4-8o The Descent of Man. Part II. 



males in the last class. Mr. Wallace believes that the males 

 have had their colours rendered less conspicuous for the sake of 

 protection during the period of incubation ; but the difference 

 between the sexes in hardly any of the foregoing cases appears 

 sufficiently great for this view to be safely accepted. In some 

 of the cases, the brighter tints of the female are almost confined 

 to the lower surface, and the males, if thus coloured, would not 

 have been exposed to danger whilst sitting on the eggs. It 

 should also be borne in mind that the males are not only in a 

 slight degree less conspicuously coloured than the females, but are 

 smaller and weaker. They have, moreover, not only acquired 

 the maternal instinct of incubation, but are less pugnacious and 

 vociferous than the females, and in one instance have simpler 

 vocal organs. Thus an almost complete transposition of the 

 instincts, habits, disposition, colour, size, and of some points of 

 structure, has been effected between the two sexes. 



Now if we might assume that the males in the present class 

 have lost some of that ardour which is usual to their sex, so that 

 they no longer search eagerly for the females ; or, if we might 

 assume that the females have become much more numerous 

 than the males — and in the case of one Indian Turnix the females 

 are said to be "much more commonly met with than the 

 " males." 26 — then it is not improbable that the females would 

 have been led to court the males, instead of being courted by 

 them. This indeed is the case to a certain extent with some 

 birds, as we have seen with the peahen, wild turkey, and certain 

 kinds of grouse. Taking as our guide the habits of most male 

 birds, the greater size and strength as well as the extraordinary 

 pugnacity of the females of the Turnix and emu, must mean 

 that they endeavour to drive away rival females, in order to gain 

 possession of the male; and on this view all the facts become 

 clear; for the males would probably be most charmed or ex- 

 cited by the females which were the most attractive to them by 

 their bright colours, other ornaments, or vocal powers. Sexual 

 selection would then do its work, steadily adding to the attrac- 

 tions of the females; the males and the young being left not at 

 all, or but little modified. 



Class III. When the adult male resembles the adult female, the 

 young of both sexes have a peculiar first plumage of their own. — In 

 this class the sexes when adult resemble each other, and differ 

 from the young. This occurs with many birds of many kinds. 

 The male robin can hardly be distinguished from the female, 

 but the young are widely different, with their mottled dusky- 

 olive ami brown plumage. The male and female of the splendid 



26 Jerdon, ' Birds of India,' vol. iii. p. 598. 



