GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 



59 



of the brackish zone. M. entomon is a typical inhabitant of this zone and of 

 the fresh waters of many closed bodies of water of the Arctic basin. 



E. F. Gurjanova (1938) considers that in the Ice Age many forms acquired 

 the capacity for a wide vertical distribution, and thus deep-water species were 

 formed which inhabited the depressions of the Arctic basin and in this way 

 escaped the surface loss of salinity. Some of these series are given in Table 24. 



Table 24. Series with capacity for wide vertical distribution {after E. F. Gurjanova) 



This distribution indicates that the formation of species adapted to various 

 degrees of salinity proceeded through several stages, and that the process 

 of the salinity change had a step-by-step character. This suggestion is confirmed 

 (E. F. Gurjanova, 1939) by the fact that 'all the links of this chain of species 

 from the typical marine stenohaline species to the fresh-water ones exist 

 simultaneously in the same basin (Kara and Laptev Seas). This indicates 

 that the formation of the present Arctic fauna, accompanied by the division 

 of the autochthonous genera into shallow-water species of a different stage 

 of brackishness and into deep-water species, took place there and that, con- 

 sequently, the region of the Kara and Laptev Seas is not only the centre of the 

 development of the modern young (Ice Age) high Arctic fauna of the con- 

 tinental shelf but also its place of origin. ' The high Arctic endemic forms of 

 the Arctic show usually a characteristic break in their circumpolar habitat 

 in the region of the Greenland Sea, the Norwegian Sea and the western part 

 of the Barents Sea. 



