$84 The Descent of Man. Part II 



hold good, we may conclude from the facts given in the eighth chapter, 

 that the period of variation is one important element in determining the 

 form of transmission. 



With birds it is difficult to decide by what standard we ought to jud<?e 

 of the earliness or lateness of the period of variation, whether by the 

 age in reference to the duration of life, or to the power of reproduction, 

 or to the number of moults through which the species passes. The 

 moulting of birds, even within the same family, sometimes diffrs 

 much without any assignable cause. Some birds moult so early, that 

 nearly all the body feathers are cast off before the first wing-feathers 

 are fully grown; and we cannot believe that this was the primordial 

 state of things. When the period of moulting has been accelerated, 

 the age at which the colours of the adult plumage are first developed 

 will falsely appear to us to be earlier than it really is. This may be 

 illustrated by the practice followed by some bird-fanciers, who pull 

 out a few feathers from the breast of nestling bullfinches, and from the 

 head or neck of young gold-pheasants, in order to ascertain their sex ; 

 for in the males, these feathers are immediately replaced by coloured 

 ones. 36 The actual duration of life is known in but few birds, so that 

 we can hardly judge by this standard. And, with reference to the 

 period at which the power of reproduction is gained, it is a remark- 

 able fact that various birds occasionally breed whilst retaining their 

 immature plumage. 37 



The fact of birds breeding in their immature plumage seems opposed 

 to the belief that sexual selection has played as important a part, as I 

 believe it has, in giving ornamental colours, plumes, &c, to the males, 

 and, by means of equal transmission, to the females of many species. 

 The objection would be a valid one, if the younger and less ornamented 

 males were as successful in winning females and propagating their 

 kind, as the older and more beautiful males. But we have no reason 

 to suppose that this is the case. Audubon speaks of the breeding of 

 the immature males of Ibis tantalus as a rare event, as does Mr. 

 Swinhoe, in regard to the immature males of Oriolus. 38 If the young 

 of any species in their immature plumage were more successful in 

 winuing partners than the adults, the adult plumage would probably 



36 Mr. Blyth, in Charlesworth's 58): but Mr. Blyth informs me 

 ' Mag. of Nat. Hist.' vol. i. 1837, p. that certain herons apparently are 

 300. Mr. Bartlett has informed dimorphic, for white and coloured 

 me in regard to gold-pheasants. individuals of the same age may be 



37 I have noticed the following observed. The Harlequin duck 

 cases in Audubon's ' Ornith. Bio- (Anas histriunica, Linn.) takes three 

 graphy.' The redstart of America, years to acquire its full plumage, 

 [Muscapica ruticilla, vol. i. p. 203). though many birds breed in the 

 The Ibis tantalus takes four years to second year (vol. iii. p. 614). The 

 come to full maturity, but some- White-headed Eagle (Falco leucoce- 

 times breeds in the second year (vol. phalus, vol. iii. p. 210) is likewise 

 .ii. p. 133). The Grus americanus known to breed in its immature 

 takes the same time, but breeds state. Some species of Oriolus (ac- 

 before acquiring its full plumage cording to Mr. Blyth and Mr. 

 (vol. iii. p. 211). The adults of Swinhoe, in 'Ibis,' July 1863, p. 

 Ardea cserulea are Hue, and the 68) likewise breed before they 

 young white; and white, mottled, attain their full plumage. 



axtd mature blue birds may all be 38 See the last foot-note. 



s-'en breeding together (vol. iv. p. 



