AMERICAN^ AND WEST INDIAN FAUNA AND FLORA. 115 



special conditions more or less favorable to certain genera would 

 be produced, and thus tend to the remarkable specialization of 

 many of the birds in individual islands. This, as suggested by 

 Wallace, would show that the islands were not peopled by 

 immigration from surrounding countries while in the condition 

 we now see them. 



The reptiles show likewise a general relation to the Central 

 American and Mexican types. One of the most interesting is a 

 gigantic land tortoise, found at Porto Rico, differing only in 

 size from the land turtle still found on Trinidad and adjoining 

 parts of South America. It is closely allied to the gigantic 

 turtles of the Galapagos and Mascarene Islands, and to the 

 fossil land turtles, of which fragments have been described by 

 the late Professor Wyman. These were collected by Mr. A. A. 

 Julien at Sombrero, in the phosphate beds of the island. 



The species of iguana characteristic of the small island of 

 Navassa and of Hayti presents a case of specialization very sim- 

 ilar to that of the Amblyrhynchus of the Galapagos, and of an 

 allied species occurring at the Fiji Islands. The aquatic habits 

 of these large saurians make a migration to a distant point quite 

 feasible. 



It is from the study of the land shells, however, which have 

 been so carefully observed by Bland and others, that we may 

 get a better idea of the great specialization which has taken 

 place in the development of the molluscan fauna characteristic 

 of the different islands. If, as we may assume, the islands have 

 received their molluscan fauna from the adjoining mainland, we 

 might naturally expect that those islands which were more re- 

 cently connected with the mainland, or to which access, owing 

 to the direction of the winds and currents, was most easy, would 

 show a greater preponderance of continental forms than those 

 which have been longer separated from the main coast, or to 

 which the winds and currents do not lead so directly. In addi- 

 tion, the more or less favorable physical features of the different 

 islands have undeniably had their influence in the increase of 

 the land shells. 



The greater West India Islands, according to Bland, are nearly 

 equally rich in land shells. The eastern islands, Porto Rico and 



