Dear Darwin

I found your letter on my return home on Saturday from Leeds, where I have been giving some lectures. I staid with D^r. Allbutt² who adopts your views in his lectures. He made a remark which struck me as having a curious bearing on sexual selection,—“that from the very lowest forms of vegetable and animal life it is the sperm cell that goes to the germ cell”, and he asked me if I knew of a single instance in which the reverse was the case, or in which the female sought the male. Now if this is so deep seated a law of nature is it not very extraordinary & very improbable that sexual selection should act always by the female choosing the male, and never by the male choosing the female? Again, as novelty is known to be a cause of sexual excitement in the male, is it not in the highest degree probable that variations of colour or form in the female would be sometimes preferred or rejected, & that if searched for facts to support this would be found.

I have not seen my paper for so long, (& the editor has not yet sent me the copies he promised) that I do not know exactly how I expressed myself, but I certainly never meant to exclude the action of the transmission of characters to the separate sexes in some cases.³ All I wished to show was, that extreme sexual differences of colour were due to the need of protection weeding out conspicuous colour in the female.

I fully admit that all sexual differences of colour cannot be explained in this way. There seems to be some production of greater vividness and condensation of colour in the male, independent of protection. In some of the Euplæas (a protected group of butterflies) this occurs.⁴ But how can we tell that this is not due to the females having been once more obscure, and having acquired their colours since they have acquired the protection?

I admit the fact of the incubating males showing only a slight deficiency of colour, but if this does not arise from protection why should it not occur occasionally among non-incubating males?⁵ The fact that through whole groups of Birds like the kingfishers, Jacamars, bee eaters, and Toucans,—all the species should have females as brilliant as the males is I think strongly against there being any more frequent appearance and more exclusive transmission of colour to the male than to the female. The whole group of Heliconidæ with its hundreds of species, furnishes a similar argument.⁶

To your question about the butterflies not having the male protected as well as the female, I think there is a tolerably satisfactory answer.⁷ First, that owing to the much less importance of the males’ continued existence, the selection of males is less rigid, (& without a certain rigidity of selection no permanent alteration of a race can be effected)— secondly; the males are more active in flight,—and thirdly—the males of the coloured ♀ Pieris ⁸ have different habits, frequenting the banks of rivers & the skirts of forests among swarms of other white & yellow Pieridæ which seem to be exempt from the attacks of insectivorous birds; these being found chiefly in the forest where the female is obliged to go to deposit her eggs. In the other most remarkable species, the Diadema bolina, it is curious that there is an allied species (D. auge) the male of which wonderfully resembles that of D. bolina, while the female is quite different,—but this female is wonderfully variable, (20 specimens might be picked out that would pass as species) while the males of both vary little.⁹ Here therefore is the material for selection to work on, to produce the protective mimicry of the female. In many other cases the females do vary in colour more than the males, therefore there is no reason why they should not be occasionally more gay than the males, and that they are so, so rarely, I impute to the greater danger of colour to them.

It is curious, if the male is more subject to the change of colour than the female, & has it transmitted exclusively, that in some few cases males alone have not acquired a protective tint;—but no such case is known, and this fact I think to some extent answers your query, for it shows that exclusive variations & transmissions of colour are not frequent enough, and the need of protection not great enough to induce the selection of special protective tints in that sex.

I think all the evidence goes to show that though special variations of colour may sometimes be transmitted to one sex only, yet no great accumulation of such variations can occur in one sex only, unless it is injurious to the other sex & therefore weeded out by nat. selectⁿ. The rarity of sexual differences of colour in Mammalia, combined with the frequency of other secondary sexual characters, shows this, as the habits of mammalia generally renders special protection of the female by colour needless.

There are certainly some cases which seem to imply selection by the female as well as by the male, such as some small parrots in which the male has red marks in the place where the female has blue; the different distribution of blue & white in ♂ & ♀ kingfishers; and the white headed ♀ to black headed ♂ of some ducks,—but in none of these cases is the ♂. more beautiful or more conspicuous than the female.

On the whole then, I think there is occasionally some tendency to brighter colours in the male, independent of protection to the female sobering her colours; but how or why that occurs there is as yet no means of determining.

Sexual selection too may occasionally produce brighter colours as well as additional ornaments & weapons in the male;—but in the great mass of cases in which there is great differentiation of colour between the sexes I believe it is due almost wholly to the need of protection to the female. I am afraid I have written in a rambling manner, & shall be glad to have more discussion with you on the subject.

Yours very faithfully | Alfred R. Wallace

C. Darwin Esq.