for the very simple reason that no unwary larger species is at all likely to make an attempt to bite them across the middle; if it did, it would soon retire with a profound respect through all its future life for the latent resources of electrical science. But the defenseless ancestor of the poor flat-fishes was quite devoid of any such offensive or defensive armor, and, if he was to survive at all, he must look about (metaphorically speaking) for some other means of sharing in the survival of the fittest. He found it in the now-ingrained habit of skulking unperceived on the sandy bottom. By that plan he escaped the notice of his ever-present and watchful enemies. He followed (unconsciously) the good advice of the Roman poet: bene latuit.
But, when the father of all soles (turbot, brill, and dabs included) first took to the family trick of lying motionless on the sea-bottom, two courses lay open before him. (That there were not three was probably due to the enforced absence of Mr. Gladstone.) He might either have lain flat on his under-surface, like the rays and skates, in which case he would, of course, have flattened out symmetrically sidewise, with both his eyes in their normal position, or he might have lain on the right or left side exclusively, in which case one side would soon practically come to be regarded as the top and the other side as the bottom surface. For some now almost incomprehensible reason, the father of all soles chose the latter and more apparently uncomfortable of these two possible alternatives. Imagine yourself to lie (as a baby) on your left cheek till your left eye gradually twists round to a new position close beside its right neighbor, while your mouth still continues to open in the middle of your face as before, and you will have some faint comparative picture of the personal evolution of an infant sole. Only you must, of course, remember that this curious result of hereditary squinting, transmitted in unbroken order through so many generations, is greatly facilitated by the cartilaginous nature of the skull in young flat-fish.
When once the young sole has taken permanently to lying on his left side, he is no longer able to swim vertically; he can only wriggle along sidewise on the bottom, with a peculiarly slow, sinuous, and undulating motion. In fact, it would be a positive disadvantage to him to show himself in the upper waters, and for this very purpose Nature, with her usual foresight, has deprived him altogether of a swim-bladder, by whose aid most other fishes constantly regulate their specific gravity, so as to rise or sink at will in the surrounding medium. Some people may indeed express surprise at learning that fish know anything at all about specific gravity; but as they probably manage the alteration quite unconsciously, just as we ourselves move our limbs without ever for a moment reflecting that we are pulling on the flexor or extensor muscles, this objection may fairly be left unanswered.
The way in which Nature has worked in depriving the sole of a swim-bladder is no doubt the simple and popular one of natural selec-
