My dear Sir,

I am extremely obliged by your kind offer to look through Fritz Müller’s letter & I enclose it herewith. If you think there are any remarks therein which he would not object being published I will make them known at the next meeting of the Entom. Soc.¹ There is a great deal of information in the letter which as you will see does not relate to mimicry.

I shall exhibit at the next meeting of the Entom. Soc. certain series of butterflies which are very interesting in my opinion as throwing some light upon the obscure problem of Dimorphism.² In the usual acceptation of the word this phenomenon is said to exist when a female insect exists in 2 distinct forms— when more than 2 distinct forms exist the species is said to be polymorphic. Thus our Colias edusa may be said to be dimorphic as there are two known forms—one having the ground colour of the wings the same as in the ♂ & the other (var. helice) much paler.³ Now this year when the insect has been so extraordinarily abundant with us I have seen several ♀ specimens intermediate between var. helice & the ordinary type of ♀. In the butterflies which I propose to exhibit the ♂ is constant in colour & marking but the ♀ similarly shows a series of gradations—some specimens being very like the ♂ others departing considerably—the extreme forms being connected by intermediate links. Now wherein does this graduated variability differ from dimorphism & polymorphism?— only in the presence of intermediate forms. To represent the phenomenon symbolically:—if A, = species

A ♂ (constant)

A ♀₁ A ♀₂ A♀₃ A♀₄ &c being forms of ♀— A♀₁ being most & A♀₄ least like the ♂. If A♀₂ & A♀₃ were to become extinct we should have a case of dimorphism. In nature we see this going on— there are species in which the dimorphism is absolute— there are others (e.g. hyale) in which intermediate ♀s are rare & there are others (e.g. some of our common sp. of Lycæna)⁴ in which regularly graduated series of ♀s are always to be found.

It seems to me therefore that dimorphism & polymorphism could be explained by your grand principle of Nat. Selec. if it could only be proved that the extreme forms were advantageous to the species. This has been done by Mr. Wallace in the case of certain Malayan Papilionidæ which are polymorphic mimics.⁵ To follow out an abstract example— A sp. possessing a ♀ which mimics some other protected sp. inhabiting the same region gives rise to ♀s which vary in every possible manner— those which resemble their mimicking mothers are preserved— those which depart from her are weeded out. Now suppose that variations should occur tending to assimilate some specimens to another protected sp. in the same region— these would have a chance of surviving almost as great as that of their already mimicking sisters & the sp. would tend to be drawn out in this direction also (being “a line of least resistance.”) In the meantime Nat. Selec. is busy extirpating intermediate forms—i.e. such forms would be prevented from transmitting their characters to offspring & finally the sp. would breed true to the 2 mimicking forms only & we should have a dimorphic mimic. Similarly with polymorphic mimics.

For such cases then it is proved that there is an advantage to the sp. & the dimorphism is explained by Nat. Selec. In cases where the ♀ does not mimic another sp. it is difficult to see what advantage can accrue to it by existing in 2 forms. Do you think it possible that there is any sexual-physiological advantage? or can you suggest any other advantage?

I hope you will forgive this long letter but I know you will be glad to see the remarkable phenomenon of dimorphism explained by Natural Selection if it can be done logically & without straining of theory.

I am dear Sir, | Yours faithfully, | R. Meldola.

Ch. Darwin Esq. M.A. F.R.S.

A propos of sexual ornamentation Mr. Wood-Mason exhibited at the last meeting of the Entom. Soc. a Mantis (Phyllothelys Westwoodi) in which the female only possessed a great frontal horn scarcely represented in the male. I think it very probable that in this group of insects the ♀s are the wooers but they generally end by devouring their mates!⁶