The Descent of Man

e 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.