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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 moUusca. 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-regions.
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 exhibits all the peculiar characteristics of
the region in their fullest development. We begin therefore
with our second division.
II. Tr(ypical 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.