 ence of voice, it is the male's that is the more powerful. These, 

 however, are minor distinctions. Selection among mammals is 

 mainly by battle and the secondary male characters are superior 

 strength and powerful horns or tusks. 



Most of the information given above and an enormous mass 

 besides, as to the difference between the sexes in the arthropods 

 and all that rank above them, is to be found in Darwin's Descent 

 of Man, in which sexual selection is discussed at great length. 

 The book is a mine of facts and proves, as do his other works 

 in support of the origin of species, that he never for the pleasure 

 of theorising desisted from the patient collection of materials. 

 Need of a It will hardly be disputed that a greater tendency to vary is 

 le characteristic of the male sex. This variability insures that there 

 are variations ready to hand, raw material in plenty. But some- 

 thing more is wanted. How is it that so much that is wonderful 

 in the way of weapons and ornaments has been manufactured out 

 of this ? We have to find some cumulative principle which could 

 gradually build up structures so large and elaborate. 

 Darwin's It is generally recognised that selection by battle accounts for 

 theory tne evolution of spurs in birds and of large canine teeth and 

 horns in mammals. But when we come to such male adorn- 

 ments as the fine plumes of the male bird of paradise, we 

 broach a subject around which controversy still continues. 

 Darwin contended that the female exercised a choice. The 

 male, whose antics were the most striking or whose endow- 

 ment in the way of plumage or voice was the richest, found 

 favour as a suitor. And so in each generation the most 

 brilliant or the best singers were selected. To make good 

 his contention, he had to show (l) that hen birds had at least 

 so much eye for colour and form that they could distinguish a 



