of Darwinians to take the question up and work it out thoroughly.  You have brought such a mass of facts to support your view, and have argued it so fully, that I hardly think it necessary for you to do more.  Truth will prevail, as you as well as I wish it to do.  I will only make one or two remarks.  The word “voluntary” was inserted in my proofs only, in order to distinguish clearly between the two radically distinct kinds of “sexual selection.”  Perhaps “conscious” would be a better word, to which I think you will not object, and I will alter it when I republish.  I lay no stress on the word “voluntary.”

Sound- and scent-producing organs in males are surely due to “natural” or “automatic” as opposed to “conscious” selection.  If there were gradations in the sounds produced, from mere noises, up to elaborate music—­the case would be analogous to that of “colours” and “ornament.”  Being, however, comparatively simple, Natural Selection, owing to their use as a guide, seems sufficient.  The louder sound, heard at a greater distance, would attract or be heard by more females, or it may attract other males and lead to combats for the females, but this would not imply choice in the sense of rejecting a male whose stridulation was a trifle less loud than another’s, which is the essence of the theory as applied by you to colour and ornament.  But greater general vigour would almost certainly lead to greater volume or persistence of sound, and so the same view will apply to both cases on my theory.

Thanks for the references you give me.  My ignorance of German prevents me supporting my views by the mass of observations continually being made abroad, so I can only advance my own ideas for what they are worth.

I like Dorking much, but can find no house to suit me, so fear I shall have to move again.

With best wishes, believe me yours very faithfully,

ALFRED R. WALLACE.

* * * * *

Down, Beckenham, Kent.  September 5, [1877].

My dear Wallace,—­“Conscious” seems to me much better than “voluntary.”  Conscious action, I presume, comes into play when two 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.

I am quite willing to admit that the sounds and vocal organs of some males are used only for challenging, but I doubt whether this applies to the musical notes of Hylobates or to the howling (I judge chiefly from Rengger) of the American monkeys.  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 and 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, and so I daresay is the roaring of the lion during the breeding season.