3l8 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



space of but lOO meters between the Estivado, a small tributary of the 

 Tapajoz, and the Tombador, which empties into the Cuyaba." 



Cuyaba on the headwaters of the Paraguay is not more than 200 m. in 

 elevation and the junction of the Cassiquiare with the Orinoco has an 

 elevation of but 280 m. One could therefore traverse the 8300 kilome- 

 ters by inland water without ever exceeding an elevation of 280 m. 



The Amazon itself, where it leaves the Andes, has an elevation of but 

 180 m. ; indeed nearly 60 per cent, of all South America consists of flat 

 lowlands, drained by this system of rivers. 



The Amazon draining more than 7,000,000 square kilometers, the La 

 Plata about 3,100,000 and the Orinoco about 1,000,000, these rivers 

 which as far as the fishes are concerned form a single system, drain an 

 area 2>'/i times as large as the Mississippi basin. 



The lowlands through which these main rivers flow are the youngest 

 part of South America. The parts that first arose out of the sea and 

 became populated with fresh-water fishes were probably two land areas. 

 The one embraces the highlands of Guiana and Northern Brazil, the 

 other the highlands of Brazil east of the Araguay and south of the falls 

 of the Tapajos. In Tertiary times the Cordilleras arose out of the ocean 

 on the west. The basins left as open seas between the three land areas 

 later, in late Tertiary, became dry land, in part by elevation and in part 

 by delta formation by the rivers coming from the surrounding land 

 masses. The rivers coming from the Andes became stocked from the 

 surrounding ocean and /afcr from one of the older eastern land areas. 

 The main part of the later colonisation very probably did not take place 

 nntil the Andes had become an effective barrier to the distribution of fresh- 

 water fishes. 



These interior rivers, chiefly the Amazons, colonized from the north- 

 east and southeast became themselves the seat of unparalleled adaptive 

 radiation and centers of distribution, as we shall see. 



The Amazons are unequalled in richness and form a province distinct 

 from the Orinoco and Guiana region to the north and the La Plata on 

 the south. The La Plata basin is well distinguished from the Amazonian, 

 but by negative characters only. ^ 



' I insisted on this distinction in 1891. Perugia registered his " disgosto " with this scheme, 

 because the fishes from the La Plata basin were nearly all Amazonian, as I had pointed 

 out. 



