I am so much pleased about the male musk Callichroma; for by odd chance I told Frank a week ago that next spring he must collect at Cambridge lots of Cerambyx moschatus, for as sure as life he would find the odour sexual!

You will be pleased to hear that I am undergoing severe distress about protection and sexual selection:  this morning I oscillated with joy towards you; this evening I have swung back to the old position, out of which I fear I shall never get.

I did most thoroughly enjoy my talk with you three gentlemen, and especially with you, and to my great surprise it has not knocked me up.  Pray give my kindest remembrances to Mrs. Wallace, and if my wife were at home she would cordially join in this.—­Yours very sincerely,

CH.  DARWIN.

I have had this morning a capital letter from Walsh of Illinois; but details too long to give.

* * * * *

Among Wallace’s papers was found the following draft of a letter of his to Darwin: 

9 St. Mark’s Crescent, N.W.  September 18, 1868.

Dear Darwin,—­The more I think of your views as to the colours of females, the more difficulty I find in accepting them, and as you are now working at the subject I hope it will not interrupt you to hear “counsel on the other side.”

I have a “general” and a “special” argument to submit.

1.  Female birds and insects are generally exposed to more danger than the male, and in the case of insects their existence is necessary for a longer period.

2.  They therefore require in some way or other a special balance of protection.

3.  Now, if the male and female were distinct species, with different habits and organisations, you would, I think, at once admit that a difference of colour serving to make that one less conspicuous which evidently required more protection than the other had been acquired by Natural Selection.

4.  But you admit that variations appearing in one sex are transmitted (often) to that sex only:  there is therefore nothing to prevent Natural Selection acting on the two sexes as if they were two species.

5.  Your objection that the same protection would to a certain extent be useful to the male, seems to me utterly unsound, and directly opposed to your own doctrine so convincingly urged in the “Origin,” “that Natural Selection never can improve an animal beyond its needs.”  So that admitting abundant variation of colour in the male, it is impossible that he can be brought by Natural Selection to resemble the female (unless her variations are always transmitted to him), because the difference of their colours is to balance the difference in their organisations and habits, and Natural Selection cannot give to the male more than is needed to effect that balance.

6.  The fact that in almost all protected groups the females perfectly resemble the males shows, I think, a tendency to transference of colour from one sex to the other when this tendency is not injurious.