212 THE EVOLUTION THEOKY 



Still less will any one be disposed to regard the marvellous 

 splendour of the plumage of the male bird of Paradise, with its 

 erectile collars — glistening like burnished metal — on the neck, breast 

 or shoulders, with its tufts, with its specially decorative feathers 

 standing singly out from the rest of the plumage, on head, wings, or 

 tail, with its mane-like bunch of loose, pendulous feathers on the 

 belly and on the sides, in short, with its extraordinary, diverse, and 

 unique equipment of feathers, as a mere unintentional accessory effect 

 of a feather dress designed for flight and protective warmth. Such 

 conspicuous, diverse, and unusual specializations of plumage must 

 have some other significance than that just indicated. 



Alfred Russel Wallace regards these distinctive features of the 

 male as an expression of the greater vigour, and the more active 

 metabolism of the males, but it is unproved that the vigour of the 

 male birds is greater than that of the females, and it is not easy to see 

 why a more active metabolism should be necessary for the produc- 

 tion of strikingly bright colours than for that of a dark or protective 

 colour. Moreover, there are brilliantly coloured females, both among 

 birds and butterflies, and in nearly allied species the males may be 

 either gorgeous or quite plain like the females. 



Darwin refers the origin of these secondary sexual characters to 

 processes of selection quite analogous to those of ordinary natural 

 selection, only that in this case it is not the maintenance of the 

 species which is aimed at, but the attainment of reproduction by the 

 single individual. The males are to some extent obliged to struggle 

 for the possession of the females, and every little variation which 

 enables a male to gain possession of a female more readily than his 

 neighbour has for this reason a greater likelihood of being trans- 

 mitted to descendants. Thus, attractive variations which once crop up 

 will be transmitted to more and more numerous males of the species, 

 and among these it will always be those j^ossessing the character 

 in question in the highest degree which will have the best chance 

 of securing a mate, and so the character will continue to be 

 augmented as long as variations in this direction appear. 



Two kinds of preliminary conditions, however, must be assumed. 

 As the ordinary natural selection could never have operated but for 

 the fact that in every generation a great many individuals, indeed the 

 majority of them, perish before they have had time to reproduce, 

 so the process of sexual selection could never have come into 

 operation if every male were able ultimately to secure a mate, no 

 matter what degree of attractiveness to the latter he possessed. If 

 the numbers of males and females were equal, so that there was 



