GEOGRAPHICAL DISTRIBUTION 259 



have been moulded by their manner of life into the 

 most diverse forms. 



Our freshwater fishes may be primarily divided 

 into {a) those which spend a part of their life in the 

 sea and {b) permanent residents in fresh water. In 

 the former class are included marine fishes such as the 

 Grey Mullets and the Bass, which frequent estuaries 

 and are occasionally found higher up the rivers, and 

 the Flounder, which migrates up stream for consi- 

 derable distances. The Three-spined Stickleback, 

 which occurs on all our coasts and in nearly all our 

 rivers, is equally at home in the sea and in fresh water, 

 and breeds in either ; in the Arctic regions it is 

 essentially a marine fish, but towards the southern 

 limits of its area, in Spain and Italy, it rarely enters 

 the sea. Catadromous fishes, which descend to the 

 sea to breed, are represented in our islands by the Eel, 

 which returns to its ancestral home in the depths of 

 the ocean for the purposes of reproduction ; the 

 abundance of Eels in our waters is due to our 

 proximity to places of the right depth and 

 temperature for their breeding, in the Atlantic to 

 the west of Ireland. Anadromous fishes are those 

 which ascend from the sea to spawn in fresh water ; 

 under suitable conditions these may lose their 

 migratory habit and may found colonies of permanent 

 freshwater residents. The Salmon, the Shads, and 

 the Sea Lamprey, when adult, feed and grow in 

 the sea, and enter our rivers only in order to spawn. 

 But it is interesting to note that Lake Wenern in 

 Sweden, and some of the larger lakes and rivers of 

 Quebec, New Brunswick, and Maine are inhabited by 

 Salmon which never go to the sea ; it is exactly the 

 same with the Sea Lampreys of some of the lakes of 



