INTRODUCTORY 39 



the broad type S. tnitia and a southern variety, S. triitta {eriox) for the 

 migratory fish ; and between 5. fario (Gaimard'i) and a southern variety, 

 5. fario {^Ausonii), for the non-migratory trout. But the " bull trout " 

 is not yet, I think, thereby explained. 



There is no reason to believe, nor is there any evidence, that since 

 the prehistoric glacial epoch changes have occurred in the types of 

 migratory trout which became localised in British waters. As the types 

 were then we may suppose them to be to-day. While it is no doubt 

 true that there existed great river basins in the European plateau, 

 whereof our islands are a fragment, and of which ancient rivers our 

 greatest streams are, it is believed, but the surviving upper tributaries, 

 or at least extensions of these tributaries, the plateau had a compara- 

 tively uniform sea-board on which the ancient rivers debouched. One 

 need not therefore ascribe to each ancient river specific races of trout. 

 But I think a fair analogy may be drawn between our modern trout and 

 those prehistoric fish in this respect. Just as to-day there is great 

 external difference between the trout of one locality and the trout of 

 another locality, to such a degree that one can hardly allege the trout 

 of one river area to be exactly like the trout of any other river area, so 

 the trout of the ancient Rhine which flowed northwards over land which 

 now forms the bed of the North Sea, the trout of the " Irish Channel 

 River " which flowed southwards, and the trout of the " English 

 Channel River " which flowed westwards, may quite well have had 

 distinctive external characteristics. It is not a very extravagant 

 speculation that some of the local characteristics of these prehistoric 

 fish may survive to this day, and that, for instance, the trout of the 

 Tweed and the Coquet, the Haddington Tyne and the Northumberland 

 Tyne, the Forth and the Aln, all, we may suppose, connected with the 

 ancient Rhine system, may still possess them. 



But setting speculation aside for the moment, the best scientific 

 opinion does not appear to concede the " bull trout's " claim to be a 



