352 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



Of the 31 species recorded from above the falls of the Parana 16 are 

 also found below it. 



There is no indication that the fauna of the Rio San Francisco, or 

 the Alta Parana, which is geographically nearer to southeast Brazil, is 

 more like the fauna of the latter region than is the La Plata fauna, for we 

 have : 



Number of species in the Upper Parana and Southeastern Brazil 3 



Number of species in the Lower Parana and Southeastern Brazil 4 



Number of species in the Alta and Baja Parana and Southeastern Brazil 4 



Number of species in the Rio Grande do Sul and Southeastern Brazil 8 



Number of species in the Rio Grande do Sul, La Plata and Southeastern Brazil .... 10 

 Number of species in all four localities 5 



33 



VII. The Origin of the Pacific Slope Fauna. 

 There are four distinct faunas on the Pacific slope of America between 

 Cape Horn and the tropic of Cancer. One of these is of common origin 

 with that on the Atlantic slope, one is autochthonus and the other two 

 are derivative from the Atlantic slope faunas opposed to them. 



1. The fauna of southern Chili is essentially like that of Patagonia, and 

 inasmuch as it is largely made up of marine forms entering fresh water, and 

 fresh-water forms entering the ocean, it seems very probable that the species 

 migrated from river to river along the coast from Patagonia to Chili or 

 from Chili to Patagonia. 



2. At the other extreme in the Rio Mezquital of the Transition Region 

 and the Yaqui just to the north of it there is a fauna essentially like that of 

 the Rio Grande east of them. As Meek has pointed out, the Yaqui and 

 Mezquital have captured tributaries of the Rio Grande together with the 

 fishes in them, and the migration of Atlantic slope northern forms to the 

 Pacific slope has been a passive one. 



Thus, types which in America north of Mexico have not succeeded in 

 reaching the Pacific slope, have, within the Tropics, crossed the divide. 

 EtJieostoina pottsi is the only representative out of over 50 North American 

 species of Percidce that has crossed the divide and has done it in this way. 

 Ameiurus dugesi and pricei and Istlarius balsanus are the only North 

 American catfishes which have crossed the divide. The Rio Mezquital 

 within the tropics contains Pantostens plebeiits, Hyboguathus episcopus, 

 Leuciscus nigrescens, Notropis ornatus, all species extending northward 



