THE DISTRIBUTION OF THE MOLLUSCA. 473 



Dr. Sclater, and Professor Edward Forbes, have done much towards 

 giving us a clear idea of zoological geography. Climate alone is 

 insufficient to account for the distribution of animals : some higher 

 cause rules here. But while we admit this, still we must acknowledge 

 that climate exerts considerable influence in modifying the qualities of 

 species. 



The distribution of the Mollusca may be considered from three 

 points of view. First, as regards geography; second as regards 

 depth ; and third as regards time ; the last belongs to geology. 



We shall now survey the principal divisions of the ocean; the line 

 of demarcation being drawn, not by latitude or longitude, but by 

 genera and species. 



The Mollusca of the Arctic seas are well known to show consider- 

 able analogy with those of the later Tertiary periods of Europe. 

 Hence the great interest connected with their comparison, as it affords, 

 provided we are satisfied with this line of argument, a proof that 

 an Arctic climate formerly existed in temperate regions. It is the 

 northern Drift of which we are speaking. Even when species are 

 found living in Britain identical with those of the Arctic regions, still 

 there is often a difference in the form or size of British and Arctic 

 specimens ; certain species, such as Cyprina Islandica, being com- 

 paratively small in the south of Britain, larger in Shetland, and 

 attaining their greatest size in Iceland. 



The countries included in the Arctic molluscan province are Lapland, 

 Iceland, Greenland, the west coast of Davis' Straits, and Behring's 

 Straits. About two hundred species are enumerated by the various 

 Arctic voyagers, as found in these seas ; of these about one-half are 

 peculiar to these seas, and the other half are either found living 

 in the temperate regions of Europe, or in their so-called glacial 

 strata. 



The Boreal province includes the North Atlantic, from Nova Scotia 

 to Iceland, and from thence to Faroe, Shetland, and the Norway 

 coast. 



The number of species is very large ; and more than one-half are 

 common both to Scandinavia and the North American coast, while a 

 great number also are found on the British coast. 



The province called Celtic by Professor Edward Forbes embraces 

 the coasts of Britain, Sweden, and Denmark. 



