CHAP. XIV.] THE NEOTROPICAL EEGION. 21 



South America is, to some extent, peopled by Oriental and Pacific 

 genera of shells. On the west coast there is hardly any coral, 

 while on the east it is abundant, showing a difference of physical 

 conditions that must have greatly influenced the development 

 of mollusca. When these various counteracting influences are 

 taken into consideration, the identity or close affinity of about 

 140 species and 40 genera on the two sides of the Isthmus 

 of Panama becomes very important; and, combined with the 

 fact of 48 species of fish (or 30 per cent, of those known) 

 being identical on the adjacent coasts of the two oceans (as 

 determined by Dr. Giinther), render it probable that Central 

 America has been partially submerged up to comparatively re- 

 cent geological times. Yet another proof of this former union 

 of two oceans is to be found in the fossil corals of the Antilles 

 of the Miocene age, which Dr. Duncan finds to be more allied 

 to existing Pacific forms, than to those of the Atlantic or even 

 of the Caribbean Sea. 



Neotropical Sub-eegions. 



In the concluding part of this work devoted to geographical 

 zoology, the sub-regions are arranged in the order best adapted 

 to exhibit them in a tabular form, and to show the affinities of 

 the several regions ; but for our present purpose it will be best 

 to take first in order that which is the most important and most 

 extensive, and which exliibits all the peculiar characteristics of 

 the region in their fullest development. We begin therefore 

 with our second division. 



II. Tropical South-America, or the Brazilian Sub-region. 



This extensive district may be defined as consisting of all the 

 tropical forest-region of South America, including all the open 

 plains and pasture lands, surrounded by, or intimately associated 

 with, the forests. Its central mass consists of the great forest- 

 plain of the Amazons, extending from Paranaiba on the north 

 coast of Brazil (long. 42° W.) to Zamora, in the province of 

 Loja (lat. 4° S., long. 79° W.), high up in the Andes, on the west ; — 

 a distance in a straight line of more than 2,500 English miles, 



