From Fritz Müller 14 June 1871
Itajahy, Sa Catharina, Brazil
June 14. 1871.
My dear Sir
Permit me to thank you cordially for your admirable book on the Descent of Man and for the great pleasure, I have derived from reading it. The wonderful discussion (Chapt. III) on “Moral Sense” has particularly struck me as good and original.—1 I was also exceedingly interested in the chapter (XI) on the sexual selection of Butterflies and on this head I hope you will permit me a few remarks.2
As to Ageronia I told you, some time ago, that I had not yet seen it here; but lately I have caught two specimens belonging to two species and I have seen, in the collection of a friend of mine, a third specimen of a third species. One of these specimens had been observed for many days by my children flying around some orange-trees near my house; it frequently alighted on the putrifying fruits on the ground, on the juice of which it seemed to feed. My children never heard any noise produced by it, nor did I; and this seems to confirm your view, that the noise is made only during the courtship of the sexes.—3
With regard to display. I observed a curious little fact with our Hesperiadæ;4 most of them are of a dull brownish colour; but there are some, in which the wings have a more or less vivid blue tint either on the upper, or on the lower surface; now the former, when alighting on a flower, always hold their wings expanded in a horizontal plane, while those of the latter are folded vertically, so that in either case the blue surface is exposed to view.—
Butterflies not only discover flowers by colour; but certain species even give an unmistakeable preference to certain colours. Thus Callidryas Philea and some other species of that genus almost exclusively visit brilliant red flowers; (Canna, Salvia, Hedychium, Quamoclit etc.).5 A red Hedychium in my garden was constantly surrounded by a multitude of Callidryas Philea (and of Papilio Thoas);6 and so are at present some plants of Quamoclit, while they never alighted on yellow or white Hedychium, nor on white or blue Ipomoeæ.—
You allude to the difficulty which some writers have felt in understanding, how the first steps in the process of mimickry could have been effected through natural selection; I must confess, that to me also this difficulty appears to be very great; for in those cases, in which a species is mimicked by three or more other species belonging to as many widely different families, or in which species of the same genus are imitating species belonging to two or three distinct families, it does not appear very probable that all the mimicked and mimicking species should have been moderately like each other, when the process of mimickry commenced.7 The former is, as you know, the case with some Ithomiæ on the Amazonas and also here with our common white Pieridæ and with Acræa Thalia,8 while on the other hand forms so dissimilar as the white butterfly (I, 1), Acræa Thalia (I, 2) and Mechanitis Lysimnia (I, 3) are, are all imitated by species of the genus Leptalis (I, 1a. 2a. 3a).—9
I was surprised by the fact, that though we have here several rather common species of Ithomia and several mimicking species of Leptalis, none of our Ithomiæ is, as far as I know, mimicked by a Leptalis, nor by any other butterfly,—while on the Amazonas Ithomiæ appear to have been imitated more frequently than any other butterflies.10 The fact may perhaps be explained by the circumstance that all our Ithomiæ are rather inconspicuous animals, in the way of Ithomia Sylvo11 (II, 5.), and so species, which wanted protection, might have been protected equally well by assuming any other inconspicuous dress.—
There are here some exceptions to the rule, that the imitating species are comparatively rare, whilst the imitated swarm in large numbers. Thus Mechanitis Lysimnia is hardly more common, than the imitating Leptalis, and the beautiful Papilio Nephalion God.12 is here so rare, that I have seen only two or three specimens last summer, whilst the imitating Euterpe Tereas13 is by far more frequent. But in other parts of Brazil the numeric relation of these species may be different; if it were the same as here, this would be an argument in favour of the view, that in this and some other cases mimickry is due to sexual selection. On this view the first steps in the process of mimickry, even with species widely dissimilar in size, shape and colour, would offer no difficulty; every rude approximation, which by no means could serve as a protection, might be preserved by sexual selection.— To give some instances. The ressemblance between Eresia Langsdorffii (II, 1) and Heliconia Phyllis (II, 2) is no doubt too imperfect, to serve as a protection; but should it be due to mere chance?14 Should it not be permitted to look at Eresia Langsdorffii as a rude attempt, through sexual selection, to copy the brilliant Heliconiæ?— Meadow-browns appear to be so well protected by their dull colours, that it is hardly probable that they should have imitated other species for this purpose; but there is here a species of that group (II, 4) which at the same time ressembles in a certain degree two other species haunting the same stations.15 The upper surface of the wings imitates tolerably well our common white Pieridæ, whilst the lower surface is crossed by a large white stripe, as in some species of Heterochroæ16 (II, 3.).—
One of the most interesting of our mimicking butterflies is Leptalis Melite (I, 1a. III, 2–13.).17 The female alone of this species is imitating one of our common white Pieridæ (I, 1. III, 14),18 which she copies so well, that even her own male is often deceived; for I have repeatedly seen the male pursuing the mimicked species, till after closely approaching and becoming aware of his error he suddenly returned.— There is a great amount of difference in the shape of the fore-wings of the two sexes and in both of them the colouring of the upper surface of these wings varies much.— Some males nearly approach, in the colouring of the upper face of the fore-wings, another species of Leptalis (II, 1), the only species of that genus out of 5 or 6, I have hitherto found here, which does not imitate any common butterfly.— Are we to assume, that the mimicking species departed from a form closely ressembling this latter Leptalis? But on this view it is not probable, that the mimicked species or its ancestors at any time should have been moderately like this primitive form of the mimicker.— As in this case the male is more brightly coloured than the female, it may be asked, whether the plain dress acquired by the latter through natural selection has been transmitted exclusively or almost exclusively to the same sex, or whether being transmitted to both sexes the transmission has been checked and the bright colouring preserved in the male through sexual selection and in this regard I may mention an apparently trifling circumstance: that part of the lower surface of the fore-wings, which is covered by the hind-wings, and where consequently the transmission of the white colour of the ♀ to their male offspring could not be checked by sexual selection, is indeed white in the male also (III, 9).— During the last summer I have caught more than 30 males and about as many females; all these males, without a single exception, had bright yellow wings. Now in the last week, when the species had almost completely disappeared for about a month, only a few females being left, I caught two males and these (see III, 8) did not differ from the females in the colouring of the wings. This may be a mere chance, though it admits of a plausible explication. Late in the season males are scarce; any male at that time wd be admitted without selection; so that the transmission of the plain dress of the ♀ not being checked by sexual selection, an autumnal race of white males might be formed.—
I have been much pleased by the ingenious explication, given by Mr. Wallace of the bright colours of caterpillars.—19 I lately found the caterpillar of a Papilio, which strikingly ressembled the head of a venomous snake; the caterpillars of some other Papilios ressemble fresh excrements of a bird.20 These caterpillars always rest on the upper surface of the leaves, on which they feed, while those of some other Papilios (P. Nephalion, Polydamas),21 which are not protected by some such ressemblance, always hide themselves on the lower surface of the leaves.—
Pardon my having wasted your time with my long scrawling and believe me, dear Sir, with the most sincere respect | very truly yours | Fritz Müller
NB. The butterflies, mentioned in this letter, have been named for me by Mr. Gerstäcker,22 of Berlin.
Discussion of mimicry and sexual selection among butterflies, occasioned by reading Descent.