CiiAP. XVI.] THE YOUNG LIKE BOTH ADULTS. 203 



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-spar- 

 row (P. domesticus) the male differs much from the fe- 

 male 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. hrachydactylus)^ 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 have all been modified in the same manner, 

 and all depart from the typical coloring of their early pro- 

 genitor. This may have been effected by a male ancestor 

 of the tree-sparrow having varied, firstly, when nearly ma- 

 ture, or, secondly, while quite young, 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 correspondiag 

 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 while young, and 

 transmitted their variations to their offspring of both 

 sexes, is perhaps the most probable. I may here add that 

 I have endeavored, with little success, by consulting vari- 

 ous works, to decide how far with birds the period of vari- 

 ation has generally determined the transmission of charac- 

 ters to one sex or to both. The two rules, often referred 

 to (namely, that variations occurring late in life are 



2* I am indebted to Mr. Blyth for infonnation in regard to this genus. 

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



