SNAILS OF THE WEST INDIES 
265 
land mollusks, it may be urged, in the first instance, that the 
value of the evidence is greatly impaired by the possibility 
of occasional or accidental dispersal. That a certain per¬ 
centage of the fauna of an island is due to importation by 
human agency is, I think, well established. That a small 
proportion of the West Indian fauna may possibly have been 
carried from island to island and from the mainland by other 
accidental means is likewise possible. But I quite concur 
with Dr. Simpson in his belief that we are not justified in 
explaining the whole distribution of the terrestrial mollusks in 
the West Indian islands by such an hypothesis. The very 
nature of the West Indian fauna, and its distinctness from 
that of the neighbouring continents, precludes the assumption 
of any extensive accidental dispersal. Hence we may take for 
granted the correctness of the theory that the main mass of 
these mollusks have migrated from island to island when the 
latter formed a united land surface. This belief is vouched 
for, moreover, by the fact that the study of practically all 
other groups of West Indian animals brings us to precisely 
the same conclusion. In their general agreement with the 
results arrived at from a geological study, all these groups 
tend to show that the study of geographical distribution is a 
science of profound importance as an indicator of former 
changes of land and water. 
The facts adduced by Dr. Simpson, that the operculate 
species form so large a proportion of the Antillean land- 
snail fauna, that a majority of the genera is found on two 
or more of the islands and the mainland, while nearly every 
species is absolutely restricted to a single island, is a strong 
testimony in favour of' a former general land connection in 
this area. Dr. Simpson * very carefully compared the mol- 
luscan fauna of the various islands with one another and with 
that of the mainland, and bases his conception of the geolo¬ 
gical history of the Antilles mainly on the results so derived. 
He believes a considerable portion of the species inhabiting 
the Greater Antilles to be ancient, and to have developed on 
the islands where they are now found. Probably some time 
* Simpson, C. T., “ Distribution of Mollusks in West Indies,” pp. 447— 
448. 
