The Evolution Theory, Vol. 1 of 2

by August Weismann

As a further example, I now select a diurnal butterfly from Africa, Papilio merope Cramer[3], which was shown by Trimen in 1868 to be mimetic. The species has a wide distribution, for, if we except slight local differences in the marking of the male, its range extends over the greater part of Africa, from Abyssinia to the Cape, and from East Africa to the Gold Coast.

[3] The West African form of Papilio merope has been quite recently distinguished from the southern form and regarded as a distinct species, the latter being now called Papilio cenea. The differences in the males are very slight—somewhat shorter wings, shorter wing-tail, and so on—differences which seem relatively unimportant in comparison with the differences between the males and the females.

The male is a beautiful large butterfly, yellowish white, with a touch of black, and with little tails to the posterior wings ([Pl. I], Fig. 1), like our own swallowtail. A very nearly related species occurs in Madagascar, and there the female is similarly coloured, though it may be distinguished by having a little more black on the wing. On the mainland of Africa, however, the females of Papilio merope are so different in colour and form of wing that it would be difficult to believe them of the same species as the male had not both sexes more than once been reared from the eggs of one mother. The females ([Pl. I], Fig. 6) in South Africa imitate a species of Amauris, A. echeria ([Pl. I], Fig. 7), of a dark ground-colour with white, or brownish-white, mirrors and spots, and they resemble it most deceptively. But what makes the case more interesting in its theoretical aspect is that Danais echeria of Cape Colony is markedly different from Danais echeria of Natal, and the female of Papilio merope has followed those two local varieties, and has likewise a Cape and a Natal local form. Even this is not all, for in Cape Colony there are two other females of Papilio merope. One of them has a yellow ground-colour, and resembles Danais chrysippus, which is extremely abundant there ([Pl. I], Fig. 3); the other is entirely different ([Pl. I], Fig. 4), for it closely mimics another Danaid occurring in the same districts of Africa, and also immune, Amauris niavius ([Pl. I], Fig. 5), not only in the beautiful pure white and deep black of the wing surface, but also in the distribution of these colours to form a pattern.

We have thus in Africa four different females of Papilio merope, each of which mimics a protected species of Danaid. They are not always locally separate, so that each is exclusively restricted to a particular region, for their areas of distribution often overlap, and, at the Cape for instance, one male form and three different forms of female have been reared from one set of eggs. In addition, we have the fact that between the two local forms of Danais echeria transition forms occur, and that the mimetic females of Papilio merope show the same transition forms locally, and we must admit that all these facts harmonize most beautifully with the selection interpretation, but defy any other. And that the last doubt may be dispelled, nature has preserved the primitive female form on the continent of Africa—namely, in Abyssinia, where, along with the mimetic females, there are others which are tailed like the males ([Pl. I], Fig. 1), and are like them in form and colour, a few minor differences excepted.

Thus we have in Papilio merope a species which, in the course of its distribution through Africa, has scarcely varied at all in the male sex, but in the female has almost everywhere lost the outward appearance of a Papilio, and has assumed that of a Danaid, which is protected by being unpalatable, and not even everywhere the appearance of the same species, but in each place that of the prevailing one, and sometimes of several in one region. These females thus show at the present day a polymorphism which consists of four chief mimetic forms, to which has to be added the primitive form—that resembling the male. This has survived in Abyssinia alone, and even there it is not the only one, but occurs along with some of the mimetic forms.

To the question why only the females are mimetic in this and other cases, Darwin and Wallace have answered that the females are more in need of protection. In the first place, the males among butterflies are considerably in the majority, and, secondly, the females must live longer in order to be able to lay their eggs. Moreover, the females, which are loaded with numerous eggs, are heavier in flight, and during the whole period of egg-laying—that is, for a considerable time—they are exposed to the attacks of numerous enemies. Whether one of the abundant males is devoured sooner or later is immaterial to the persistence of the species, since one male is sufficient to fertilize several females. The death of a single female, on the other hand, implies a loss of several hundred descendants to the species. It is, therefore, intelligible that, in species already somewhat rare, the female must first of all be protected; that is to say, that all variations tending in the direction of her protection would give rise to a process of selection resulting in an augmentation of the protective characters.

But there are also butterflies in which both sexes mimic a protected model. Thus many imitators of the unpalatable Acræides ([Pl. II], Fig. 21) resemble the model in both sexes, and of the South American Whites which mimic the Heliconiidæ there are some which have the appearance of the Heliconiidæ even in the male sex ([Pl. II], Fig. 18, 19), while others look like ordinary Whites (for instance, Archonias potamea). But in many of these species, which are mimetic in the female sex, we find also in the male some indications of the mimetic colouring, but in the first instance only on the under surface. Thus the females of Perhybris pyrrha ([Pl. II], Fig. 17) resemble in their black, yellow, and orange-red colour-pattern the immune American Danaid, Lycorea halia ([Pl. II], Fig. 12), but their mates are, on the upper surface, like our common Whites, though they already show on the under surface the orange-red transverse stripes of the Lycorea ([Pl. II], Fig. 16). In other mimetic species of Whites a similar beginning is even more faintly hinted at, and in others, again, the upper surface of the male is also provided with protective colours, and only a single white spot on the posterior, or sometimes even on the anterior wing as well, shows the original white of the Pieridæ ([Fig. 18]).

I do not know how any one can put any other construction on these facts than that the females first assumed the protective colouring, and that the males followed later, and more slowly. Whether this is due to inheritance on the female side, and thus ensues as a mechanical necessity, in virtue of laws of inheritance still unknown to us, or whether it arose because there was a certain advantage in protection to the males—though not such a marked one—and that these, therefore, followed independently along the same path of evolution as the females, has yet to be investigated. Personally, I incline to the latter view, because there are protected mimetic species, in which the female mimics one immune model, and the male another, quite different from the female's. A case in point is that of an Indian butterfly, Euripus haliterses, and also Hypolimnas scopas, in the latter of which the male resembles the male of Euplœa pyrgion, and the female is like the somewhat different female of the same protected species. The Indian Papilio paradoxus, too, seems to show the independence of the processes of mimetic adaptation, for the male is like the blue male of the immune Euplœa binotata ([Pl. III], Fig. 25), while the female resembles the radially-striped female of Euplœa midamus ([Pl. III], Fig. 27), and this double adaptation is repeated in another of the persecuted butterflies, Elymnias leucocyma ([Pl. III], Fig. 26, 28).

Many objections have been made to the interpretation of mimicry by selection. It has been asserted that butterflies are exposed to injury from birds only to an inconsiderable extent, not sufficient to account for such an intense and persistent process of selection, because they are not very welcome morsels, on account of the large and uneatable wings and the relatively small body. Doubt has also been raised as to the immunity of the models, which has not been proved in many of the species in regard to which it is assumed. Finally, it is maintained that the advantage which resemblance to an immune model brings is not proved, but is purely hypothetical; and that it is probable that the birds do not distinguish the colours and markings of the flying butterflies at all, but are at the most only deceived by resemblances in their manner of flight.