Dear Sir,

I duly received your kind letter with enclosure—¹ pray accept my best thanks. The question seems to be this:—is mimetic colouring & marking a product of natural or sexual selection? If exclusively of natural selection the mimetic characters being of the most vital importance to the species possessing them one would expect to find these characters tolerably stable—while on the other hand if these characters are produced by the sole action of sexual selection we should expect to find them (as you have demonstrated in the Origin) very variable.² In my proposed paper I intend to adopt the view that mimetic colouring is a product of both natural & sexual selections for just as natural selection will prevent sexual selection from developing any character hurtful to a species so I believe that sexual selection will always govern the mimetic colouring of a species; thus only permitting a mode of colouring admired by the opposite sex to survive.³ In some cases as we see, the mimetic colouring is limited in its transmission to the female sex only & in such cases—the female being more brightly coloured than the male, natural selection only appears to have been at work—the mimetic colouring being sexually limited in its inheritance. From your kind remarks I feel convinced of the difficulty of studying the variability of mimetic characters for these being as I believe due to the joint action of Natural & Sexual Selection one hardly knows whether to expect great variability or not. I therefore think it advisable to leave this point untouched till I can have the means of examining long series of most of the known mimics. In all cases of mimicry where the resemblance is limited to one sex it is the female only which mimics. Now as the female is in most classes of animals the selecting sex we can I venture to think, understand why in cases such as those just mentioned the female is brighter than her male—for while natural selection has been modifying & brightening the female for the purpose of protection she has remained faithful to the general taste displayed by other species of her genus & gone on coupling with males of the ordinary colour—perhaps even rejecting any male specimen that had partly acquired through inheritance some of her gaudy colours. By this joint action of Natural & Sexual selections it appears to me that the mimetic characters may become fixed to the female sex only & transmitted in a latent state through the males in accordance with your admirable hypothesis of pangenesis.⁴

I have perused with much pleasure Fritz Müller’s letter for which I am much obliged.⁵ I enclose it to you but am so bold as to ask you to allow me to retain the specimens for a week or so as they illustrate a very important point which F. Müller does not allude to—viz. a gradation in the secondary sexual characters in the same species of which I already possess an excellent example in Teracolus Danaë collected by my friend Mr. J. K. Lord in Egypt.⁶ The ♂ of this species has bright scarlet tips to the fore wings while I have a series of females showing a complete gradation from colourless tips to tips as brilliant as those of the males. Here & in F. Müller’s series of Leptalis Melite we have a glorious example of secondary sexual characters in all stages of transmission, from sexually limited inheritance to complete transmission to the female.⁷

I must confess that with Mr. Wallace, I do not see much difficulty in explaining the fact of several remotely allied forms all imitating one species.⁸ The extraordinary amount of variability in that most variable of all characters, colour, in the Lepidoptera is assuredly great enough for the purpose. If Fritz Müllers explanation—that the females select colours similar to those of other species which they constantly see before them⁹—be adopted we are still met with the difficulty that varieties approximating to the imitated forms do sometimes occur & if we allow this, then why not at once admit that the variation in that direction is sufficiently large to be of service to the species immediately? Perhaps as Mr. Wallace remarks sufficiently near the imitated form to be mistaken at a distance for one of the latter & so gain a few hours life—enough perhaps to deposit eggs.¹⁰ The species from the Satyridæ which imitates & haunts the same stations as two other species is adduced by Müller in support of his view but in such a case as this there is certainly not sufficient resemblance to entitle one to rank it as a case of mimicry, nor are we obliged to suppose that the female Satyrus through seeing two species with a white bar on the under surface of the hind wings continually about her acquired a similar taste & began to select males of her own species presenting white barred variations.¹¹

How mysterious are the laws that govern the colouration of organisms. Natural Selection or Sexual Selec. may sieze upon any particular mode of colouring or marking that may be useful to a species, but how ignorant we are of the cause of any particular colour appearing in an organism. May not Müllers example be explained by what you have termed the direct action of external conditions? We have several species inhabiting the same district & haunting the same stations— is it surprising then that a character common to all the species should be possessed by them? Surely the presence or absence of a minute white spot on the wing of a butterfly cannot affect the welfare of the species— yet Lycæna Agestis of the English list is represented by a var. Artaxerxes in Scotland which differs from its English ally only by the possession of a white spot on the wings. These two extremes are connected by another variety Salmacis—which not only connects them genetically but also geographically being found between England & Scotland.¹²

On the view of mimicry being produced by Nat. & governed by sexual selection I can conceive that in certain districts the mimic should be commoner than the mimicked species for in these districts another kind of foe is probably keeping down the numbers of the imitated form: a new class of persecutors to the larvæ of this latter form might explain it. I have come to the conclusion that the imitated form must be protected in the imaginal state & not owe its abundance to causes favourable to it in any of its earlier stages of growth.

In explaining phænomena of this nature by Nat. Selec. protective resemblance becomes also included & the whole class of phænomena I have applied the term “mimetic adaptation” which is divided into “mimicry” & “protective resemblance”.¹³

I must really apologise for stretching my communication to this inordinate extent.

With many thanks for your kind wishes concerning the success of my work— | I remain, | Yours obediently, | Raphael Meldola.

C. Darwin Esq, M.A. F.R.S. &c