212 
SEXUAL selection: bikds. 
Part II. 
and legs, wliich 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. These resemble 
each other, and likewise to a large extent both sexes 
and the young of the sparrow of Palestine (P. brachy- 
dadylus), 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, 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 
‘ 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 this- 
genus. The sparrow of Palestine belongs to the sub-genus Petronia.. 
