Another instance which may be cited is the asymmetrical condition of the heads of the flat-fishes (Pleuronectidae), 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 {38} 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 (Squalidae), have been found abundantly even down so far as the carboniferous rocks, and traces of them in the Upper Silurian. Another difficulty seems to be the first formation of the limbs of the higher animals. The lowest Vertebrata[34] are perfectly limbless, and if, as most Darwinians would probably assume, the primeval vertebrate creature was also apodal, how are the preservation and development of the first rudiments of limbs to be accounted for--such rudiments being, on the hypothesis in question, infinitesimal and functionless?