L56 SEXUAL SELECTION: BIRDS. [Part II 



fully-developed spurs, we may conclude that all the succes- 

 sive variations were transferred to them; and that they 

 gradually acquired the inherited habit of not disturbing 

 their nests. 



The vocal organs and the variously-modified feathers 

 for producing sound, as well as the proper instincts for 

 using them, often differ in the two sexes, but are sometimes 

 the same in both. Can such differences be accounted for 

 by the males having acquired these organs and instincts ? 

 while the females have been saved from inheriting them, 

 on account of the danger to which they would have been 

 exposed by attracting the attention of birds or beasts of 

 prey ? This does not seem to me probable, when we think 

 of the multitude of birds which with impunity gladden the 

 country with their voices during the spring. 8 It is a safer 

 conclusion that as vocal and instrumental organs are of spe- 

 cial service only to the males during their courtship, these 

 organs were developed through sexual selection and con- 

 tinued use in this sex alone — the successive variations and 

 the effects of use having been from the first limited in their 

 transmission in a greater or less degree to the male off- 

 spring. % 



Many analogous cases could be advanced ; for instance, 

 the plumes on the head, which are generally longer in the 

 male than in the female, sometimes of equal .length in both 

 sexes, and occasionally absent in the female — these several 

 cases sometimes occurring in the same group of birds. It 

 would be difficult to account for a difference of this kind 

 between the sexes on the principle of the female having 

 been benefited by possessing a slightly shorter crest than 



6 Daines Barrington, however, thought it probable (' Phil. Transact.' 

 1Y73, p. 164) that few female birds sing, because the talent would have 

 been dangerous to them during .incubation He adds, that a similar 

 view may possibly account for the inferiority of the female to the male 

 in plumage. 



