Chap. XVI. Birds — Inheritance Limited by Age. 4O3 



I conclude, therefore, that the habit of moulting twice in the 

 year was in most or all cases first acquired for some distinct 

 purpose, perhaps for gaining a warmer winter covering ; and that 

 variations in the plumage occurring during the summer were 

 accumulated through sexual selection, and transmitted to the off- 

 spring at the same season of the year ; that such variations were 

 inherited either by both sexes or by the males alone, according to 

 the form of inheritance which prevailed. This appears more 

 probable than that the species in all cases originally tended to 

 retain their ornamental plumage during the winter, but were 

 saved from this through natural selection, resulting from the 

 inconvenience or danger thus caused. 



I have endeavoured in this chapter to shew that the arguments 

 are not trustworthy in favour of the view that weapons, bright 

 colours, and various ornaments, are now confined to the males 

 owing to the conversion, by natural selection, of the equal trans- 

 mission of characters to both sexes, into transmission to the male 

 sex alone. It is also doubtful whether the colours of many 

 female birds are due to the preservation, for the sake of protec- 

 tion, of variations wiiich were from the first limited in their 

 transmission to the female sex. But it will be convenient to 

 defer any further discussion on this subject until I treat, in the 

 following chapter, of the differences in plumage between the 

 young and old. 



CHAPTEE XVI. 

 Bieds — concluded. 



The immature plumage in relation to the character of the plumage in 

 both sexes when adult — Six classes of cases — Sexual differences between 

 the males of closely-allied or representative species — The female as- 

 suming the characters of the male — Plumage of the voung in relation 

 to the summer and winter plumage of the adults — On the increase of 

 beauty in the birds of the world — Protective colouring — Conspicuously- 

 coloured birds — Novelty appreciated — Summary of the four chapters on 

 Birds. 



We must now consider the transmission of characters, as limited 

 by age, in reference to sexual selection. The truth and im- 

 portance of the principle of inheritance at corresponding ages 

 need not here be discussed, as enough has already been said on 

 the subject. Before giving the several rather complex rules or 

 classes of cases, under which the differences in plumage between 

 the young and the old, as far as known to me, may be included, 

 it will be well to make a few preliminary remarks. 



