554 THE DESCENT OF MAN. 



by the males during the breeding-season having been limited intheiv 

 transmission to the corresponding season. When the adults have v 

 distinct summer and winter plumage, an t the young differ from both, 

 the case is more difficult to understand. We may admit as probable 

 that the young have retained an ancient state of plumage; we can 

 account by sexual selection for the summer or nuptial plumage of the 

 adults, but how are we to account for their distinct winter plumage ? 

 If we could admit that this plumage serves in all cases as a protec- 

 tion, its acquirement would be a simple affair; but there seeins no 

 good reason for this admission. It may be suggested that the 

 widely different conditions of life during the winter and summer 

 have acted in a direct manner on the plumage ; this may have had 

 some effect, but I have not much confidence in so great a difference 

 as we sometimes see between the two plumages having been thus 

 caused. A more probable explanation is, that an ancient style of 

 plumage, partially modified through the transference of some char- 

 acters from the summer plumage, has been retained by the adults 

 during the winter. Finally, all the cases in our present class appa- 

 rently depend on characters acquired by the adult males having 

 been variously limited in their transmission according to age, season, 

 and sex; but it would not be worth while to attempt to follow out 

 these complex relations. 



CLASS VI. The young in their first plumage differ from each other 

 according to sex; the young males resembling more or less closely the 

 adult males, and the young females more or less closely the adult 

 females. The cases in the present class, though occurring in various 

 groups, are not numerous; yet it seems the most natural thing that 

 the young should at first somewhat resemble the adults of the same 

 sex, and gradually become more and more like them. The adult 

 male blackcap (Sylvia atricapilla} has a black head, that of the 

 female being reddish-brown; and I am informed by Mr. Blyth, that 

 the young of both sexes can be distinguished by this character even 

 as nestlings. In the family of thrushes an unusual number of simi- 

 lar cases have been noticed ; thus, the male blackbird (Turdus 

 merula) can be distinguished in the nest from the female. The two 

 sexes of the mocking-bird (Turdus polyglottus, Linn.) differ very little 

 from each other, yet the males can easily be distinguished at a very 

 early age from the females by showing more pure white.* The 

 males of a forest-thrush and of a rock-thrush (Orocetes erythrogastra 

 and Petrocincla cyanea) have much of their plumage of a fine blue, 

 while the females are brown; and the nestling males of both species 

 have their main wing and tail feathers edged with blue, while those 

 of the female are edged with brown. f In the young blackbird the 

 wing-feathers assume their mature character and become black after 

 the others; on the other hand, in the two species just named the 

 wing- feathers become blue before the others. The most probable 

 view with reference to the cases in the present class is that the males, 



* Audubon " Ornith. Biography," vol. i, p. 113. 



tMr. C. A. Wright, in "Ibis," vol. vi. 1864, p. 65. Jerdon, "Birds of 

 India," vol. i, p. 515. See also on the blackbird, Blyth, 'In Charles worth's 

 " Mag. of Nat. History/' vol. i, 1837, p. 113. 



