550 DEPTHS OF THE OCEAN 



most from those of the Norwegian Sea. It seems, then, that 

 the arctic-boreal species have not been able to adapt themselves 

 to such conditions, or in other words that their power of 

 adaptation is limited. 



Outside the Norwegian Sea the species of this group 

 have another area of distribution on the western side of 

 the Atlantic, north of Cape Cod. The cold polar current 

 sweeps down over the shallow parts of the American coast, and 

 produces conditions that vary from arctic to boreo-arctic. As 

 a result we find there arctic species, such as normally occur in 

 the boreo-arctic areas of the Norwegian Sea and similar waters, 

 and also the majority of the arctic-boreal species of the 

 Norwegian Sea, a few of the latter being found as well a 

 little to the south of Cape Cod, where conditions are more 

 boreal. 



(2) The second group consists of the boreal species, that is 

 to say, those species which are limited to boreal areas within 

 the Norwegian Sea, and those which are able to penetrate as 

 well into boreo - arctic areas, though not into arctic tracts. 

 Most of them are widely distributed over the northern 

 Atlantic, either in its littoral and sub-littoral or in its deeper 

 zones. We find their southern limit accordingly in the Medi- 

 terranean or at the Azores and the Canary Islands, while the 

 deep-sea forms also go a long way south on the American side. 

 Very few of the shallow-water forms, however, which extend 

 southwards along the coasts of Europe are to be met with on 

 the American side of the Atlantic, either because they cannot 

 pass across the profound depths separating the two continents, 

 or because they are debarred from advancing over the shallow 

 northern parts of the Atlantic by the arctic conditions prevailing 

 there. No satisfactory explanation can, therefore, be given for 

 the presence of the very few boreal shallow-water forms which 

 are common to both sides. 



I have already stated that most of the species of this group 

 have migrated into the Norwegian Sea in post-glacial times, 

 and their present distribution is evidence of this ; but there are 

 some species nowadays confined on the eastern side to the 

 boreal and boreo-arctic areas of the Norwegian Sea, and on 

 the western side occurring to the north, and in some cases 

 also a little to the south, of Cape Cod. As to their origin it is 

 difficult to form an opinion, but most probably a number of them 

 have been developed from arctic species after the ice-period 

 came to an end, and have adapted themselves to their boreal 



