CHAP. XIV.] 
THE NEOTROPICAL REGION. 
51 
mainland. The species are here so very few, that the greatest 
advocate for continental extensions would hardly call such vast 
causes into action, to account for the presence of these three 
birds on so small and so remote an island, especially as the 
union must have continued down to the time of existing species. 
But if accidental immigration has sufficed here, it will also 
assuredly have sufficed where the islands are larger, and the 
chances of reaching them proportionately greater; and it is 
because an important principle is here illustrated on so small 
a scale, and in so simple a manner as to be almost undeniable, 
that we have devoted a paragraph to its elucidation. 
A few Coleoptera from Juan Fernandez present analogous 
phenomena. All belong to Chilian genera, while a portion of 
them constitute peculiar species. 
Land-shells are rather plentiful, there being about twenty 
species belonging to seven genera, all found in the adjacent 
parts of South America; but all the species are peculiar, 
as well as four others found on the island of Mas-a-fuera. 
III. Tropical North America, or the Mexican Sub-region. 
This sub-region is of comparatively small extent, consisting of 
the irregular neck of land, about 1,800 miles long, which 
connects the North and South American continents. Almost 
the whole of its area is mountainous, being in fact a con- 
tinuation of the great range of the Rocky Mountains. In 
Mexico it forms an extensive table-land, from 6,000 to 9,000 
feet above the sea, with numerous volcanic peaks from 12,000 to 
18,000 feet high; but in Yucatan and Honduras, the country is 
less elevated, though still mountainous. On the shores of the 
Caribbean Sea and Gulf of Mexico, there is a margin of low 
land from 50 to 100 miles wide, beyond which the mountains 
rise abruptly ; but on the Pacific side this is almost entirely 
wanting, the mountains rising almost immediately from the sea 
shore. With the exception of the elevated plateaus of Mexico 
and Guatemala, and the extremity of the peninsula of Yucatan, 
the whole of Central America is clothed with forests ; and as its 
surface is much broken up into hill and valley, and the volcanic 
E 2 
