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, endeavoured, 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
(34. For instance, the males of Tanagra aestiva and Fringilla cyanea
require three years, the male of Fringilla 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.
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.), second, and fourth classes of
cases; but they fail in the third, often in the fifth (35. Thus the
Ibis tantalus and Grus americanus 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.), 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 generalisation 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 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 colours of the adult plumage are
first developed will falsely appear to us to be earlier than it really
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