4 
ZOOLOGICAL GEOGRAPHY. 
[part ii r. 
guay, and of the Orinooko out of what were once, no doubt, arms 
of the sea, separating the large islands of Guiana, Brazil, and the 
Andes. From these concurrent favourable conditions, there has 
resulted that inexhaustible variety of generic and specific forms 
with a somewhat limited range of family and ordinal types, 
which characterise neotropical zoology to a degree nowhere else 
to be met with. 
Together with this variety and richness, there is a remarkable 
uniformity of animal life over all the tropical continental portions 
of the region, so that its division into sub-regions is a matter 
of some difficulty. There is, how T ever, no doubt about separating 
the West Indian islands as forming a well-marked subdivision; 
characterised, not only by that poverty of forms which is a 
general feature of ancient insular groups, but also by a number 
of peculiar generic types, some of which are quite foreign to the 
remainder of the region. We must exclude, however, the islands 
of Trinidad, Tobago, and a few other small islands near the coast, 
which zoologically form a part of the main land. Again, the 
South Temperate portion of the continent, together with the high 
plateaus of the Andes to near the equator, form a well-marked 
subdivision, characterised by a peculiar fauna, very distinct both 
positively and negatively from that of the tropical lowland dis- 
tricts. The rest of Tropical South America is so homogeneous in 
its forms of life that it cannot be conveniently subdivided for the 
purposes of a work like the present. There are, no doubt, con- 
siderable differences in various parts of its vast area, due partly to 
its having been once separated into three or more islands, in part 
to existing diversities of physical conditions ; and more exact 
knowledge may enable us to form several provinces or perhaps 
additional sub-regions. A large proportion of the genera, how- 
ever, when sufficiently numerous in species, range over almost 
the whole extent of this sub-region wherever the conditions are 
favourable. Even the Andes do not seem to form such a barrier 
as has been supposed. North of the equator, where its western 
slopes are moist and forest-clad, most of the genera are found on 
both sides. To the south of this line its western valleys are arid 
and its lower plains almost deserts ; and thus the absence of a 
