BIRDS— YOUNG LIKE BOTH ADULTS. 477 



when adult, may have been transmitted at first to the adults 

 alone, and at some subsequent period transferred to the young. 

 For it is known that, when the law of inheritance at correspond- 

 ing ages fails, the offspring often inherit characters at an earlier 

 age than that at which they first appeared in their parents.'"' 

 Cases apparently of this kind have been observed with birds in 

 a state of nature. For instance Mr. Blyth has seen specimens of 

 Lanius rufus and of Colymbus glacialis which had assumed whilst 

 young, in a quite anomalous manner, the adult plumage of their 

 parents." Again, the young of the common swan (Cygnus olor) 

 do not cast off their dark feathers and become white until 

 eighteen months or two years old; but Dr. F. Forel has de- 

 scribed the case of three vigorous young birds, out of a brood 

 of four, which were born pure white. These young birds were 

 not albinoes, 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 resemble to a large extent both sexes and 

 the young of the sparrow of Palestine (P. br^chydactylus), as well 

 as some allied species. We may therefore assume that the female 

 and young of the house-sparrow approximately show us the plu- 

 mage 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 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, 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 trans- 

 mitted his plumage to both adult sexes, and, owing to the failure 

 of the law of inheritance at corresponding ages, at some subse- 

 quent period to his young. 



It is impossible to decide which of these three modes has gen- 



30 'Variation of Animals and Plants under Domestication,' vol. ii. 

 p. 79. 



31 Charlesworth's 'Mag-, of Nat. Hist.' vol. i. 1837, pp. 305, 306. 



^'- 'Bulletin de la Soo. Vaudolse des Sc. Nat.' vol. x. 1SG9, p. 132. The 

 young of the Polish swan Cygnus Immutahills 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). 



^ 1 am indebted to Mr. Blyth for information in regard to this 

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



