We see, then, that the much-persecuted and easily injured butterflies make use of a poisonous substance (in the widest sense), prepared in the plant for its own protection, and, wherever their own metabolism makes it possible, they use it to protect themselves. We need not wonder, therefore, that so many butterflies are immune, nor that among the numerous palatable species a small proportion have endeavoured to become like the protected species, as far as natural selection was able to bring such a resemblance about.
There is hardly any adaptation phenomenon so widely distributed and diverse in its manifestations, which has been at the same time so much observed and followed out into all its details, as Mimicry; and it must surely be regarded as a justification of the validity of interpreting it in terms of Natural Selection that all the observed phenomena tally so beautifully with the deductions from the theory. I at least know of no facts which contradict the theory, but of many which might have been predicted from it.
For instance, it might have been predicted from the theory alone that an immune species would often have several mimics, as, in point of fact, is frequently the case, and it would be easy to give numerous examples of this. Thus the two Danaids of South and Central Africa, Amauris echeria and Amauris niavius, are mimicked, not only by the two female forms of Papilio merope, as we have already described in detail, but the latter is also mimicked by Nymphalid, which requires protection, Diadema anthedon, and the former by two diurnal butterflies of different families, Diadema nuina and Papilio echerioides.
Similarly, the black-and-red coloured Heliconius melpomene in Brazil is mimicked both by the female of a White (Archonias teuthamis), and by a Papilio, which has received the name of P. euterpinus on account of this resemblance. Thus, too, the immune Methona psidii, Cr. of Brazil, with its half-transparent wings marked with black bands, has five mimics, belonging to five different genera, and one of these is not a true diurnal butterfly at all, but one of the day-flying species of the genus Castnia, whose systematic position is doubtful.
Fig. 18. Upper surfaces of A, Acræa
egina, from the Gold Coast, immune.
B, Papilio ridleyanus, from Gaboon, not
immune. C, Pseudacræa boisduvalii,
from the Gold Coast, not immune.
The West African immune Acræid, Acræa gea ([Pl. II], Fig. 21), is deceptively mimicked, both as to the narrow, long shape of the wing and its blackish-brown and white mottled markings, by a Nymphalid, Pseudacræa hirce, by the female of a Papilio (P. cynorta) whose mate is quite different, and by the female of a Satyrid (Elymnias phegea) ([Pl. II], Fig. 20). In the Papilio the resemblance extends to the peculiar pitch-black shining spot on the under side of the base of the posterior wing, and all three are like the model on both surfaces, and therefore in flight as well as in the resting attitude.
On the same West African coast occurs the strange greyish-black Acræa egina, with brick-red spots and bands, and coal-black dots (Fig. 18, A). This immune species is deceptively mimicked in its native country by two other butterflies—a Nymphalid, Pseudacræa boisduvalii (Fig. 18, C), and by a female Papilio (P. ridleyanus) (Fig. 18, B), by the latter not so exactly as by the former, but quite sufficiently to be confused with its model in flight.
It would have been less easy to predict with certainty from the theory that, conversely, the different species of a genus which stood in need of protection would be able to mimic quite different immune models, for who would have ventured to prophesy how far the capacity of a species for variation might go, and how many different kinds of coloration it was able to assume? But the facts teach us that there is a wide range of possibility in this respect.