(S^ The Descent of Man. Pj\itT 11 



hold good, WG iQay onnclu.le from the facts given in tbe eighth chapter, 

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

 form of transmission. 



With birds i1; is difficult to decide by what standard we onght tn judi;e 

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

 age in reference to tlie dnrntion of life, or to trie power of repioduetion, 

 or to tlie number of ninults throujih which the species passes. The 

 moaitiiig of biids, even wthin the same family, sometimes diif rs 

 much without any assignable cause. Some birds moult so e.irly. that 

 neiirly all the body feathers- are cast oiff before the first wing-featln rs 

 »re fully grown; and we cannut believe that this was the primordial 

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

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

 will frtlsely appear to us to be earlier than it really is. Th'S uiay be 

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

 out a few feathers from the briast of nestling bullfinches, an 1 frum the 

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

 for in the male,^, these feathers are immediately replaced by aJoured 

 ones.'° Tlie 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 whiUt retaining thtir 

 immature plumage." 



The fact of birds breeding in their immature plumage seems opposed 

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

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

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

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

 miles were as successful in winning females and propagating their 

 kind, as the (dder 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 Ihh tuntalus as a rare event, as does Mr. 

 Swinlioe, in regard to the immature males of Oriolus.^' If the young 

 of any species in their immature plumage were more successful in 

 winning partners than ttie adults, the adult plumage would probably 



" Mr. Blyth, in Charlesworth's 58): but Mr. Blyth informs ine 



* 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 



^^ I have noticed the following observed. The Harlequin duck 



cases in Audubon's ' Ornith. Bio- {Ariai histrionica, Linu.) takes three 



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



{Muscapica ruticiUa^ 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. pJialus, vol. iii. p. 210) is likewise 



.ii. p. 133). The Grus ainericanus 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 cseruUa are liue, and the 68) likewise breed before thej 



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



a-id mature blue birds may all be ^^ See the last foot-note, 

 i-vn breeding t<igether (vol. iv. p. 



