246 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



Mr. Dickson adds a discussion of the forces at work in producing the 

 observed conditions and changes, taking separately the local forces in 

 the Xorth Sea and the external. He finds that the chief influence at 

 work Tocally is the wind. His conclusions are that the existence of 

 a continuous strip of oceanic water along the central axis of the North 

 Sea, is due to westerly and south-westerly winds strengthening the 

 oceanic current. Strong northerly winds tend to broaden the northern 

 area of oceanic water, and to blunt its extremity, and also have the effect 

 of sending coast water southwards along the west coast of Norway : it 

 is probable that this is the cause of the inflow of coast water into the 

 Skagerack. Calm weather favours the spread of North Sea water over 

 a great part of the North Sea, while easterly and south-easterly winds 

 spread the fresher waters of the east side of the sea over the surface. 



With regard to the inflowing or oceanic waters, as they are all 

 of about the same salinity, Mr. Dickson infers their movements from 

 the different temperatures. He finds that the Atlantic streams are on 

 the whole strongest in summer, but the cold streams moving southwards 

 from the eastern coasts of Iceland and Greenland are also strongest in 

 summer. It is a mixture of the two which takes place in the Faroe- 

 Shetland Channel, and this mixture is driven into the North Sea in the 

 manner already mentioned. 



I do not think that, for my present purpose, it would be of much 

 advantage to give a more detailed account of Mr. Dickson's discussion 

 of the physical conditions. He does not enter upon their relation to the 

 migrations and distribution of fish, and it seems to me that, with respect 

 to this relation, the changes and distribution of actual temperature and 

 salinity in detail are of more importance than the forces by which these 

 changes are caused. The points of view of the physicist and the 

 biologist are different. The former is chiefly interested in tracing the 

 causes of the observed movements of waters, regarding temperature and 

 salinity rather as qualities by which the different currents are to 

 be identified. The biologist's business, on the other hand, is to ascertain 

 how the degrees of temperature and salinity affect the migrations and 

 distribution of different species of fish. It must be understood, there- 

 fore, that in this paper I have, to a large extent, expressed conclusions 

 of my own concerning the relation of the observations published by the 

 physicists to those made by zoologists. 



We have then to consider more particularly what is known concern- 

 in" the actual distribution of fishes and other animals in the North Sea, 

 and the waters communicating with it. With regard to two well-defined 

 portions of the regions, namely, the Baltic and the Heligoland Bight, 

 the facts have been collected and analysed in a very interesting manner 

 by the German naturalists associated with the Kiel Commission, and the 



