550 THE DESCENT OF MAN. 



gradually acquired colors, either equally or almost equally, to their 

 offspring of both sexes. 



Another point is more doubtful, namely, whether the successive 

 variations first appeared in the males after they had become nearly 

 mature or while quite young. In either case sexual selection must 

 have acted on the male when he had to compete with rivals for the 

 possession of the female; and in both cases the characters thus 

 acquired have been transmitted to both sexes and all ages. But 

 these characters, if acquired by the males 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 corresponding 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 Golymbus glacialis which had 

 assumed while young, in a quite anomalous manner, the adult 

 plumage of their parents, f 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 

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

 four, which were born pure white. These young birds were not 

 albinos, 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. hrdchydactyhis), 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. mon- 

 ianus) 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- 

 genitors. This may have been effected by a male ancestor of the tree- 

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

 while 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 transmitted his plumage to both adult 

 sexes, and, owing to the failure of the law of inheritance at corre- 

 sponding ages, at some subsequent period to his young. 



It is impossible to decide which of these three modes has generally 



* " Variation of Animals and Plants under Domestication," vol. 11, p. 79. 



t Charlesworth's " Maj?. of Nat. Hist.," vol. i, 1837, pp. 30.5, 306. 



t " Bulletin de la Soc. Vaudoise des Sc. Nat.," vol. x, 1869, p. 132. The 

 young of the Polish swan, Cygnvs immvtafjUit^ of Yarrell, are always white; 

 out this species, as Mr. Sclater informs me, is believed to be nothing more than 

 a variety of the domestic swan {Cygnvs olar). 



I am indebted to Mr. Blyth for in formation in regard to this genus. The 

 sparrow of Palestine belongs to the sub-genus Petronia. 



