620 THE DESCENT OF MAN 



the females which were the most attractive to them by theit 

 bright colors, other ornaments, or vocal powers. Sexual 

 selection woald 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 (he adult male resembles the adult fs' 

 male, 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 and brown plu- 

 mage. The male and female of the splendid scarlet ibis are 

 alike, while the young are brown; and the scarlet color, 

 though common to both sexes, is apparently a sexual char- 

 acter, for it is not well developed in either sex under con- 

 finement; and a loss of color often occurs with brilliant 

 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 botSl 

 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 de- 

 pend, as in the last two classes, on the young having re- 

 tained 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 limited to 

 the same age, has not been limited to the same sex. Con- 

 sequently, the sexes when mature resemble each other and 

 differ from the young. 



