INVERTEBRATE BOTTOM FAUNA 553 



while a purely boreal area (the deeper parts of the plateaus) 

 extends to lat. 71 N. How little latitude affects faunal marine 

 areas is evident when we compare the conditions on either side 

 of the northern Atlantic, for on the American side the southern 

 limit of the arctic shallow-water area lies about lat. 42' N., 

 whereas on the European side it lies about lat. 67° N. 



It has already been mentioned that intervening areas of a 

 different hydrographical character can always prevent connec- 

 tion between two, marine areas. The northernmost parts of 

 the Pacific and Atlantic are arctic, and so also is the sea 

 between them lying to the north of America. As a result the 

 arctic faunas of the two areas have an uninterrupted connection 

 and resemble each other. It is otherwise with the temperate 

 parts of these oceans, for their boreal forms are isolated by the 

 arctic tracts which intervene, though they share a few boreal 

 species like Crangon vulgaris, as well as some others that are too 

 closely allied for any one to doubt that they have formerly been 

 identical. This probably arises from hydrographical changes in 

 what are now arctic areas, which caused an isolation of specimens 

 belonging to the same species in both areas, for there are 

 indications that higher temperatures prevailed during post-glacial 

 times in the coast-waters of some of these arctic tracts, and we may 

 assume that the boreal species now occurring normally in boreo- 

 arctic areas could exist then in what have since become purely 

 arctic waters, and that by way of the shores of Canada and Alaska 

 they had uninterrupted connection from ocean to ocean. When 

 subsequently arctic conditions set in, the individuals of these 

 boreal boreo - arctic species were compelled to retire south- 

 wards either to the Atlantic or to the Pacific, and all connection 

 between them ceased. There is, of course, the possibility that 

 these species lived as long ago as the tertiary age — in which 

 case their present distribution can be easily explained — for 

 tertiary fossils make it perfectly certain that a warm climate 

 existed at that time in these latitudes. 



The theory of a warmer post-glacial period is based upon . 

 the sub-fossil boreal molluscs found in certain arctic areas, like JTenod. ^^"^ 

 those from the south-west coast of Greenland described by 

 Adolf Jensen, comprising shells of present-day boreal species 

 no longer found there (Anornia ephippium, Cyprina islandica, 

 ZirphcFa crispata). In the Gulf of St. Lawrence, too, where 

 conditions are nowadays arctic or boreo-arctic, we get quantities 

 of empty mussel-shells belonging to undoubtedly southern forms. 

 In the purely arctic waters of Spitsbergen there are sub-fossil 



Warm climate 



