Domestication, vol. ii. p. 79.) Cases apparently of this kind have
been observed with birds in a state of nature. For instance Mr. Blyth
has seen specimens of Lanius rufus and of Colymbus glacialis which had
assumed whilst young, in a quite anomalous manner, the adult plumage of
their parents. (31. Charlesworth s Magazine of Natural History, vol.
i. 1837, pp. 305, 306.) Again, the young of the common swan (Cygnus
olor) do not cast off their dark feathers and become white until
eighteen months or two years old; but Dr. F. Forel has described the
case of three vigorous young birds, out of a brood of four, which were
born pure white. These young birds were not albinos, as shewn by the
colour of their beaks and legs, which nearly resembled the same parts
in the adults. (32. Bulletin de la Soc. Vaudoise des Sc. Nat. vol. x.
1869, p. 132. The young of the Polish swan, Cygnus immutabilis of
Yarrell, are always white; but this species, as Mr. Sclater informs me,
is believed to be nothing more than a variety of the domestic swan
(Cygnus olor).)
It may be worth while to illustrate the above three modes by which, in
the present class, the two sexes and the young may have come to
resemble each other, by the curious case of the genus Passer. (33. I am
indebted to Mr. Blyth for information in regard to this genus. The
sparrow of Palestine belongs to the sub-genus Petronia.) In the
house-sparrow (P. domesticus) the male differs much from the female and
from the young. The young and the females are alike, and resemble to a
large extent both sexes and the young of the sparrow of Palestine (P.
brachydactylus), as well as of some allied species. We may therefore
assume that the female and young of the house-sparrow approximately
shew us the plumage of the progenitor of the genus. Now with the
tree-sparrow (P. montanus) both sexes and the young closely resemble
the male of the house-sparrow; so that they have all been modified in
the same manner, and all depart from the typical colouring of their
early progenitor. This may have been effected by a male ancestor of the
tree-sparrow having varied, firstly, when nearly mature; or, secondly,
whilst quite young, and by having in either case transmitted his
modified plumage to the females and the young; or, thirdly, he may have
varied when adult and transmitted his plumage to both adult sexes, and,
owing to the failure of the law of inheritance at corresponding ages,
at some subsequent period to his young.
It is impossible to decide which of these three modes has generally
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