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78 ZOOLOGICAL GEO&RAPHY. [part iii.
there is good reason to believe that many species have become
extinct since the European occupation of them. When small
islands are much cultivated, many of these molluscs which can
only live under the shade of forests, are soon extirpated. In
St. Croix many species have become extinct at a comparatively
recent period, from the burning of forests ; and as we know that
in all the islands many of the species are excessively local, being
often confined to single valleys or ridges, we may be sure that
wherever the native forests have disappeared before the hand of
man, numbers of land-shells have disappeared with them. As
some of the smaller islands have been almost denuded of their
wood, and in the larger ones extensive tracts have been cleared
for sugar cultivation, a very considerable number of species have
almost certainly been exterminated.
General Conclusions as to the Past History of the West Indian
Islands.—The preceding sketch of the peculiarities of the animal
life of these islands, enables us to state, that it represents the
remains of an ancient fauna of decided Neotropical type, having
on the whole most resemblance to that which now inhabits the
Mexican sub-region. The number of peculiar genera in all
classes of animals is so great in proportion to those in common
with the adjacent mainland, as to lead us to conclude that,
subsequent to the original separation from the Mexican area, a
very large tract of land existed, calculated to support a rich and
varied fauna, and, by the interaction of competing types, give
rise to peculiar and specially modified organisms. We have
already shown that the outline of the present islands and the
depths of the surrounding seas, give indications of the position
which not improbably occupied
and extent of this ancient land ;
the space enclosed by uniting Western Cuba with Yucatan, and
Jamaica with the Mosquito Coast. This land must have
stretched eastward to include Anguilla, and probably northward
to include the whole of the Bahamas. At one time it perhaps
extended southward so as to unite Hayti with northern
Venezuela, while Panama and Costa Eica were sunk beneath the
Pacific. At this time the Lesser Antilles had no existence.
The only large island of whose geology we have any detailed