Chap. XVI. THE YOUNG LIKE BOTH ADULTS. 
213 
both sexes is perhaps the most probable. I may here 
add that I have endeavoured, with little success, by 
consulting various works, to decide how far with birds 
the period of variation 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 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 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 
34 For instance, the males of Tanagra sestiva and Fringilla cyanea 
require three years, the male of Fringilla ciris four years, to complete 
their beautiful plumage. (See Audubon, ‘ Ornith. Biography,’ vol. i. 
p. 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 does not acquire his full splendour until the end 
of the September in the following year. 
35 Thus the Ibis tantalus and Grus Americanus take four years, the 
Flamingo several years, and the Ardea Ludovicana two years, before 
dhey acquire their perfect plumage. See Audubon, ibid. vol. i. p. 221 ; 
^Yol. iii. p. 133, 139, 211. 
