were not albinoes, as shewn by the colour of their beaks and legs, which nearly resembled the same parts in the adults.[1]
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.[2] 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 sparroow 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 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,[3] second, and fourth classes of cases; but they fail in the third, often in the fifth,[4] 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
- ↑ '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)
- ↑ I am indebted to Mr. Blyth for information in regard to genus. The sparrow of Palestine belongs to the sub-genus Petronia.
- ↑ For instance, the males of Tanagra æstica 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. 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.
- ↑ 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.
