PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NOKTH SEA. 257 



mentioned in the above list because not occurring in the Baltic, It 

 is common in the northern part of the Xorth Sea, in the deeper water, 

 and also at Iceland, and at depths over 30 fathoms on the south-west 

 coast of England. 



In the northern region the species most abundant in individuals are 

 haddocks, whiting, cod, plaice, dabs, lemon dabs, witches, long rough 

 dabs, megrims, cat-fishes, ling, while on the southern ground the only 

 northern species which are abundant are whiting, plaice, and dabs. 



In this discussion I have omitted all mention of a number of species, 

 such as Raia clavata, the thornback ray; Baia hatis, the skate; Lophiua 

 2yiscatorius, the angler ; which are classed by Heincke and Mobius as of 

 indefinite distribution, because they either extend both to the Arctic 

 Ocean and the Mediterranean, or to neither. 



In the further consideration of the distribution of northern species, 

 three subjects may be taken separately : (1) the general physical con- 

 ditions, (2) the migrations of the herring, (3) the difference in the size 

 of fish of the same species in different parts of its habitat. 



(1) The general ijhydccd conditions. In general terms, the physical fact 

 which determines the distribution of northern species of fish in the North 

 Sea, is that deep water in open communication with the Arctic Ocean 

 extends along the east coast of Britain towards the coast of Norfolk. If 

 we look at the contour lines of the sea-bottom, we see that the 100 

 fathom line passes round the north of the Shetlands and bends round 

 to the south, parallel to the Norwegian coast and at no great distance 

 from it. The 50 fathom line passes dowu the east side of the Shetlands, 

 Orkneys, and the east coast of Britain to the latitude of the Farn Islands, 

 and runs north again along the west side of the Great Fisher Bank, to 

 the edge of the Norwegian depression. The 40 fathom line runs further 

 south off the east coast of England, and to the west of the Dogger Bank 

 and Great Fisher Bank. The 30 fathom line runs outside the Dogger 

 Bank, and the whole of the Fisher Bank is more than 30 fathoms in 

 depth. But the 20 fathom line isolates the Dogger Bank, and leaves a 

 valley between it and the slope of the mainland. To the south of this 

 valley the 20 fathom line runs across the North Sea from Flamborough 

 Head to the continental slope. 



Apart from movement of the water, this depression must contain at 

 the bottom water which is continuous with the cold bottom water of the 

 Arctic Ocean, and which cannot be much affected or raised in tempera- 

 ture by the warm current of the Gulf Stream, both because that is a 

 current of surface water, and because it Hows past the north of Britain 

 to the Norwegian coast. We see thus that the roads of the northern 

 forms and southern forms actually cross one another to the east of the 

 Shetlands and Orkneys, the southern species travelling in the warm 



T 2 



