398 Darwin, and after Darwin. 



ings. When not thus inflated, it hangs down, as 

 shown in the second figure, which represents the 

 plumage of a young male. (Fig. 124.) 



In another species of the genus there are three of these 

 appendages the two additional ones being mounted 

 on the corners of the mouth. (Fig. 12,5.) In all species 

 of the genus (four in number) the tubes are inflated 

 during courtship, and therefore perform the function 

 of sexual embellishments. Now the point to which I 

 wish to draw attention is, that so specialized and mor- 

 phologically elaborate a structure cannot be regarded 

 as merely adventitious. It must have been developed 

 by some definite cause, acting through a long series of 

 generations. And as no other function can be as- 

 signed to it than that of charming the female when it 

 is erected in courtship, the peculiarity of form and 

 mechanism which it presents like the elaboration of 

 patterns in cases where colour only is concerned 

 virtually compels us to recognise in sexual selection 

 the only conceivable cause of its production. 



For these reasons I think that Mr. Wallace's main 

 objection falls to the ground. Passing on to his sub- 

 sidiary objections, I do not see much weight in his 

 merely negative difficulty as to there being an absence 

 of evidence upon hen birds being charmed by the 

 plumage, or the voice, of their consorts. For, on the 

 one hand, it is not very safe to infer what sentiments 

 may be in the mind of a hen ; and, on the other hand, 

 it is impossible to conceive what motive can be in the 

 mind of a cock, other than that of making himself 

 attractive, when he performs his various antics, displays 

 his ornamental plumes, or sings his melodious songs. 

 Considerations somewhat analogous apply to the 



