CHAP, xiv.] THE NEOTKOPICAL 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 Helicidse is represented by 22 

 genera, of which 6 are peculiar. Spiraxis is confined to 

 Central America and the Antilles ; Stenopus and Sagda are 

 Antillean only ; Ortlialicus, Macroceramus, and Bulimulus have 

 a wider range, the last two extending into the southern United 



