GENERAL BIOLOGY 



705 



is peculiar in so far that southern species of molluscs, for 

 instance, occur as isolated specimens even far north in the 

 Norwegian Sea, while northern species have a sharp southern 

 limit (see Chapter VII.). Vast numbers of small pelagic 

 organisms are introduced into the Norwegian Sea from the 

 Atlantic. 



As the water-masses of a current are carried along, the 



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Fig. 50S.— Currents of the North Atlantic. (From Schott's " Valdivia" Report and 

 Helland-Hansen and Nansen's memoir on The Norwegian Sea.) 



conditions of existence for certain animals change, and as a 

 consequence the fauna gradually changes in character. This 

 change of fauna from place to place in the same expanse of 

 water has always presented interesting problems in oceanic 

 research. Sir John Murray writes upon this point as follows : 

 "Where cold and warm currents meet at the surface of the Effect of large 

 ocean, there is a rise of temperature for the animals of the cold ff"^!.°H,-„ 



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current, and a fall of temperature for the animals of the warm in the surface 

 current, which results in a plentiful destruction of organisms. ^^^^^'^^• 



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