268 ORIGIN OF LIFE IN AMERICA 



this old land was submerged having since gradually regained 

 its present position. 



The great age of the West Indian fauna and the inter-re- 

 lationship between the islands and the mainland is well exem- 

 plified by the ancient family of operculate snails the 

 Cyclophoridae.* The genus Neocyclotus inhabits principally 

 northern South America and the Antilles. From this 

 apparently very old centre of dispersal some members of 

 the genus have pushed southward as far as Peru in the west 

 and Rio de Janeiro in the east. A few have entered Central 

 America. One distinct group (Plectocyclotus) has no less 

 than thirty-two species in Jamaica and only one in Portorico. 

 Another genus (Crocidopoma) is entirely confined to Jamaica, 

 Haiti and eastern Cuba. This indicates strikingly the re- 

 lationship of the three Great Antilles and their distinctness 

 'from western Cuba, which was already pointed out by Dr. 

 Pilsbry, while geologists maintain that western Cuba was 

 submerged quite independently from the remainder of the 

 islands. It also illustrates the extreme slowness with which 

 the dispersal of these mollusks takes place. 



Still more instructive is the whole group to which 

 Crocidopoma belongs. With Cyrtotoma, Amphicyclotus and 

 Buckleya, it forms, as already mentioned (p. 256), a group 

 of closely related genera of operculate snails. I alluded 

 also to the fact that three of them had a discontinuous range 

 in Central America, and that Amphicyclotus had apparently 

 travelled eastward from Ecuador, invading Venezuela and 

 Guiana, and had thence passed into the islands of Martinique, 

 Guadeloupe and Dominica, when the latter were connected 

 with one another, and with the mainland. It might be 

 urged that accidental dispersal is responsible for their 

 presence on these islands. 'But we have no reason for such a 

 supposition, because the species occurring on the islands are 

 quite distinct from one another and from those of Venezuela. 

 Some evidence is afforded by these snails for the belief that 

 the Lesser Antilles are remnants of older land which ex- 

 tended northward from Venezuela, although all the visible 

 parts of the islands are covered by modern volcanic deposits. 



* Kobelt, W., " Cyclophoridae." 



