148 TlMEHRI. 
the north. For our present purpose it will, however, 
be better to describe its position somewhat differently. 
The artificially formed political divisions of the conti- 
nent corresponding very closely, for obvious reasons, 
with the tracts naturally differentiated each by its own 
river system, and it being along the river systems that 
the migration of animals and plants chiefly occurs, there- 
fore the customary and convenient names of these poli- 
tical divisions really correspond somewhat closely with 
the natural and important differences in flora, as also 
in fauna, which distinguish the various river basins. 
Thus, as Venezuela is essentially the tract, drained 
by the great river Orinoco, and as Brazil is essen- 
tially the tract drained by the great river Amazon, 
and as Guiana, intermediate between these two, consists 
essentially of the parallel tracts drained by certain com- 
paratively, but only comparatively, small rivers, of which 
the Essequibo, the Demerara, the Berbice, the Corentyn, 
the Saramacca and the Maroni may be mentioned, so the 
political names, to mention them in their order from north 
to south, of Venezuela, Guiana, and Brazil, represent 
also natural tracts which are really more or less diffe- 
rentiated, each from the other in its flora and fauna. 
Now, as the whole of the tract under consideration — 
that drained by the Orinoco, the Amazon and the inter- 
mediate rivers — rises gradually, or more generally by 
steplike ascents, from the sea-level on its east, toward 
the table-land on its west— the table-land of the centre 
of the continent — it is of course on this table-land that 
the rivers take their origin. And, as owing to the ir- 
regularity of the surface of this table-land, and still more 
that of its slope toward the eastern sea, it happens that 
