202 SEXUAL SELECTION: BIRDS. [Part II. 



transmitted their 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 corre- 

 sponding ages fails, the offspring often inherit characters 

 at an earlier age than that at which they first appeared in 

 their parents. 3 " 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 while young, in a 

 quite anomalous manner, the adult plumage of their 

 parents. 31 Again, the young of the common swan (Cyg- 

 nus 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 albinoes, as shown by the 

 color of their beaks and legs, which nearly resembled the 

 same parts in the adults, 



32 



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

 p. 79. 



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



32 'Bulletin de la Soc. Vaudoise des Sc. Nat.' vol. x. 1869, p. 132. 

 The young of the Polish swan, Cygnus immulabilis 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). 



