PELAGIC ANIMAL LIFE 637 



communities of northern waters to distinguish between the 

 various types of areas of distribution. In accordance with all 

 previous descriptions of the animal life of northern waters, we 

 may recognise three typical faunas, viz. (i) the arctic, (2) the 

 boreal, and (3) the temperate Atlantic. 



The arctic communities include those forms which are 

 propagated and attain their maximum abundance in waters 

 belonging to the ice - covered area at temperatures below 

 2 'C. 



The temperate Atlantic communities comprise those forms 

 which occur mainly in the warm layers of the Atlantic, and only 

 at certain seasons or in small quantities occur in the north. 

 Most of these forms are entirely oceanic. 



The boreal communities include those forms having their 

 maximum frequency in waters at temperatures between 4° and 

 8° C. It is the boreal region which specially interests us, but 

 the nature of boreal communities can only be fully grasped 

 when we know the " strange elements " — the Atlantic and arctic 

 "visitors." 



The boreal region includes several areas, each limited by 

 natural borders, one of which lies between the west coast 

 of Britain and South Iceland, extending to the Faroe-Shetland 

 channel, the upper layers being occupied by the North 

 European branch of the Gulf Stream. Another area is the 

 Norwegian Sea, separated from the first-mentioned by the 

 submarine ridges between Shetland and Faroe and Iceland ; 

 a third area is found round Greenland, Davis Straits, and the 

 Newfoundland banks. 



We will discuss the Norwegian Sea first, because this area 

 has been most thoroughly investigated. 



The Noi'zuegian Sea. — The borders of the ice may be con- Arctic animal 

 sidered as indicating roughly the limits of distribution of pelagic communities. 

 arctic communities. It is therefore interesting to examine the 

 ice-limits as shown by the charts published by the Danish 

 Meteorological Institute. Fig. 481 represents some of these 

 ice-limits for different months of the years 1902, 1903, and 1906, 

 showing considerable variations from season to season and 

 from year to year. Vast areas of the Barents Sea and White 

 Sea are closed in winter and open in summer, as also the sea 

 off Spitsbergen, and the Greenland Sea between Jan Mayen 

 and Greenland. The Polar Sea north of Spitsbergen is in 

 certain years ice-covered all the year round, but sometimes a 



