SCIENCE OF FLAT-FISH, OR SOLES AND TURBO T. 109 



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 an- 

 cestor 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 sur- 

 vival of the fittest. He found it in the now-ingrained habit of skulk- 

 ing unperceived on the sandy bottom. By that plan he escaped the 

 notice of his ever-present and watchful enemies. He followed (uncon- 

 sciously) 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 proba- 

 bly 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 prac- 

 tically come to be regarded as the top and the other side as the bot- 

 tom 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 con- 

 tinues 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 un- 

 dulating 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-blad- 

 der, 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 altera- 

 tion quite unconsciously, just as we ourselves move our limbs without 

 ever for a moment reflecting that we are pulling on the flexor or ex- 

 tensor 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- 



