482 The Descent of Man. 1\art 11. 



I may remark before proceeding that, rmdei the present and 

 next two classes of cases, the facts are so complex and the con- 

 clusions so doubtful, that any one who feels no especial interest 

 in the subject had better pass them over. 



The brilliant or conspicuous colours which characterise many birds 

 in the present class, ca:i rarely or never be of service to theni as a pro- 

 tection ; so that they have probably been .trained by the males through 

 st xual selection, and then transferred to the females and the young. 

 It is, however, possible that the males may have selected the more 

 attractive females ; and if these transmitted their characters to their 

 offspring of both sexes, the same results would follow as from the 

 selection of the more attractive males by the females. But there is 

 evidence that tliis contingency has rarely, if ever, occurred in any of 

 those groups of birds in which the sexes are generally alike ; for, if 

 even a few of the successive variations had failed to be transmitted to 

 both sexes, the females would have slightly exceeded the males in 

 btauty. Exactly the reverse occurs under nature; for, in almost every 

 large group in which the sexes generally resemble each other, the 

 males of some few species are in a slight degree more brightly cloured 

 than the females. It is again possible that the females may have 

 selected the more beautiful males, these malts having reciprocally 

 selected the more beautiful females; but it is doubtful whether this 

 double process of selection would be likely to occur, owing to the 

 greater eagerness of one sex than the other, and whether it would be 

 more efficient than selection on one sfle alone. It is, therefore, the 

 most probable view that sexual selection h is acted, in the present class, 

 as far as ornamental characters are concerned, in accordance with the 

 geneial rule throughout the animal kingdom, that is, on the males; 

 and that these have transmitted their gradually-acquired colours, 

 ei her 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 whilst 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. 30 

 Cases apparently of ibis kind have been observed with birds in a state 

 of nature. For instance Mr. Blyth has seen specimens of Lanius rufus 

 and of Colymbus cjlacialis which ha 1 assumed whilst young, in a quite 

 anomalous manner, the adult plumage of their parents. 31 Again, the 

 young of the common swan (Cycjnus olor) do not cast off their dark 

 feathers and become white until eighteen mouths 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 



30 'Variation of Animals and '' Charlesworths' ' Mag. of Nat. 



Plants under Domestication ' vol. ii. Hi .' vol. i. 1837. pp. 305, 306. 

 p 79 



