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CHAP, xiv.l THE NEOTROPICAL REGION. 27
quity) and perhaps even to the elevation of the continuous land
which forms the base of the mountains. It was, no doubt, during
their slow elevation and the consequent loosening of the surface,
that the vast masses of debris were carried down which filled up
the sea separating the Andean chain from the great islands of
Brazil and Guiana, and formed that enormous extent of fertile
lowland forest, which has created a great continent ;
given space
for the free interaction of the distinct faunas which here met
together, and thus greatly assisted in the marvellous development
of animal and vegetable life, which no other continent can match.
But this development, and the fusion of the various faunas into
one homogeneous assemblage must have been a work of time ;
and it is probable that most of the existing continent was dry
land before the Andes had acquired their present altitude. The
blending of the originally distinct sub-faunas has been no doubt
assisted by elevations and depressions of the land or of the ocean,
which have alternately diminished and increased the land-area.
This would lead to a crowding together at one time, and a dis-
persion at others, which would evidently afford opportunity for
many previously restricted forms to enter fresh areas and become
adapted to new modes of life.
From the preceding sketch it will appear, that the great sub-
region of Tropical South America as here defined, is really formed
of three originally distinct lands, fused together by the vast
lowland Amazonian forests. In the class of birds sufficient materials
exist for separating these districts ; and that of the Andes
contains a larger series of peculiar genera than either of the
other sub-regions here adopted. But there are many objections
to making such a sub-division here. It is absolutely impossible
to define even approximate limits to these divisions—to say for
example where the "Andes" ends and where "Brazil" or
" Amazonia " or " Guiana " begins ; and the unknown border
lands separating these are so vast, that many groups, now appar-
ently limited in their distribution, may prove to have a very
much wider range. In mammalia, reptiles, and insects, it is
even more difficult to maintain such divisions, so that on the
whole it seems better to treat the entire area as one sub- region,