SEXUAL SELECTION 622 



alis which had assumed while young, in a quite anomalous 

 manner, the adult plumage of their parents." Again, the 

 young of the common swan {Cygnua 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 pui-e white. These young birds were not 

 albinos, as shown by the color of their beaks and legs, 

 which nearly resembled the same parts in the adults." 



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. '° 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 

 reseriible 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 show 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 nave all been modified in the same manner, and all 

 depart from the typical coloring of their early progeni- 

 tor. This may have been effected by a male ancestor of the 

 tree-sparrow having varied, first, when nearly mature; or, 

 secondly, while 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 while young, and transmit- 

 ted their variations to their offspring of both sexes, is the 

 most probable. I may here add that I have, with little 



" Charlesworth's "Mag. of Nat. Hist.," vol. i., 183t, pp. 305, 306. 



22 "Bulletin de la Soc. Vaudoise des Sc. BTat.," vol. x., 1869, p. 132. The 

 young of the Polish swan Gygnus vmmutabilis of Tarrell are always white; 

 but this species, as Mr. Sclater informs me, is believed to be nothing more than 

 a variety of the domestic swan ( Gygnus olor). 



^ I am indebted to Mr. Blyth for information in regard to this genus. The 

 sparrow of Palestine belongs to the sub-genus Petronia. 



