BIOLOGIC BEARING OP CHANGES OF LEVEL. 135 



"^'hich prevailed on the continent have emboldened the writer to present 

 this })aper, many of tlie data of which are new or given with new interpre- 

 tations. There have l)een many opinions of a generalized character, but 

 few have been accompanied by detailed evidence. On the other hand, 

 there has been a great deal of most valuable information relating to the 

 geology published, without which this paper could not have been written. 

 The key to the dynamic problems was found in Cuba and the southern 

 states ; but in Jamaica and San Domingo it could have been obtained 

 just as well, after making the study of the drowned valleys. 



The changes in the physical conditions have had an effect on the distri- 

 bution and preservation of life. To land shells and mammals let us 

 refer. 



Long ago Mr Thomas Bland was led to conclude that Florida and the 

 West Indies had been once united, on account of the relation of the land 

 shells. According to Mr Charles T. Simpson, tlie land shells belonging 

 to the West Indies aggregate 1,700 species. Those of the Greater Antilles 

 (Cuba, Haiti, Puerto Rico and Jamaica) are generically and by minor 

 groups closely related, but the species are mostly restricted to each island. 

 The type of the fauna ai)parently belongs to the islands, but having rela- 

 tions with Yucatan, Mexico and somewhat with Florida. The fauna of 

 the smaller islands is poorer, and appears to have been derived from South 

 America, having little relationshii) with the greater islands. The fauna 

 of l^ahamas is almost identical with that of Cuba and Haiti. Mr Simp- 

 son regards the type as dating back to the early Tertiary period. 



Let us see what relation existed between the physical conditions of the 

 West Indian region and the shell-life. At no time since the later Creta- 

 ceous period have the Greater Antilles been completely submerged, 

 although they were several times reduced to very small islets, now repre- 

 sented by the higher mountain districts which rise above the Tertiary 

 formations. Consequently the earliest types of shells could have migrated 

 to the unsubmerged lands, and, owing to their habits, found sufficient 

 food and favorable conditions for perpetuating their types. This retreat 

 to small islands was favorable for the specialization into local species. 

 The difference between the types of the larger and smaller islands is not 

 ren:iarkaljle, for the great physical break between the Virgin islands and 

 the other Windward gnjups was of such a character as to interfere with 

 migrations. Thus, while the continent was perhaps more than two miles 

 higli in that region, the deep depression to sealevel could interfere with 

 migrations. The i)reservation of related land sliells in Yucatan and 

 Mexico is not remarkable. Those regions have suffered the same insula- 

 tions as the Antilles, thus favoring the perpetuation of old types, for 

 which the less isolated conditions of Florida would afford less protection. 



XIX-Brr.i,. (ir.nj.. fine. Am.. Voi,. fi. 18ii4. 



