46 



support the general opinion that during the PUocene period the land 

 gradually rose to a higher level, and this afiected not merely the 

 area now under consideration but the whole of the North Atlantic 

 region. But whatever was the cause it is now clear that during 

 the glacial period the Wyville -Thomson ridge occupied a higher 

 level, so much so as to prevent the passage of Atlantic water beyond 

 it.* It was evidently during Pliocene time that the change of 

 level gradually took place, so as more and more to prevent Atlantic 

 water crossing into the Arctic Ocean, thus producing the advancing 

 cold cHmate of that epoch, and as a culmination, when the Atlantic 

 water was altogether prevented from crossing the ridge, the glacial 

 period. 



The Arctic Ocean was thus converted into a land-locked sea, 

 with a small opening to the Pacific if the Behring Strait existed 

 then. The prevention of the passage of the Atlantic water into this 

 region would obviously have been sufficient to bring about a glacial 

 epoch in the northern hemisphere. 



Durmg the glacial period the land, or the bank and island, 

 connexion between Scotland and Greenland was occupied by or 

 fronted by an ice barrier, f which must have extended along the 

 coast of Ireland for some distance (fig. 7). North and east of this 

 the whole area under consideration was covered by ice. It is 

 probable, however, that an overflow of Atlantic water nay have 

 occurred at intervals during the period of glaciation, causing 

 variations of climatic conditions and of the extent of the ice covering. 



It is not at all likely therefore that plaice or any other living 

 creature occupied the North Sea during the ice age. If, as seems 

 to have been the case, the Arctic Ocean was covered by ice con- 

 tinuous with the land ice all round its borders, it is not any more 

 likely that there was a fauna and flora in the whole marine area 

 involved in the glaciation. It will be assumed in what follows 

 that such was the case. 



The results of the migration experiments, as has been seen, 

 point to the conclusion that plaice entered the North Sea from the 

 Atlantic. It may be concluded therefore that durmg the ice age 

 they were confined to such portion or portions of the continental 



* For a description of glaciated stones from the crest of the Wyville-Thomson Ridge, see 

 Murray, " The Physical and Biological Conditions of the Seas and Estuaries about North 

 Britain. Proc. Phil. Soc, Glasgow, vol. XVII., pp. 311-312, 1886. 

 t Murray and Hjort, Depths of the Ocean. 



