PDF - Wallace Online

528 GEOGRAPHICAL ZOOLOGY. .
to the contrary, recent immigrants into the Old World !
[part
iv.
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