49 



retain their larval state until they get into the neighbourhood of 

 the coast and water of low saluiity which probably supplies the 

 stimulus for the change. As is now well known the eels remain in 

 the fresh waters of north-western Europe until maturity impels 

 them to migrate to the spawnmg region in the Atlantic. 



With the further extension and deepening of the North Sea, 

 and perhaps still further hydrographical changes, it would no longer 

 be necessary for the plaice to migrate into the Atlantic on reaching 

 maturity. In addition to the passive migration of the species in 

 the larval condition, active migrations would tend to spread the 

 plaice still further south and east. New spawning areas would 

 therefore be formed within the North Sea, and these would become 

 fresh centres of dispersal. It may also be inferred that during the 

 period of submergence plaice were compelled to adopt the coastal 

 distribution which so characterises the species in the North Sea at 

 the present time. 



Such passive and active migrations would in similar manner 

 bring about the present distribution of plaice along the Norwegian 

 coast to the Barents Sea. Iceland and the Faroes may have already 

 been occupied by schools in these regions. Whether the Alaskan 

 plaice are pre- or post-glacial in origin cannot yet be said. 



During the same period Atlantic plaice would have penetrated 

 into the Channel bay, and when the bay was connected vv^ith the 

 North Sea, a second invasion of plaice into the latter area would 

 occur. The history would be similar to that just sketched for the 

 northern race, only in this case the feeding migration would be 

 north and east, and the Avinter migration south and west. This 

 southern race would also have penetrated into the Irish Sea, 

 although it is possible also that a second invasion came from the 

 north. 



Migration experiments have demonstrated, as has been stated, 

 that at maturity plaice migrate in many cases if not always outwith 

 the region of the school in which they were reared. This means in 

 the case of the northern North Sea race that the spawning grounds 

 are to a large extent if not completely recruited from the schools to 

 the south. Two questions arise from this. The first is : do the 

 plaice after spawning remain in association with the school near 

 which they happen to be, or do they return to the school to which 

 they belong ? Fulton has recorded examples which have migrated 

 long distances to the south from the Forth region. There is there- 



