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CHAP, xxiii.] SUMMARY AND CONCLUSION. .
551
organisms have spread over the earth, but owing to their small
size and rapid multiplication, they have made use of some which
are exclusively their own. Such are the passage along moun-
tain ranges from the Arctic to the Antarctic regions, and the
dispersal of certain types over all temperate lands. It will
perhaps be found that insects have spread over the land surface
in directions dependent on our surface zones—forests, pastures,
and deserts ;—and a study of these, with a due consideration of
the fact that narrow seas are scarcely a barrier to most of the
groups, may assist us to understand many of the details of
insect-distribution.
Terrestrial Mollusca.
The distribution of land-shells agrees, in some features, with
that of insects, while in others the two are strongly contrasted.
In both we see the effects of great antiquity, with some special
means of dispersal ;
but while in insects the general powers of
motion, both voluntary and involuntary, are at a maximum, in
land-molluscs they are almost at a minimum. Although to
some extent dependent on vegetation and climate, the latter are
more dependent on inorganic conditions, and also to a large
extent on the general organic environment. The result of these
various causes, acting through countless ages, has been to spread
the main types of structure with considerable uniformity over
the globe ; while generic and sub-generic forms are often
wonderfully localized.
Land-shells, even more than insects, seem, at first sight, to
require regions of their own ; but we have already pointed out
the disadvantages of such a method of study. It will be far
more instructive to refer them to those regions and sub-regions
which are found *to accord best with the distribution of the
higher animals, and to consider the various anomalies they pre-
sent as so many problems, to be solved by a careful study of
their habits and economy, and especially by a search after the
hidden causes which have enabled them to spread so widely
over land and ocean.
The lines of migration which land-shells have followed, can
YoL. II.—36