78 
ZOOLOGICAL GEOGRAPHY. 
[part in. 
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 
and extent of this ancient land ; which not improbabty occupied 
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 Rica 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 
