260 BRITISH FRESHWATER FISHES 



the State of New York ; in times past the abundance 

 of food in the lakes has induced them to give up 

 their journey to the sea, and if nowadays they could 

 get there they would certainly not be able to get 

 back again, and so they remain, freshwater colonies 

 or races of anadromous species. 



The Trout and the River Lamprey, which are re- 

 spectively somewhat smaller species than the Salmon 

 and the Sea Lamprey, differ from the latter also in 

 that they feed and grow in fresh water as well as in 

 the sea, and in all suitable rivers and lakes which 

 they can reach a proportion of them will become per- 

 manent freshwater residents. 



From a consideration of those fishes which spend 

 part of their life in the sea we have thus gradually 

 arrived at those which pass the whole of their lives 

 in fresh water. Fluviatile colonies of the Three-spined 

 Stickleback and the Trout may often be reinforced 

 from the sea, and in our islands only show pecu- 

 liarities immediately associated with their changed 

 environment, but the Char and Whitefish, long 

 isolated in their lakes, are very different from their 

 marine allies. 



The range in the sea of a migratory species such as 

 the Salmon seems to be mainly a matter of tempera- 

 ture; there is a pretty definite southern limit, and 

 we may take it that if the climate of Europe were 

 colder this limit would be farther south. Thus we 

 may explain the presence of a Three-spined Stickle- 

 back and a Trout in the rivers of Algeria, farther south 

 than the present range of these fish in the sea, on 

 the hypothesis that they reached those rivers from 

 the sea when the climate was colder, and we may 

 suppose that their isolation from the parent stock 



