Mr. Darwin explains the imitation of some species by others more or less nearly allied to it, by the common origin of both the mimic and the mimicked species, and the consequent possession by both (according to the theory of "Pangenesis") of gemmules tending to reproduce ancestral characters, which characters the
mimic must be assumed first to have lost and then to have recovered. Mr. Darwin says,[[29]] "Varieties of one species frequently mimic distinct species, a fact in perfect harmony with the foregoing cases, and explicable only on the theory of descent." But this at the best is but a partial and very incomplete explanation. It is one, moreover, which Mr. Wallace does not accept.[[30]] It is very incomplete, because it has no bearing on some of the most striking cases, and of course Mr. Darwin does not pretend that it has. We should have to go back far indeed to reach the common ancestor of the mimicking walking-leaf
insect and the real leaf it mimics, or the original progenitor of both the bamboo insect and the bamboo itself. As these last most remarkable cases have certainly nothing to do with heredity,[[31]] it is unwarrantable to make use of that explanation for other protective resemblances, seeing that its inapplicability, in certain instances, is so manifest.
Again, at the other end of the process it is as difficult to account for the last touches of perfection in the mimicry. Some insects which imitate leaves extend the imitation even to the very injuries on those leaves made by the attacks of insects or of fungi. Thus, speaking of one of the walking-stick insects, Mr. Wallace says:[[32]] "One of these creatures obtained by myself in Borneo (Ceroxylus laceratus) was covered over with foliaceous excrescences of a clear olive-green colour, so as exactly to resemble a stick grown over by a creeping moss or jungermannia. The Dyak who brought it me assured me it was grown over with moss although alive, and it was only after a most minute examination that I could convince myself it was not so." Again, as to the leaf butterfly, he says:[[33]] "We come to a still more extraordinary part of the imitation, for we find representations of leaves in every stage of decay, variously blotched, and mildewed, and pierced with holes, and in many cases irregularly covered with powdery black dots, gathered into patches and spots, so closely resembling the various kinds of minute fungi that grow on dead leaves, that it is impossible to avoid thinking at first sight that the butterflies themselves have been attacked by real fungi."
Here imitation has attained a development which seems utterly beyond the power of the mere "survival of the fittest" to produce. How this double mimicry can importantly aid in the struggle for life seems puzzling indeed, but much more so
how the first faint beginnings of the imitation of such injuries in the leaf can be developed in the animal into such a complete representation of them—a fortiori how simultaneous and similar first beginnings of imitations of such injuries could ever have been developed in several individuals, out of utterly indifferent and indeterminate infinitesimal variations in all conceivable directions.
Another instance which may be cited is the asymmetrical condition of the heads of the flat-fishes (Pleuronectidæ), such as the sole, the flounder, the brill, the turbot, &c. In all these fishes the two eyes, which in the young are situated as usual one on each side, come to be placed, in the adult, both on the same side of the head. If this condition had appeared at once, if in the hypothetically fortunate common ancestor of these fishes an eye had suddenly become thus transferred, then the perpetuation of such a transformation by the action of "Natural Selection" is conceivable enough. Such sudden changes, however, are not those favoured by the Darwinian theory, and indeed the accidental occurrence of such a spontaneous transformation is hardly conceivable. But if this is not so, if the transit was
gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is indeed far from clear. It seems, even, that such an incipient transformation must rather have been injurious. Another point with regard to these flat-fishes is that they appear to be in all probability of recent origin—i.e. geologically speaking. There is, of course, no great stress to be laid on the mere absence of their remains from the secondary strata, nevertheless that absence is noteworthy, seeing that existing fish families, e.g. sharks (Squalidæ), have been found abundantly even down so far as the carboniferous rocks, and traces of them in the Upper Silurian.