174 THE DESCENT OP MAN. 



immature plumage resembling the adult males instead of the 

 adult females, as in the previous class, are not numerous, though 

 they are distributed in various Orders. The amount of differ- 

 ence, also, between the sexes is incomparably less than that 

 which frequently occurs in the last class; so that the cause of 

 the difference, whatever it may have been, has here acted on the 

 females either less energetically or less persistently than on the 

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

 have had their colors 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 colored, 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 colored 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 in- 

 stincts, habits, disposition, color, 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 ardor 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. "^° — 

 then it is not improbable that the females would have been led 

 to court the males, instead of being courted by them. This in- 

 deed is the case to a certain extent with some birds, aa 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 endeavor 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 excited by the females which were 

 the most attractive to them by their bright colors, other orna- 

 ments, or vocal powers. Sexual selection would then do its work, 

 steadily adding to the attractions 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 



» Jerdon, 'Birds of India,' vol. ili. p. 598. 



