Down, Bromley, Kent, S.E.  April 15, 1868.

My dear Wallace,—­I have been deeply interested by your admirable article on Birds’ Nests.  I am delighted to see that we really differ very little—­not more than two men almost always will.  You do not lay much or any stress on new characters spontaneously appearing in one sex (generally the male) and being transmitted exclusively, or more commonly only in excess, to that sex.  I, on the other hand, formerly paid far too little attention to protection.  I had only a glimpse of the truth.  But even now I do not go quite as far as you.  I cannot avoid thinking rather more than you do about the exceptions in nesting to the rule, especially the partial exceptions, i.e. when there is some little difference between the sexes in species which build concealed nests.  I am now quite satisfied about the incubating males; there is so little difference in conspicuousness between the sexes.  I wish with all my heart I could go the whole length with you.  You seem to think that such birds probably select the most beautiful females:  I must feel some doubt on this head, for I can find no evidence of it.  Though I am writing so carping a note, I admire the article thoroughly.

And now I want to ask a question.  When female butterflies are more brilliant than their males, you believe that they have in most cases, or in all cases, been rendered brilliant so as to mimic some other species and thus escape danger.  But can you account for the males not having been rendered equally brilliant and equally protected?  Although it may be most for the welfare of the species that the female should be protected, yet it would be some advantage, certainly no disadvantage, for the unfortunate male to enjoy an equal immunity from danger.  For my part, I should say that the female alone had happened to vary in the right manner, and that the beneficial variations had been transmitted to the same sex alone.  Believing in this, I can see no improbability (but from analogy of domestic animals a strong probability):  the variations leading to beauty must often have occurred in the males alone, and been transmitted to that sex alone.  Thus I should account in many cases for the greater beauty of the male over the female, without the need of the protective principle.  I should be grateful for an answer on this point.

I hope that your Eastern book progresses well.—­My dear Wallace, yours sincerely,

C. DARWIN.

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Sir Clifford Allbutt’s view, referred to in the following letter, probably had reference to the fact that the sperm-cell goes, or is carried, to the germ-cell, never vice versa.  In this letter Darwin gives the reason for the “law” referred to.  Wallace has been good enough to supply the following note (May 27, 1902):  “It was at this time that my paper on ‘Protective Resemblance’ first appeared in the Westminster Review, in which I adduced the greater, or, rather, the more continuous, importance of the female (in the lower animals) for the race, and my ‘Theory of Birds’ Nests’ (Journal of Travel and Natural History, No. 2), in which I applied this to the usually dull colours of female butterflies and birds.  It is to these articles, as well as to my letters, that Darwin chiefly refers.”