230 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



mouth of the Rio Negro with Santiago, Chili. No systematic collecting has 

 been done in the Rio Negro, but the few specimens recorded from this 

 region are, with the exception of Astyanax, Patagonian ; on the other hand, 

 the few specimens recorded from just south of Buenos Aires are Brazilian. 



The southern boundary of the Brazilian fauna has been extended on 

 the east by the La Plata-Paraguay river. The headwaters of this river, 

 the Parana and especially the Paraguay, are in contact with the southern 

 tributaries of the Amazon, and have opened a way for the southward 

 migration of equatorial forms like members of the Gymnotidce, the Ser- 

 rasalmonincE and the CharaciucB, which have not succeeded in migrat- 

 ing so far south along the east coast, where there does not exist a 

 continuous water way. (See the chapter on southeastern Brazil.) The 

 distance which the La Plata- Paraguay has extended the tropical fauna 

 can be measured by comparing the fauna of Buenos Aires, which is com- 

 posed entirely of Amazonian types, with the fauna of the Rio Grande 

 at the boundary of the United States, whose mouth is 10° nearer the 

 equator and the Amazon than Buenos Aires, and only 6^ per cent, of whose 

 fauna is made up of equatorial types. This southward extension of the 

 equatorial fauna was also facilitated by the entire absence of competitors. 

 The entire fauna of the Parahyba near Rio de Janeiro is equatorial, while 

 only 15 -fper cent, of the Rio Panuco fauna in the same latitude in Mexico 

 is equatorial, the rest being mostly intrusives from the north. Similar 

 irregularities in the distribution of species are introduced in other parts 

 of the globe, where a river has a north and south extent over different 

 climates or regions, as in the case of the Nile. 



On the Pacific slope one genus of tropical lowland forms, Cheirodou, 

 reaches the southern limit of the Chilian plateau, and one mountain genus, 

 Pygidinin, reaches as far south as Santiago, Chili, and it is continued still 

 further south as Hatcheria. The southern boundary of the northern fauna 

 is here, so far as records show, the desert of Atacama. The southern 

 boundary of the Brazilian fauna seems to be determined by climate and 

 the means of ready migration. 



The factors determining the northward extent of the Patagonian fauna 

 are not so apparent. With the exception of Geotria and Exoniegas, no 

 Patagonian forms reach so far north as Buenos Aires on the Atlantic side ; 

 on the Pacific side the fauna in the latitude of Buenos Aires is still dis- 

 tinctly Patagonian. 



