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76 ZOOLOGICAL GEOGRAPHY. [part hi.
are, they will be more effective where the islands have been long
separated from the mainland, as is here undoubtedly the case.
It seems most probable that the great development of land-
shells in islands, is due to the absence or deficiency of the verte-
brata, which on continents supply a variety of species adapted
to prey upon these molluscs. This view is supported by the fact,
that in such islands as have been united to a continent at no
very distant epoch, and still maintain a continental variety of
vertebrate, no such special development of land-shells has taken
place. If we compare the Philippine islands with the Sunda
group, we find the development of vertebrata and land-molluscs
in inverse ratio to each other. The same thing occurs if we
compare New Zealand and Tasmania ; and we have a still more
striking example in the Antillean group itself, continental
Trinidad having only 20 genera and 38 species, while the
highly insular Jamaica has about 30 genera and more than 500
species.
The other causes favourable to the increase and development
of land-shells are of a physical nature. A great extent of lime-
stone-rock is one ; and in the larger West Indian islands we have
a considerable proportion of the surface consisting of this rock.
But perhaps equally or more important, is the character of the
land surface, and the texture of the exposed rock itself A
much broken surface, with numerous deep ravines, cutting up
the whole country into isolated valleys and ridges, seems very
favourable to the specialization of forms in this very sedentary
class of animals. Equally favourable is a honeycombed and
highly-fissured rock-surface, affording everywhere cracks and
crannies for concealment. Now, taking Jamaica as an example
of the archipelago, we find all these conditions in a wonderful
degree. Over a large part of this island, a yard of level ground
can hardly be found; but ridges, precipices, ravines, and rockbound
valleys, succeed each other over the whole country. At
least five-sixths of the entire surface is limestone, and under the
influence of tropical rains this rock is worn, fissured, and honey-
combed, so as to- afford ample shelt'er and concealment for land-
shells.