AUGUST 195 



their presence in any particular region throws no sure 

 light upon the remote geological past. Fishes, also, are 

 both mobile and subject to transport by human agency. 

 We have it on the authority of Linnaeus that the puny 

 stone-loach owes its presence in Swedish waters to 

 Frederick i., who caused it to be introduced because he 

 loved its delicate flavour. British trout fishers entertain 

 anything but gratitude for the monks of old who so 

 diligently carried the omnivorous pike from river to 

 river and lake to lake with the view of an easy 

 and abundant supply for fast days. With these facts 

 before us it may seem that the present distribution of 

 fishes has as little bearing upon geological problems 

 as that of beasts and birds. In the case, however, 

 for which I am asking consideration, it is not the 

 presence of the burbot, but its absence from the Thames, 

 which seems to have significance. That absence must 

 be owing to some agency, and if it could be proved that 

 ice was the agent, the theory of the mode in which the 

 Thames was severed from the Rhine system would be 

 made clearer. 



The grayling is another Rhine fish which is con- 

 spicuous by its absence from the Thames and its 

 tributaries, or was so until recently when, as I am 

 informed, grayling were introduced to the head waters. 

 It is probably impossible now to ascertain the original 

 distribution of the grayling in British waters, so freely 

 has it been transplanted by anglers and naturalised in 

 streams where it was certainly not indigenous. This 

 has been especially the case within the last fifty or 

 sixty years. Grayling now swarm in the Tweed, the 



