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. (Vig. 125.) Inall 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 conccivable 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 mclodious songs, 
Considerations somewhat analogous apply to the 
