of West Lidla Land Shells. 339 



sidence of the surrounding laud. The character and arrange- 

 ment of its fauna are probably nearly the same now as when it 

 formed part of a continent." Tiie origin and condition of the 

 West Indies are very different. The presumption is, if indeed 

 there be not evidence of the fact, that the elevation of the 

 islands was gradual — that the tertiary beds which occur there 

 were deposited after the elevation of the central mountains, 

 and the inland cliffs and terraces plainly traceable (I have per- 

 sonally noticed them in Jamaica, Haiti, and Barbados) indicate 

 subsequent elevatory movements, synchronous probably with 

 those which upraised the adjoining continents- There is no 

 reason for believing that subsidence of contiguous lands caused 

 the geographical isolation of this group of islands, certainly 

 not within the period subsequent to the introduction of existing 

 species. The characteristics of the insular faunas were pro- 

 bably the same at the time of their origin on the islands as at 

 present. 



As the number of species of land shells in insular provinces 

 is generally much greater proportionally to the area of the 

 provinces than in a continental province, of which the Madeira 

 group is itself a striking example, it may with more reason be 

 assumed that the land shells originated there after, and not 

 anterior to the isolation of the group. Continents by submerg- 

 ence become islands, and islands by emergence become conti- 

 nents, but, as Prof. C. B. Adams* remarks, "such changes 

 require an amount of time exceeding one geological period, 

 during which time there is a change of faunas." 



Dr. Hooker states, as a general result of investigation, that 

 the sinking islands, those (so determined by Darwin) character- 

 ized as atolls, or as having barrier reefs, contain comparatively 

 fewer species, and fewer generic types than those which are 

 rising. The West Indies are in the latter category, while the 



* The paper entitled " Hints on the Geographical Distribution of Animals with 

 special reference to the Mollusca," by C. B. Adams, in the Contributions, is 

 extremely interesting, and worthy of more notice than it appears to have received. 



