BREEDING 291 



feed and grow at or fairly near to the surface of the sea, and 

 gradually move in an easterly direction, approaching the coasts 

 of Western Europe when about three inches long and a little 

 more than two years old. They now undergo a metamorphosis 

 and turn into little elvers or glass eels, about two and a half 

 inches in length. These move inshore and commence the 

 ascent of the rivers when about three years of age. The number 

 of elvers passing up a river during these migrations or "Eel-fares " 

 is enormous; upwards of three tons are said to have been 

 captured in a single day in the Gloucester district in 1886, 

 and it has been estimated that more than fourteen thousand 

 individuals go to make a pound weight. Few obstacles seem 

 too great to be overcome by the elvers in their ascent, and they 

 will wriggle over weirs, etc., and even travel overland if the 

 ground be wet in order to reach a suitable resting-place. Here 

 they will feed and grow for some years until the time arrives 

 for them to set off on their own breeding migration. 



On the coasts and in the rivers of the Atlantic slope of North 

 America is another closely related species of Eel (A. rostrata), 

 distinguished from its European ally by the smaller number of 

 vertebrae in its backbone. The breeding area of this species 

 overlaps that of the European Eel, but its centre lies rather more 

 to the south-west (Fig. 106). In the Western Atlantic, however, 

 larvae of both species are found living together. How is it, 

 then, that these larvae sort themselves out, one kind going to 

 America, the other migrating across the Atlantic to Europe ? The 

 explanation lies in the fact that the American Eel grows more 

 rapidly, and the development from egg to fully metamorphosed 

 elver occupies only one year, as against three years in the case 

 of the European species. Thus, if the larva of the European 

 Eel travels in a westerly instead of an easterly direction it will 

 reach the coast of America long before it is ready to change 

 into an elver; and conversely, if the larva of the American Eel 

 migrates in an easterly direction it will undergo its meta- 

 morphosis in the middle of the Atlantic. In other words, the 

 larval life in each case is suited to the distance to be travelled, 

 and the length of that of the European species is to be regarded 

 as a special adaptation related to the great distance of the 

 breeding-ground from the coasts. 



Among the true fresh-water fishes the members of the large 

 and varied Carp family {Cyprinidae) nearly all produce large 

 numbers of ova, which adhere to weeds, stones, and other 

 objects. After spawning no further care of the offspring is 



