548 THE DESCENT OF MAN. 



female, tlie young of both sexes have a peimliar first phimage 

 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 and brown plum- 

 age. The male and female of the splendid scarlet 

 ibis are alike, wliile the young are brown; and the scarlet 

 color, though common to both sexes, is apparently a sexual 

 character, for it is not well developed in either sex nnder 

 confinement ; and a loss of color often occurs with brill- 

 iant males when they are confined. With many species of 

 herons the young differ greatly from the adults; and the 

 summer plumage of the latter, though common to both 

 sexes, clearly has a nuptial character. Young swans are 

 slate-colored, while the mature birds are pure white; but it 

 would be superfluous to give additional instances. These 

 differences between the young and the old apparently 

 depend, as in the last two classes, on the young having 

 retained a former or ancient state of plumage, while the old 

 of both sexes have acquired a new one. When the adults 

 are bright colored, we may conclude from the remarks 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 

 colors have been acquired through sexual selection by the 

 nearly mature males ; but that, differently from what 

 occurs in the first two classes, the transmission, though lim- 

 ited to the same age, has not been limited to the same sex. 

 Consequently, the sexes when mature resemble each other 

 and differ from the young. 



Class IV. When the adult male resembles the adult 

 female, the young of both sexes in their first plumage resem- 

 ble the adults. In this class the young and the adults of 

 both sexes, whether TDrilliantly or obscurely colored, resem- 

 ble each other. Such cases are, I think:, more common 

 than those in the last class. We have in England instances 

 in the kingfisher, some woodpeckers, the jay, magpie, 

 crow, and many small dull-colored birds, such as the 

 hedge-warbler or kitty-wren. But the similarity in plum- 

 age between the young and the old is never complete, and 

 graduates away into dissimilarity. Thus the young of 

 some members of the kingfisher family are not only les 



