PAST GEOGRAPHICAL MUTATIONS 17 



from the Continent by a wide sea in the south, which 

 was probably tiien the dominant line of Emigration of 

 all species moving north with the changing climate. 

 The 40-fathom contour, however, would leave Ireland 

 joined to Scotland by a narrow isthmus — of no conse- 

 quence as regards southern Emigration, as I hope to 

 show in a future chapter — and submerge about half of 

 the North Sea plain beneath the waves. The sea at this 

 contour would encroach considerably upon the English 

 Channel area, but still offer little obstacle to the northern 

 or western Em.igration of species by that route ; whilst 

 sufficient land would be left between Denmark and 

 England to allow of Emigration and Migration on a 

 vast scale, as the sequel will show. Proof of the rich 

 mammalian fauna dwelling at this time in Western 

 Europe is suggested by the great quantities of its remains 

 which have been dredged from the bottom of the North 

 Sea, especially from the Dogger Bank, from which 

 latter locality bones, teeth, and antlers have been ob- 

 tained, relics of the reindeer, Irish elk, stag, woolly 

 rhinoceros, mammoth, hyaena, bear, wolf, etc., some of 

 which must have roamed that ancient plain during 

 prehistoric ages. 



But the sea still continued to encroach upon the land, 

 terrestrial submergence or marine upheaval steadily pro- 

 gressed. The North Sea crept southwards or spread 

 east and west, and the Atlantic meantime encroached 

 more and more up the English Channel, the Bristol 

 Channel, and the Irish Sea, until the contour of 20 

 fathoms was reached, and the Dogger and other banks 

 in the North Sea became islands, on some of which 

 many large animals appear to have been imprisoned by 



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