SUMMARY OF DISTRIBUTION. 257 



Binney, " inhabits all the Atlantic States, from Maine to South 

 Carolina, and from Vermont to Council Bluffs on the Missouri." 

 Of Zua sub cylindrical, he says : " It is distributed over a vast 

 expanse of country ; it has been noticed in the North- western Ter- 

 ritory, near the Lake of the Woods and Lake Winnipeg, in Ohio, 

 in all the Middle States, and in every State of New England." 

 Now it is a curious fact that the two last-named species are quite 

 abnormal forms in the Caucasian molluscan fauna. Not one of 

 the other twenty-three British Helices have any typical affinity 

 with Helix pulchella ; and there are no other species of Zua in 

 Britain. Concomitant with this, they have the most extended dis- 

 tribution both in space and elevation of all mollusks in the Eastern 

 Hemisphere, — both species ranging throughout Europe, and in Asia 

 from Northern Siberia to Thibet. 



§ 3. 



The range of land, and freshwater species over areas (zoological pro- 

 vinces) indicated by uniformity of type, is not influenced by the inter- 

 vention of sea. 



It was remarked by Kirby, that physical conditions are not the 

 primary causes of the zoological provinces, which he regarded as 

 fixed by the Will of the Creator. The intervention of sea appears 

 to have no influence on the type or diffusion of species. The boun- 

 daries of two distinct provinces meet across a continent, as in West 

 Africa, or across an island, as in Japan, while parts of a province 

 may be widely separate by an intervening sea, as in the case of 

 the Mediterranean. There are, it is true, fewer Caucasian species 

 in North Africa than in Southern Europe, so would there be if 

 the intermediate space were dry land. The North African species 

 are not members of another fauna ; they are outlying southern 

 species of the Caucasian type, just as the British species are out- 

 lying western species of that type. North Africa has, however, 

 from fifty to sixty species of the Caucasian type, chiefly in Algeria 

 and Egypt, of its own ; while Britain has only three species of 

 its own, Assiminia Grayana, Geomalacus maculosus, and Lymncea 

 involuta. The sea around the islands of the Asiatic Archipelago, 

 which enclose the specific centre of the Malayan province of dis- 

 tribution, predominates in marked typical force in Burniak, Siam, 



