208 SEXUAL selection: birds. PartIL 



the male ; and on this view, all tlie facts become clear; 

 for tlie males would probably be most charmed or ex- 

 cited by the females which were the most attractive to 

 them by their brighter colours, other ornaments, or 

 Tocal powers. Sexual selection would then soon 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 resemhles the adult 

 female, the tjoung ofhoth sexes have a i^ecidiar first plum- 

 age of their oivn. — In this class both sexes when adult 

 resemble each other, and differ from the vouno'. This 

 occurs with many birds of many Ivinds. The male robin 

 can hardly be distinguished from the female, but the 

 young are widely different with their mottled dusky- 

 olive and brown plumage. The male and female of the 

 splendid scarlet Ibis are alike, wliilst the young are 

 brown; and the scarlet-colour, though common to both 

 sexes, is apparently a sexual character, for it is not well 

 developed with birds under confinement, in the same 

 manner as often occurs in the case of brilliantly co- 

 loured male birds. AVith many species of herons the 

 young differ greatly from the adults, and their summer 

 plumage, though common to both sexes, clearly has 

 a nuptial character. Young swans are slate-coloured, 

 whilst the mature birds are pure white ; but it would be 

 superfluous to give additional instances. These differ- 

 ences between the young and the old apparently de- 

 pend, as in the two last classes, on tlie young having 

 retained a former or ancient state of plumage, which has 

 been exchanged for a new plumage by the old of both 

 sexes. When the adults are brightly coloured, we may 

 conclude from the lemarks just made in relation to the 

 scarlet ibis and to many herons, and from the analogy of 

 the species in the first class, that such colours have been 



