HASEMAN, GEOGRAPHICAL DISTRIB UTION IN SOUTH AMERICA 69 



dilian Caiman niger, the large Amazonian turtles Podocenemis expansa 

 and P. tracaxa and the red porpoise Inia geoffroyensis. All of these 

 forms are found as far up Eio Guapore as Bastos, Eio Alegre. If any- 

 direct swamp connections existed, I should certainly expect to find these 

 animals in the Paraguayan pantanals. The smaller species of caiman, 

 Caiman sclerops, is found all over South America excepting Patagonia 

 and west of the Andes, but it has been seen six miles away from water, 

 and hence an overland trip is not impossible for it. 



2. Large species of fishes found in both the Guapore-Amazon and the 

 Paraguay-La Plata, but never near their headwaters, i. e., at least fifty 

 miles apart in a straight line between the two basins. A few typical 

 examples of such fishes are Sorubim lima, Hemisorubim platyrhynclius, 

 Sciades pictus, Mylossoma aureus, Charax gibbosus, the giant Paulacea 

 jahu of La Plata and Paulacea luikeni of Amazon (I consider the last 

 two species synonymous). These and at least fifty other species which 

 are found both in the Amazon and the Paraguay have not interchanged 

 as such between these basins, for the following reasons : 



1. No connection has existed. 



2. They are not found in the headwaters, i. e., above the waterfalls 

 and at such high altitudes as exist between these rivers. 



3. The distance between the headwaters is so great that an accidental 

 distribution is not possible. 



4. Even if a connection had existed, it would not explain why the spe- 

 cies have remained the same and why Eio Uruguay and Parana, belong- 

 ing to the same river basin as Eio Paraguay, do not possess all of these 

 species but, on the contrary, harbor many species not found in Eio Para- 

 guay. Why, also, did not other species interchange, if a connection has 

 existed ? 



5. The Sao Francisco Eiver is connected with the Amazon Valley, yet 

 it does not have nearly as many Amazonian species as does the Paraguay. 

 In fact, its common or Amazonian species are cosmopolitan forms. Eio 

 Sao Prancisco has an unfavorable environment (dry, hot, high, muddy, 

 fewer swamps, etc.) and therefore has fewer species than the large Para- 

 guay, with its favorable cichlid environments which have produced more 

 cenotelic changes in the ancestral stock. 



In view of all this, it appears that the only answer which can be given 

 to the question why the Paraguay has at least 53 per cent of Amazonian 

 fishes is 



1. About 50 per cent of the similarity is due to the cosmopolitan forms, 

 i. e., to overland distribution of the small generalized highland genera 

 which are widely distributed. 



