524 GEOGRAPHICAL ZOOLOGY. [part iv. 



absence of genera confined to, and characteristic of Africa and 

 India. One small sub-genus of Helix, (Rachis), and one of Acha- 

 tina, {Homorus), appear to have this distribution, — a fact of but 

 little significance when we find another sub-genus of Helix, 

 (Hapalus), common and confined to Guinea and the Philippine 

 Islands ; and when we consider the many other cases of scattered 

 distribution which cannot be held to indicate any real connection 

 between the countries implicated. No genus is confined to the 

 Pala3arctic and Nearctic regions as a whole. A large number 

 of sub-genera, many of them of considerable extent, are peculiar 

 to one or other of these regions, but only 3 sub-genera of Helix 

 and 2 of Pupa are common and peculiar to the two combined, 

 and these are always such as have an Arctic range and whose 

 distribution therefore offers no difficulty. 



We find, then, that each of our six regions and almost all of 

 our sub-regions are distinctly confirmed by the distribution of the 

 terrestrial mollusca ; while the different combinations of them 

 which have at various times been suggested, receive little or no 

 support whatever. Even those remarkably isolated sub-regions, 

 New Zealand and Madagascar, have no strictly peculiar genera of 

 land- shells, although they both possess several peculiar sub- 

 genera ; being thus inferior in isolation to some single West 

 Indian Islands, to the Sandwich Islands, and even to the North 

 Atlantic Islands (Canaries, Madeira, and Azores), each of which 

 have peculiar genera. This of course, only indicates that the 

 means by which land moUusca have been dispersed are some- 

 what special and peculiar. To determine in what this speciality 

 consists we must consider some of the features of the specific 

 distribution of this group. 



The range of genera, and even of sub-genera is, as we have 

 seen, often wide and erratic, but as a general rule the species 

 have a very restricted area. 



Hardly a small island on the globe but has some land-sheUs 

 peculiar to it. Juan Fernandez has 20 species, all peculiar. 

 Madeira and Porto Santo have 109 peculiar species out of a total 

 of 134. Every little valley, plain, or hill-top, in the Sandwich 

 Islands, though only a few square miles in extent, has its 



