My dear Wallace

“Conscious” seems to me much better than “voluntary”. Conscious action, I presume comes into play when 2 males fight for a female; but I do not know whether you admit that for instance the spur of the cock is due to sexual selection.²

I am quite willing to admit that the sounds & vocal organs of some males are used only for challenging, but I doubt whether this applies to the musical notes of Hylobates or the howling (I judge chiefly from Rengger) of the American monkeys.—³ In no account that I have seen of the stridulation of male insects shows that it is a challenge.— All those who have attended to birds consider their song as a charm to the females & not as a challenge. As the males in most cases search for the females I do not see how their odoriferous organs will aid them in finding the females.

But it is foolish in me to go on writing, for I believe I have said most of this in my book: anyhow I well remember thinking over it.— The “belling” of male stags, if I remember rightly, is a challenge, & so I daresay is the roaring of the lion during the breeding season.—⁴

I will just add in reference to your former letter that I fully admit with birds that the fighting of the males cooperates with their charms; & I remember quoting Bartlett that gaudy colouring in the males is almost invariably concomitant with pugnacity.⁵ But thank Heavens what little more I can do in science will be confined to observation on simple points. However much I may have blundered, I have done my best, & that is my constant comfort.—

Most truly yours C. Darwin