478 THE DESCENT OP MAN. 



erally prevailed throughout the present class of cases. That the 

 males varied whilst young, and transmitted their variations to 

 their offspring of both sexes, is the most probable. I may here 

 add that I have, with little success, endeavored, by consulting 

 various works, to decide 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 

 variations occurring late in life are transmitted to one and the 

 same sex, whilst those which occur early in life are transmitted 

 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 considerable majority of the species; and we 

 must not forget the striking generalization by 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 transmis- 

 sion. 



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 feath- 

 ers 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 are first developed 

 will falsely appear to as 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 imme- 



'* For instance, the males of Tanagra aestiva and Frlngilla cyanea 

 require three years, the male of Fringrilla ciris four years, to com- 

 plete their beautiful plumage. (See Audubon, 'Ornith. Biography,' vol. 

 i. pp, 233, 280, 378.) The Harlequin duck takes three years (ibid. vol. 

 ill. p. 614). The male of the Gold pheasant, as I hear from Mr. Jenner 

 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 Ibis tantalus and Grus americanus take four years, 

 the Flamingo several years, and the Ardea ludovlcana two years, 

 before they acquire their perfect plumage. See Audubon, ibid. vol. i. 

 p. 221; vol. iii. pp. 133, 139, 211. 



