THE BAHAMA ISLANDS 31 



has never been entirely submerged since the Eocene epoch, by which exemp- 

 tion they were preserved to again spread ont over the Bahamas when their 

 elevation again rendered them suitable for colonization. That this was com- 

 paratively recent is rendered probable by the extreme variability of the species 

 now occupying the Bahamas and which are frequently connected by inter- 

 grading forms, suggesting tliat the elimination which would be brought about 

 in the course of time by natural selection and other factors, has not yet had 

 time to complete its work of restricting the specific forms. 



This immigration took place from Haiti and Cuba, judging by the 

 analogies of the fauna, but the marked similarity to the Florida Oligocene 

 fauna is due not only to the presence of representative types but to the 

 absence of a multitude of others which are very characteristic of either Haiti 

 or Cuba. This is to be accounted for by the similarity of the environment. 

 The types represented in the Bahamas, as formerly in Florida, are lowland 

 forms, which for the most part affect the vicinity of the sea. These would 

 be the first to be transferred to the new land and would find it congenial, 

 while the species of the higher land and moister forests of Cuba and Haiti 

 would be transferred much less readily, and, if successfully transplanted, 

 would find the conditions of existence much less favorable. 



With the opportunity for occupying a favorable district in which there 

 were no competing organisms we find, as in insular faunas generally, a gi-eat 

 increase in variability, the development of a multitude of slightly varying 

 types, and the greatest profusion of individuals. 



To this is due the fact that in Cuba and Haiti, from which the immi- 

 grants came, the number of species of their type now living is much less in 

 proportion to the total fauna than in the Bahamas. Cerion and Cepolis 

 form about half the species of the Bahama landshell fauna, while in Cuba 

 and Haiti these two genera form an insignificant percentage of the molluscan 

 population. 



Another feature which confirms the view above stated is the fact that 

 several of the fossil species of the Bahamas more nearly resemble some of 

 those now living in Cuba or Haiti than they do the prevalent Bahama living 

 forms; or, at best, are intermediate between them. 



The landshells obtained by the Bahama Expedition in a fossil state are 

 the following species : 



