1 64 BRITISH FRESHWATER FISHES 



whilst a few larger ones, about 3 feet long, were ascer- 

 tained to have lived ten and a half to twelve and a 

 half years since the elver stage. From this we may 

 infer that some of the very large examples mentioned 

 above may well have been twenty or thirty years 

 old. 



The different rate of growth of the male and 

 female Eels affects the structure of the scales ; 

 as a male fish is usually older and has grown 

 more slowly than a female of his own size, his 

 scales will have narrower and more numerous 

 rings. 



In addition to the two species of the North 

 xA.tlantic there are several kinds of Eels inhabiting 

 the Indo-Pacific from the Cape of Good Hope to 

 the Sandwich Islands, but none are found on the 

 Pacific coast of North America, nor in South 

 America, nor in West Africa south of Morocco. 



As Schmidt has lately shown, the extraordinary 

 life-history of the Eel furnishes the clue to this 

 curious distribution of the species. He has found 

 that at their breeding-places the water never has 

 a temperature of less than 7° centigrade, and that 

 they are only found on coasts and in rivers which 

 are within reach of suitable breeding-places ; thus 

 their absence from South America and West Africa 

 is due to the coldness of the depths of the South 

 Atlantic. 



The Black Sea contains water of the right depth 

 and temperature for the propagation of Eels, yet 

 none are found in it or in the rivers connected with 

 it ; but the salinity is here much lower than in the 

 ocean and the deep water contains sulphuretted 

 hydrogen in such quantities as to exclude all higher 



