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. 



