GEOGRAPHICAL DISTRIBUTION 277 



from the Severn, or other Western rivers, or it may 

 have obtained part of its fauna by a temporary 

 confluence of the rivers of the Irish and English 

 Channels. It may here be remarked that the sub- 

 merged river-beds do not necessarily represent the 

 courses of the rivers during the short period of our 

 last union with the Continent. 



If the routes by which freshwater fishes reached 

 our islands have been indicated, their further 

 distribution has been accomplished chiefly by those 

 geological changes which lead to the capture of the 

 tributaries of one river by another, or by the agency 

 of man, who at one time greatly appreciated many 

 of the species as food, and has indeed spread the 

 Carp, a native of Eastern Asia, over the greater part 

 of our islands, and must be held in some degree 

 responsible for the present range of the Tench, Pike, 

 and Perch. The Minnow and Loach, probably the 

 only really indigenous freshwater fishes in Scotland 

 north of Loch Lomond and the Firth of Forth, are 

 small species which thrive in little brooks, and are 

 therefore the more likely to spread rapidly, being 

 transferred from one system to another by slight 

 changes in the head-waters which would not 

 affect the inhabitants of the lower parts of the 

 rivers. 



Lest it be thought I am too ready to assume 

 such changes, I may say that geologists seem to 

 regard it as established that the Severn System has 

 enormously aggrandized itself by the capture of 

 tributaries of the Thames, Trent, and other rivers, 

 and it is quite probable that the Irish Channel 

 River got its fishes by the cutting back of one or 



