130 ZOOLOGICAL OEOGRAPET. 



to the interior. A sufficient proportion of this moisture reaches the higher 

 slopes of the Andes, where its condensation gives rise to innumerable 

 streams, which cut deep ravines and carry down such an amount of sedi- 

 ment that they have formed the vast plains of the Amazon, of Paraguay 

 and of the Orinooko out of what were once, no doubt, arms of the sea, sepa- 

 rating the large islands of Guiana, Brazil, and the Andes. From these 

 concurrent favorable conditions, there has resulted that inexhaustible va- 

 riety of generic and specific forms with a somewhat limited range of family 

 and ordinal types, which characterise neotropical zoology to a degree no- 

 where else to be met with. 



Together with this variety and richness, there is a remarkable uni- 

 formity of animal life over all the tropical continental portions of the re- 

 gion, 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; characterized, 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 island* of Trini- 

 dad, Tobago, and a few other small islands near the coast, which zoological- 

 ly 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, characterized by a peculiar fauna^, 

 very distinct both positively and negatively from that of the tropical low- 

 land districts. 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, considerable differences 

 in various parts of its vast area, due partly to its having been once sepa- 

 rated into three or more islands, in part to existing diversities of physical 

 conditions; and more exact knowledge may enable us to form several prov- 

 inees or perhaps additional sub-regions. A large proportion of the genera, 

 however, when sufficiently numerous in species, range over almost the 

 whole extent of this sub-region wherever the conditions are favorable. 

 Even the Andes do not seem to form such a barrier as has been supposed. 

 Korth 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 number of groups to which verdant forests are essential, can 

 be traced to the unsuitable conditions rather than to the existence of the 

 mountain barrier. All Tropical South America, therefore, is here consid- 

 ered to form but one sub-region. 



The portion of North America that lies within the tropics, elosely re- 

 sembes the last sub-region in general zoological features. It possesses 

 hardly any positive distinctions; but there are several of a negative char- 

 acter, many important groups being wholly confined to South America. On 

 the other hand many genera range into Mexico and Gautemala from th© 



