72 ANIMAL LIFE AND HUMAN PROGRESS 



The Twaite Shad (Alosa finta) of the Atlantic coasts of Europe 

 and its Mediterranean representative (A. nilotica) are fishes 

 of the Herring family that run up the rivers to breed. In 

 the large lakes of northern Italy there are Shad that never go 

 to the sea, and are quite distinct from the migratory form that 

 enters the lakes at the breeding season. It is not generally 

 known that a Shad inhabits the Lakes of Killarney ; it is 

 closely related to the Twaite Shad but has a deeper body 

 and more numerous gill-rakers, characters that may be adap- 

 tive, for the form of the fish is related to its way of swimming 

 and the number of the gill - rakers to its food. We may 

 regard the migratory Shad as the parent species and the 

 lacustrine forms as their children, and it seems clear that the 

 initial step in the formation of these local races was a change 

 of habits ; some of the Shad that entered the lakes to breed 

 stayed there to feed. 



Another case of some interest is that of the Char (Salve- 

 linus), fishes of the Salmon family. Char are essentially fishes 

 of the Arctic Ocean, running up the rivers to spawn and 

 often forming lacustrine colonies of fish that do not visit the 

 sea ; in the sea they do not come nearer to the British Isles 

 than the coasts of Iceland and northern Norway, but 

 non - migratory races of Char are found in the lakes of 

 Scandinavia and of the Alps, and in our islands in Ireland. 

 Scotland, the Lake District and North Wales. Similarly 

 Sea-trout are found from Iceland and Norway to the Bay 

 of Biscay, but non-migratory Trout so far south as the rivers 

 of Algeria and Morocco. 



It is evident that during the Glacial Epoch Char occurred 

 on our coasts and ran up our rivers, and Sea-trout ranged 

 southward to northern Africa, and that when the climate 

 became warmer, and the southern limit of the marine range of 

 these anadromous species shifted northward, Trout remained 

 in the rivers of the Atlas Mountains and of southern Europe, 

 and Char in many lakes of the British Isles and of the con- 

 tinent, as relict forms. The Char of different lakes are dis- 

 tinguished from each other especially by form, coloration, 

 scaling, size of the fins, size of the eye and structure of the 



