2 72 BRITISH FRESHWATER FISHES 



other streams of the West of England and Wales. 

 Moreover, these submerged river channels, towards 

 the edge of the plateau, get deeper and deeper and 

 can be traced down nearly to its base, so that it is 

 clear that we have here the ancient coast-line of 

 Western Europe, which must have been bounded by- 

 steep cliffs, perhaps seven or eight thousand feet high, 

 and as our islands are connected with Iceland and 

 Greenland by a submarine ridge of less than half that 

 depth those countries also probably formed at that 

 time part of the European continent. There is reason 

 to believe that the conditions just described may have 

 existed throughout a considerable part of the Tertiary 

 Epoch, and that at times during this period Greenland 

 may have been connected with America, thus uniting 

 the two continents; these were also connected via 

 Asia, and there is good evidence of at least one 

 remote and one comparatively recent union of Asia 

 and North America over Bering's Straits. Thus the 

 general similarity of the freshwater fishes of the 

 Palaearctic and Nearctic regions is not to be wondered 

 at. 



During the greater part of this time the climate 

 was warmer than it is at the present day, but then 

 came the Glacial Epoch. It would be out of place 

 here to enter into the causes of this climatic change, 

 but it seems clear that at a comparatively recent date 

 the whole of Northern Europe, including our islands 

 except the south of England, became covered with 

 ice ; then a submergence of the land perhaps brought 

 the sea right to the edge of the ice-sheets, so that any 

 true freshwater fishes which may have been in our 

 rivers would have perished. At the end of this cold 

 period, a gradual elevation of the land took place. 



