212 
SEXUAL SELECTION: BIRDS. 
Part ‘ 
and legs, which nearly resembled the same parts 
the adults . 32 
It may he worth while to illustrate the above tbre® 
modes by which, in the present class, the two se^ 1 ” 
and the young may have come to resemble each otlif' 1 '' 
by the curious case of the genus Passer . 33 In ^ 
house-sparrow (P. domesticus ) the male differs ro« c ^ 
from the ternale and from the young. These resendd® 
each other, and likewise to a large extent both sex e * 
and the young of the sparrow of Palestine (P. bracM' 
daetylm), as well as ot some allied species. We m !l < 
therefore assume that the female and young of ^ e , 
house-sparrow approximately shew us the plumage 
the progenitor of the genus. Now with the tree-spart'°" r 
(P. montanus) both sexes and the young closely resend 1 ' 1 ' 
the male of the house-sparrow; so that they have 11 
been modified in the same manner, and all depart f 1 ' 0 ^ 1 
the typical colouring of their early progenitor. 
may have been effected by a male ancestor of the tre e * 
sparrow having varied, firstly, when nearly mature, ° r ’ 
secondly, whilst quite young, having in either case tra 1)S ' 
mitted his modified plumage to the females and & 0 
young ; or, thirdly, he may have varied when adult ^ 
transmitted his plumage to both adult sexes, and, 
to the failure of the law of inheritance at correspond' 11 ^ 
ages, at some subsequent period to his young. 
It is impossible to decide which of these three 
has generally prevailed throughout the present class 0 
cases. The belief that the males varied whilst young’ 
and transmitted their variations to their offspring 0 
32 ‘ Bulletin <te la See. Vaudoise des So. Nat,.’ vol. x. ISGO, !>• ^ 
The young of the Polish swan, Cygnua immutahilis of Yarrell’ "j 
always white; but this species, as Mr. Sclater informs me, is b<h e ' , 
to be nothing more than a variety of the Domestic Swan (C^«« s 
33 I am indebted to Mr. Blyth for information in iegard to 
genus. The sparrow of Palestine belongs to the sub-genus Petrol' 
