DISTRIBUTION AND MIGRATIONS 267 



fishes of three or four ounces inhabiting the mountain streams 

 of Wales, are one and the same species, as are the silvery Sea 

 Trout and the non-migratory Brown Trout. 



The presence of stocks of land-locked and non-migratory 

 Salmon, and of fresh-water colonies of Trout is not very difficult 

 to understand. It is clear that in former times all Salmon and 

 Trout lived in the sea, entering rivers at times to spawn. Some 

 of the young fish on their way back to the sea must have found 

 a sufficiency of food, and perhaps a smaller number of enemies, 

 in the lakes and rivers through which they passed, and, regard- 

 ing these as good substitutes for the sea, they remained behind 

 to found a fresh- water colony of an anadromous species, which 

 would be reinforced from time to time by fresh additions from 

 the sea. In other cases the change in habit may have been 

 involuntary, some physical barrier preventing the young fish 

 from descending to the sea in the normal manner. It has been 

 already stated that the range of both species in the sea extends 

 southwards only as far as the Bay of Biscay, and it is of interest 

 to find that there are fresh-water colonies of Trout in the 

 Atlas Mountains of North Africa, in the islands of Corsica and 

 Sardinia, and in the countries north of the Mediterranean as 

 far east as the Adriatic Sea. Further, in the rivers of Albania 

 and Dalmatia is a small species (S. microstoma), never exceeding 

 a length of fifteen inches, which is very closely related to the 

 Salmon. There can be little doubt that the present marine 

 distribution of the Salmon and Trout is limited mainly by 

 temperature, and it is this factor which prevents them from 

 entering the Mediterranean to-day. It may be assumed, 

 however, that if the climatic conditions in Europe were colder, 

 as they were known to be during the period known as the 

 Ice Age, the limit of their range would be farther south. It 

 is certain that during the glacial period both Salmon and 

 Trout occurred in the Mediterranean and ran up suitable 

 rivers to spawn, and that, when the migratory fish once more 

 retreated northwards on the return of milder climatic con- 

 ditions in Europe, fresh-water colonies were left behind in 

 some of the rivers. The presence of a fluviatile race of Three- 

 spined Sticklebacks {Gasterosteus aculeatus) in Algeria may be 

 explained on the same hypothesis. 



The fresh-water colonies or races of White-fish {Coregonus) and 

 Char {Salvelinus) have originated in much the same way. The 

 Char are primarily marine fishes, inhabiting the Arctic Ocean, 

 and running up the rivers to spawn. They have at some time 



