5i6 DEPTHS OF THE OCEAN 



{Nepkrops norvegicus, Nymphon stromi, and Hippasterias plana) 

 furnish unmistakable evidence of the dissimilarity of these areas, 

 for they are widely distributed over the North Sea, occurring 

 even on the coasts of Great Britain in depths both greater 

 and less than lOO metres, and if they existed in the central 

 portion of the North Sea, where we frequently towed our 

 big trawls, they could hardly have avoided capture. Then 

 why should a considerable part of the central area of the 

 North Sea be closed to a number of forms more or less widely 

 distributed elsewhere ? We must, I think, conclude that in 

 this central area there are special hydrographical conditions 

 which exclude these forms and their larvae. As a matter 

 of fact, Helland- Hansen has shown that in the deeper layers 

 there is a circular current of Atlantic water in the North 

 Sea, a branch of the Gulf Stream following the east coast of 

 Scotland, turning north-east just before reaching the Dogger 

 Bank, and afterwards sweeping northwards on reaching the edge 

 of the Norwegian depression. As a result, the periphery of the 

 central portion of the North Sea is bathed by water of much the 

 same composition as the warmer water of the Atlantic, enclos- 

 ing an area covered by more stagnant and on the whole colder 

 water, having a fauna of its own.^ Repeated investigations 

 will be necessary to ascertain whether this faunal dissimilarity 

 observed in the summer of 1904 is permanent or not, 



Arctic and Boreo-Arctic Regions of the Norwegian Sea 



When we speak of an arctic and a boreal fauna it must be 

 clearly understood that there is not always a distinct line of 

 demarcation between the two, either in regard to topographical 

 boundaries or to forms. There are undoubted intermediate 

 areas, where boreal and arctic forms meet, and many forms 

 are as much boreal as arctic, being impartially distributed 

 over either region, and able to thrive amidst very different 

 natural conditions. It is interesting to note, however, that the 

 same species sometimes occurs in two distinct varieties, usually 

 connected by transition forms, and that the varieties conform 

 to the region in which they occur, a fact indicative in all prob- 

 ability of the influence of physical conditions upon organisms. 



A circumstance that has especially attracted the attention 

 of arctic investigators is that some animal forms are apt to 



^ I must add that the entire northern part of the North Sea plateau is also covered by Atlantic 

 water. 



