548 DEPTHS OF THE OCEAN chap. 



distinctive character chiefly to the low temperature of its bottom 

 water, and to its isolated position due to the submarine ridges, 

 which are responsible for the low temperature. 



Though the cold area of the Norwegian Sea must be re- 

 garded on these grounds as a separate faunal region, it un- 

 doubtedly had formerly more direct connection with the deep 

 water of the Atlantic. The many closely allied species in both 

 Sea and North areas point to a common origin. Most probably the fauna was 

 Atlantic. ^^ ^^^ ^j^^ homogeneous in both areas, and the bottom water 

 of the Norwegian Sea had then the same temperature as we 

 -find in the Atlantic nowadays. When physical conditions 

 changed in the Norwegian Sea, either owing to the formation 

 of the submarine ridges or from other causes, the fauna re- 

 sponded in two ways. Some of the warm water forms, including 

 a number of present Atlantic forms, died out, while others were 

 able to adapt themselves to the altered physical conditions and 

 survived. Their adaptation, however, led to morphological 

 alterations in the species, and in some cases these alterations 

 were considerable enough to produce distinct species differing 

 from the primitive Atlantic forms. Naturally, the isolation 

 brought about by the submarine ridges had much to do with 

 the development and establishment of their characteristics. In 

 fact, it seems like an experiment carried out by nature herself 

 on a large scale, and shows that external conditions can probably 

 alter the bodily structure of a species, and consequently give 

 rise to the formation of new species and varieties. 



To understand properly the composition of the fauna in the 

 Norwegian Sea at the present time we must go back to the 

 Glacial Age, when uniform arctic conditions prevailed, and the 

 fauna was everywhere arctic. This is confirmed by the marine 

 deposits of the Glacial Age, containing exclusively arctic animal 

 forms, met with in what are now boreal areas. When sub- 

 sequently the ice melted, and the climate became milder, southern 

 forms were able to immigrate, gradually distributing themselves 

 throughout the boreal (and boreo-arctic) waters. 



The question as to what happened to the arctic fauna of the 

 Glacial Age admits of a thoroughly satisfactory answer. In 

 areas which at the present day are arctic, we still find arctic 

 species, but in boreal areas the changes have been great. 

 Some of the arctic forms which formerly inhabited what 

 are now boreal areas have gradually died out from failure 

 to adapt themselves to the new conditions ; their remains may 



