88 THE SEAS 



a similar life-history to that just described for our eel, with 

 the only difference that the spawTiing ground is slightly to the 

 west of that of the European eel, and the leaf-shaped 

 larvae take only one, instead of three years before they are 

 ready to ascend the American streams as typical elvers 

 (Plate 35). 



This difference in the life-histories of the European and 

 American eels is of great interest in showing the speeding 

 up or slowing down of development to suit the environment. 

 If the larvae of the European eel took only one year to reach 

 metamorphosis they would still be far from their destination 

 on the European coasts, when they had assumed the shape 

 at which they normally ascend the rivers ; while the 

 American eels by a reduction of the period of larval existence 

 are ready for metamorphosis by the time they have arrived 

 at the American coasts. 



A somewhat similar, but reversed, life-history is that of 

 the Atlantic salmon {Salmo salar). In the case of this fish, 

 while the adults undergo most of their growth in the sea, 

 they move up into the rivers to lay their eggs. The young 

 live for two or three years in the fresh water before migrating 

 down to the open sea to feed on the large food supplies 

 available there and to make that very great growth that 

 distinguishes them from their relatives the trout, who spend 

 all their lives in the rivers. 



Distribution 



It can be seen that, because of the large journeys carried 

 out by many fishes, the area of their distribution must vary 

 at different times of the year and be rather widespread. 

 Nevertheless there is noticeable, in the seas of the world, 

 quite a definite zoning of the distribution of different species 

 of fishes. There are, of course, differences in distribution 

 to be found between such fishes as live always in the tidal 



