528 GEOGRAPHICAL ZOOLOGY. . [part iv. 



to the contrary, recent immigrants into the Old World ! This 

 example alone must convince us, that it is impossible to form 

 any conclusion as to the origin of a genus, from the distribution 

 of existing species only. 



The general conclusion we arrive at, therefore, is, that the 

 causes that have led to the existing distribution of the genera 

 and higher groups of the terrestrial moUusca are so complex, and 

 have acted through such long periods, that most of the barriers 

 which limit the range of other terrestrial animals do not apply to 

 them, although the species are, in most cases, strictly limited 

 by them. Some means of diffusion — which, though probably 

 acting very slowly and at long intervals, and more powerfully 

 on continents than between islands, is yet highly efficient when 

 we consider the long duration of genera — has, to a considerable 

 extent, dispersed them across continents, seas, and oceans. On 

 the other hand, those mountain barriers which separate many 

 groups of the higher vertebrates, are generally less ancient than 

 the genera of land-shells, which are thus often distributed inde- 

 pendently of them. In order to compare the distribution of the 

 terrestrial mollusca on equal terms with those of land animals 

 generally, we must take genera of the former as equivalent to 

 family groups of the latter ; and we shall, I believe, then find 

 that the distribution of the sub-genera and smaller groups of 

 species do accord mainly with those divisions of the earth into 

 regions and sub-regions which we have here indicated. Mr, 

 Harper Pease, in a communication on Polynesian Land Shells 

 in the Proceedings of the Zoological Society for 1871 (p. 449), 

 marks out the limits of the Polynesian sub-region, so as exactly 

 to agree with that arrived at here from a consideration of the 

 distribution of vertebrata ; and he says that this sub-region, (or 

 region, as he terms it) is distinctly characterised by its land- 

 shells from all the surrounding regions. The genera (or sub- 

 genera) Partula, Pitys, Achatinella, Palaina, Omphalotropis, 

 and many others, are either wholly confined to this sub-region 

 or highly characteristic of it. Mr. Binney, in his Catalogue of 

 the Air-hreathing Molluscs of North America, marks out our 

 Nearctic region (with almost identical limits) as most clearly 



