45 



direction of the winter grounds, but reach a position usually much 

 be^^ond that region. In the case of the mature of the northern 

 race the migrants pass north and in some cases west to or towards 

 the Atlantic. The mature of the southern race migrate towards 

 the Atlantic via the Chamiel. 



The migrations of the schools, and particularly those of the 

 mature plaice must obviously be associated Avith the origin of the 

 two races. The northern race must have entered the North Sea 

 from the north and the southern race by the Channel. 



The Ice Age. — Whatever the history of the plaice may have 

 been prior to the existence of the North Sea in an approach to its 

 present condition, it will be acknowledged that durmg the glacial 

 period the species and indeed the whole fauna and flora were excluded 

 from the area. It is not j^et quite clear in fact whether the North 

 Sea, owing to a cuhnination of the general uprismg of the land 

 which is generally supposed to have occurred during the PHocene 

 period, had not as such practically disappeared. 



Jukes Browne * does not believe that the low level of the pre- 

 glacial valleys necessarily means that the land stood at a higher 

 level, but rather that the North Sea and the Irish Sea occupied 

 deep valleys which during the ice age were filled with glacial drift. 

 That such valleys as did occur in the North Sea prior to the ice age 

 were filled with glacial detritus there can be no doubt. The Nor- 

 wegian deep maj^ indeed be post-glacial in origin and associated 

 with the formation of the Baltic, but the whole of the North Sea 

 has nob had such an origin, for it seems to be an estabhshed fact 

 that pre-glacial fossils have repeatedly been dredged from its 

 bottom in the neighbourhood of the Dogger Bank. There was a 

 period, my colleague, Professor Lebour, assures me, that Crag 

 fossils of various kinds were regularly obtamed by fishermen, and 

 examples are scattered in North Shields and doubtless occur m 

 the collections of the late Curator of the Hancock Museum, K. 

 Howse. The Dogger Bank probably marks therefore a portion of 

 the land which m early post-glacial times stiU connected England 

 with the Contment. 



Such considerations as these, and facts of distribution f which 

 point to important land connections in north-western Europe 



The Building of the British Isles, 1911. 

 t Schartf, European Animals. 



