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 mollusean 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 
