CHAP. XIV.] 
TI-IE 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 ofthelandor 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 mate- 
rials 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, 
