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 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, 



