GEOGRAPHICAL DISTRIBUTION 271 



Rudd ; in Devonshire and Cornwall the Chub is 

 absent, Roach and Rudd occur only in Slapton Ley 

 and a few other localities in Devonshire, and Tench 

 are said not to be indigenous ; in Cornwall the Perch 

 is local and introduced. Quite a number of species 

 are absent from Wales west of the Severn system, 

 and in addition to the species already mentioned as 

 confined to eastward streams of England the Gudgeon, 

 the Bleak, and the Ruffe are notable absentees from 

 the Lake District. 



The fact that species which have a very wide and 

 a very similar distribution on the mainland of Europe 

 and Asia, have very dissimilar distributions in the 

 British Islands, can only be explained by supposing 

 that our islands were connected with each other and 

 with continental Europe comparatively recently, when 

 our eastern, and probably our southern, streams were 

 tributaries of continental rivers and received from 

 them the fishes which they contained ; only nine or 

 ten of these had reached Ireland before it became a 

 separate island, and the distribution of the rest in 

 Britain at varying rates according to circumstances 

 has not yet proceeded long enough to spread them 

 all over the island. 



The British Isles stand upon a plateau, the edge 

 whereof coincides roughly with the lOO-fathom line. 

 From this the bottom of the sea slopes rather suddenly 

 down to depths of 1000 to 1500 fathoms; ancient 

 river-beds can be traced in the Irish and English 

 Channels, which prove conclusively that at one time 

 the streams of northern France and southern England 

 were tributaries of one " English Channel " River, and 

 that those of the south and east of Ireland joined the 

 same main " Irish Channel " River as the Severn and 



