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CHAP. XIV.] THE NEOTROPICAL REGION. 51
mainland. Tlie 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.
///. Tropical North America, m 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 Eocky 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 stiU 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
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