296 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



II. ICHTHYOGEOGRAPHICAL REGIONS. 



America, south of the Tropic of Cancer, contains four (or five) distinct 

 faunas. These faunas are the Transition, the Mexican, the South Amer- 

 ican and the Patagonian. Only the last two are intimately concerned 

 with the Archiplata-Archhelenis theory. 



The northernmost, or Transition Fauna, is characterized largely by intru- 

 sive elements from both the north and the south. It is found on the At- 

 lantic slope from the tropic to the Isthmus of Tehuantepec and on the 

 Pacific slope of this isthmus. It is of comparatively recent origin and 

 not an independent fauna but a mixture of the advance guards of the 

 North American and South American faunas. 



The Mexican fauna is unique, and occupies a narrow strip including the 

 valley of the city of Mexico and the Lerma basin, draining to the west, 

 and the Rio San Juan, a tributary of the Panuco, draining to the east. 

 While containing intrusive elements from the north, it contains none from 

 the south, and its fauna is so distinct from either that there is slight hesi- 

 tation in considering it as equivalent to the North American, South Amer- 

 ican and Patagonian faunas. It is a very old fauna, but has at no time 

 influenced the South American fauna. 



The third, the South American Fauna, is sharply divisible into the 

 Brazilian and Andean. The Brazilian occupies the rivers from Southern 

 Mexico to Buenos Aires and from the Atlantic to the Pacific, exclusive of 

 the high Andes and Chili. This fauna is the richest in species in the 

 world. From this region about 10 per cent, of all the known fishes have 

 been recorded. The Andean, from 3,000 to 5,000 feet and over above 

 sea level, while possessing some forms in common with the Brazilian, is 

 quite distinct. The species inhabiting this region, while derived from the 

 Brazilian fauna, have become so far modified that they would cause 

 surprise if found at Manaos; those of the lower portions of the Pacific 

 slope would not. 



The Patagonian fauna, in distinct contrast to the South American, is 



one of the poorest in the world. It occupies the Rio Negro basin and 



.everything lying south of it and a line joining it with Valparaiso. Its 



fauna is distinct from the Brazilian and has been considered in detail, 



pp. 227-292. 



For convenience these faunal areas may be enumerated as the following 

 "regions" of unequal value. 



