76 
ZOOLOGICAL GEOGRAPHY. 
[part III. 
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 
vertebrata, 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 v T e 
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 rock- 
bound 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 shelter and concealment for land- 
shells. 
