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.