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 com- 
petition of the chance immigrants 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 pre- 
dominant 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 Helicidas is represented by 22 
genera, of which 6 are peculiar. Spiraxis is confined to 
Central America and the Antilles ; Stenojms and Sagda are 
Antillean only ; Orthalicns , Macroceramus , and Bulimulm have 
a wider range, the last two extending into the southern United 
C 2 
