Living Silver 



he came to consider the shark's tail, he realised how much more 

 effective was the tail of a cod, how little it tended to raise or 

 lower the head of the fish in the water, how well it was adapted 

 for keeping an even keel. And then, investigating further, he dis- 

 covered that the cod's tail was not as he had thought, a simple 

 structure but a very cunning simplification of many complex fac- 

 tors. He learned, in fact, that the tail of a cod was as simple and 

 only as simple as the paddling limbs of a whale. They both pre- 

 sented an even surface to the world but they contained both a 

 baffling intricacy of bone and muscle. The spinal column of the 

 cod, for example, was uptiltedat its hind end like that of the shark. 

 Only a greater evolutionary art on the part of the cod's ancestors 

 had simplified its external anatomy so that its tail fin looked like 

 an even paddle, and no superficial hint of the millions of years of 

 change was left untidily behind as a clue. 



His courses of discovery grew longer and more difficult. Al- 

 ways there were new things to be found in fish that he had thought 

 he already understood and these discoveries were always revealed 

 by some difference between the apparent simplicity of the bony 

 fish and the complexity of the cartilaginous ones. Yet, as he well 

 knew, the cartilaginous fish were not the ancestors of the bony 

 ones, any more than the lizards were the ancestors of birds. They 

 belonged to very different zoological groups. If the whole truth 

 of heredity were ever to be told, Jan decided that the cod might 

 well be proved to be as distant a relative of the sharkfish as it was 

 of the whale. 



Both of the great groups of fish had evolved in fresh water, not 

 in the sea. The gadoids and pleuronectids actually drank fresh 

 water from salt in order to compensate for the desiccation of their 

 blood through the gill filaments and the skin. How this was man- 

 aged remained unknown. But the sharks were simpler. They too 

 should have lost water if only the normal mineral salts in their 

 bodies had been taken into consideration. Their blood was a good 

 deal less salt than the sea so that there was a continual tendency for 

 the sea to pull water out of their bodies and thus to concentrate 



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