204 SEXUAL SELECTION: BIRDS. [Part IL 



transmitted to one and the same sex, while those which 

 occur early in life are transmitted to both sexes), ap- 

 parently hold good in the first," second, and fourth classes 

 of cases ; but they fail in an equal number, namely, in the 

 third, often in the fifth," and in the sixth small class. 

 They hold good, however, as far as I can judge, with a 

 considerable majority of the species of birds. Whether 

 or not this be so, we may conclude from the facts given 

 in the eighth chapter that the period of variation has 

 been one important element in determining the form of 

 transmission. 



With birds it is difficult to decide by what standard 

 we ought to judge of the earliness or lateness of the period 

 of variation, whether by the age in reference to the du- 

 ration of life, or to the power of reproduction, or to the 

 number of moults through which the species passes. The 

 raoulting of birds, even within the same family, sometimes 

 differs 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 colors of the adult plumage were first 

 developed would falsely appear to us to have been earlier 



•^ For instance, the males of Tanagra cestiva and FringiUa cyanea re- 

 quire three years, the male of FrmgiUa ciris four years, to complete their 

 beautiful plumage. (See Audubon, ' Ornith. Biography,' vol. i. pp. 233, 

 280, 378.) The Harlequin duck takes three years (ibid. vol. iii. p. 614). 

 The male of the Gold pheasant, as I hear from Mr. J. Jenner Weir, can 

 be distinguished from the female when about three months old, but he 

 docs not acquire his full splendor until the end of the September in the 

 following year. 



^ Thus the Ibis taiilahis and Grus Amcricanus take four years, the 

 Flamingo several years, and the Ardea Ludovicana two years, before they 

 acquire their perfect plumage. See Audubon, ibid. vol. i. p. 221 ; vol. 

 iii. pp. 133, 139, 211. 



