you have so admirably (as I believe it will prove) explained.  I have got one capital case (genus forgotten) of an [Australian] bird in which the female has long-tailed plumes and which consequently builds a different nest from all her allies.[60] With respect to certain female birds being more brightly coloured than the males, and the latter incubating, I have gone a little into the subject and cannot say that I am fully satisfied.  I remember mentioning to you the case of Rhynchaea, but its nesting seems unknown.  In some other cases the difference in brightness seemed to me hardly sufficiently accounted for by the principle of protection.  At the Falkland Islands there is a carrion hawk in which the female (as I ascertained by dissection) is the brightest coloured, and I doubt whether protection will here apply; but I wrote several months ago to the Falklands to make inquiries.  The conclusion to which I have been leaning is that in some of these abnormal cases the colour happened to vary in the female alone, and was transmitted to females alone, and that her variations have been selected through the admiration of the male.

It is a very interesting subject, but I shall not be able to go on with it for the next five or six months, as I am fully employed in correcting dull proof-sheets; when I return to the work I shall find it much better done by you than I could have succeeded in doing.

With many thanks for your very interesting note, believe me, dear Wallace, yours very sincerely,

CH.  DARWIN.

It is curious how we hit on the same ideas.  I have endeavoured to show in my MS. discussion that nearly the same principles account for young birds not being gaily coloured in many cases—­but this is too complex a point for a note.

Postscript.  Down.  April 29.

My dear Wallace,—­On reading over your letter again, and on further reflection, I do not think (as far as I remember my words) that I expressed myself nearly strongly enough as to the value and beauty of your generalisation, viz. that all birds in which the female is conspicuously or brightly coloured build in holes or under domes.  I thought that this was the explanation in many, perhaps most cases, but do not think I should ever have extended my view to your generalisation.  Forgive me troubling you with this P.S.—­Yours,

CH.  DARWIN.

* * * * *

Down, Bromley, Kent, S.E.  May 5, 1867.

My dear Wallace,—­The offer of your valuable notes is most generous, but it would vex me to take so much from you, as it is certain that you could work up the subject very much better than I could.  Therefore I earnestly and without any reservation hope that you will proceed with your paper, so that I return your notes.