208 
SEXUAL SELECTION: BIEDS. 
Part II. 
the male ; and on this view, all the facts become clear ; 
for the males ivould probably be most charmed or ex- 
cited by the females which were the most attractive to 
them by their brighter colours, other ornaments, or 
vocal 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 resembles the adult 
female, the young of loth sexes have a jgeeuliar first plum- 
age of their own, — In this class both 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 plumage. The male and female of the 
splendid scarlet Ibis are alike, whilst 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. With 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 the 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 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 colours have been 
