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 
