ZOOLOGICAL GEOGEAPHY. [partiii. 



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, however, 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 numljer 

 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 



