624 THE DESCENT OF MAN 



success, endeavored, by consultiDg various works, to de- 

 cide how far the period of variation in birds has generally 

 determined the transmission of characters to one sex or to 

 both. The two rules, often referred to (namely, that vari- 

 ations occurring late in life are transmitted to one and the 

 same sex, while those which occur early in life are trans- 

 mitted to both sexes), apparently hold good in the first,"* 

 second, and fourth classes of cases; but they fail in the 

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

 They apply, however, as far as I can judge, to a consider- 

 able majority of the species, and we must not forget the 

 striking generalization oy Dr. W. Marshall with respect to 

 the protuberances on the heads of birds. Whether or not 

 the two rules generally 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 judge 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 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 col- 

 ors of the adult plumage are first developed will falsely ap- 

 pear to us to be earlier than it really is. This may be illus- 

 trated by the practice followed by some bird-fanciers, who 

 pull out a few feathers from the breast of nestling bull- 

 finches, and from the head or neck of young gold pheasants, 

 in order to ascertain their sex ; for in the males these f eath- 



^ For instance, the males of Tanagra cestiva and Pringitla cycmea require 

 three years, the male of Pringilla cms 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. Jenuer Weir, can be distinguished from 

 the female when about three months old, but he does not acquire his full 

 splendor until the end of the September in the following year. 



'* Thus the His tantalus and G^us americanua take four years, the Fla- 

 mingo several years, and the Ardea ludovicana two years, before they acquire 

 their perfect plumaga See Audubon, ibid., vol. i. p. 221; voL Iii. pp. 133, 

 139. 211. 



