PDF - Wallace Online

CHAP. XIV.] THE NEOTROPICAL REGION. 19
over the globe is not nearly so great as we might expect. This
points to a long period of isolation, during which the various
forms of life have acted and reacted on each other, leading to such
a complex yet harmoniously-balanced result as to defy the competition
of the chance imniigrants that from time to time must
have arrived. This is quite in accordance with the very high
antiquity we have shown most insect-forms to possess ; and
it is no doubt owing to this antiquity, that such a complete
diversity of generic forms has been here brought about, without
any important deviation from the great family types which pre-
vail over the rest of the globe.
Land Shells.—The Neotropical region is probably the richest
on the globe in Terrestrial Mollusca, but this is owing, not to any
extreme productiveness of the equatorial parts of the continent,
where almost all other forms of life are so largely developed, but
to the altogether exceptional riches of the West India Islands.
The most recent estimates show that the Antilles contain more
species of land shells than all the rest of the region, and almost
exactly as many as all continental America, north and south.
Mr. Thomas Bland, who has long studied American land shells,
points out a remarkable difference in the distribution of the
Operculated and Inoperculated groups, the former being predominant
on the islands, the latter on the continent. The
Antilles possess over 600 species of Operculata, to about 150
on the whole American continent, the genera being as 22 to 14.
Of Inoperculata the Antilles have 740, the Continent 1,250, the
genera being 18 and 22. The proportions of the two groups in
each country are, therefore
West India Islands.
American Continent.
Operculata Gen. 22 Sp. 608 ; 14 151
Inoperculata „ 18 ,,737 22 1251
The extensive family of the Helicidae is represented by 22
genera, of which 6 are peculiar. Spiraxis is confined to
Central America and the Antilles ; Stenopus and Sagda are
AntiUean only ;
Orthalicus, Macroceramus, and Bulimulus have
a wider range, the last two extending into the southern United