48 



The preceding sketch has been made not with a view of attempt- 

 ing to explain the ice age, but to draw attention to what appears 

 to be a natural conclusion that the present distribution and 

 migratory habits of the fauna of the Xorth Sea and Arctic Ocean 

 are post-glacial in origin, and at the same time to form a background 

 for the consideration of these. It is obvious also that while the 

 passing of the glacial period »began a new era for the fauna of the 

 North Sea and Arctic Ocean, the causes which brought about the 

 epoch, brought to an end a not very different fauna in the precedmg 

 period. 



Migrations of Plaice. — It was therefore durmg the time 

 that the early post-glacial changes were happening that the North 

 Sea received its first fauna and flora. The Atlantic water which 

 was bringing about the climatic change would carry into the region 

 and into the Arctic Ocean plankton as it does to-day. It is pro- 

 bable that early in post-glacial times all may have perished, for 

 the winter conditions for a time may have been too severe, and m 

 summer, owing to the melting of the ice, the North Sea would have 

 been occupied by water of low salinity, but with the amehoration 

 of the climate such would be able to successfully winter in the region, 

 and would therefore form fresh centres of dispersal. Only the more 

 active on attaining maturity would be able to retreat to the Atlantic 

 for spawning purposes. 



Among these latter for a period, it may Avith reason be pre- 

 sumed, were the plaice. As soon as the conditions permitted, it 

 is fairly evident that plaice would spread along the plateau to. the 

 west of Scotland, both actively in the adult condition and passively 

 by the eggs and larvae carried by the new direction of the current. 

 From a spawning ground in that region eggs and larvae would be 

 similarly carried into the North Sea, and sooner or later the young 

 plaice would be able to pass the winter there. During the period 

 of their growth to maturity they would form a school migrating 

 outwards to the deeper water for winter, and inwards again with 

 the pulsations of the Atlantic stream. But on reaching maturity 

 they would have to migrate to the Atlantic for spawning. 



The plaice at this post-glacial period of the history of the North 

 Sea would have been therefore practically in the condition of the 

 eel at the present day. Every year the larvae of the eel are carried 

 around the north of Scotland into the North Sea, and are able to 



