528 
GEOGKAPHICAL 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 mollusca 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 , Omphalotmpis, 
and many others, are either wholly confined to this sub-region 
or highly characteristic of it. Mr. Binney, in his Catalogue of 
the Air-breathing Molluscs of North America, marks out our 
Nearetic region (with almost identical limits) as most clearly 
