212 SEXUAL selection: BIKDS. PartIL 



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. domestieus) the male differs much 

 from the female and from the young. These resemble 

 each other, and likewise to a large extent both sexes 

 and the young of the sparrow of Palestine (P. hrachy- 

 dadyhis), 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 efi'ected by a male ancestor of the tree- 

 sparrow having varied, firstly, when nearly mature, or, 

 secondly, whilst quite young, having in either case trans- 

 mitted 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. The belief that the males varied whilst young, 

 and transmitted their variations to their offspring of 



32 ' Bulletin de la Soc. Yaudoise des Sc. Nat/ vol. x. 1869, p. 132. 

 The young of the Polish swan, Cygnus immutahilis 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). 



33 I am indebted to Mr. Blyth for information in legard to this 

 genus. The sparrow of Palestine belongs to the sub-genus Petronia. 



