< 



EIGENMANN : FRESH WATER FISHES. 37 1 



and Protistiits remain in the high Cordilleras of southern Peru as relicts 

 of these marine species. Later, these mountain streams, especially those 

 of Ecuador and Colombia, became populated by stragglers or accidental 

 visitors from the land areas to the east. These in their turn, with the ele- 

 vation of the Andes, became modified and gave rise to the genera now 

 peculiar to both slopes of the high Andes, Pygidium, Eremophiliis, C/ics- 

 lostoniits, Arges, Cyclopinm, Astroblepus, etc. 



With the further elevation of the Cordilleras into a continuous barrier 

 and the formation of the Orinoco, Amazon and La Plata valleys through 

 elevation and the debris brought from the land masses, and the development 

 of the enormous fresh-water system occupying these valleys, this system, 

 particularly the Amazon, became colonized from the older land areas and 

 became the center of unparalleled adaptive radiation and a new center 

 for distribution, which it has remained to the present time.^ The com- 

 paratively few types inhabiting the old eastern land masses found them- 

 selves in possession of a continent and diverged along every conceivable 

 direction, the characins alone giving rise to over 500 species and over 

 100 genera. 



From the Amazon species moved in all directions till they met barriers 

 of one sort or another. The Pacific slope fauna is derived to a very large 

 extent from this latter divergent migration over the isthmus of Panama 

 and through the Atrato valley between the western and coast Cordilleras 

 of Colombia. Others possibly crossed over the Andes east of Guayaquil 

 before the Andes reached their present height. The Pacific slope fauna is 

 less different from the Amazon fauna than that of the coastwise streams of 

 Minas, if the number of peculiar genera is used as a measure of difference. 

 Amazonian types moved south till climate and barriers checked them 

 south of Buenos Aires. They migrated northward till they came in com- 

 petition with emigrants from the north in the lowlands of Mexico. 



The origin of the fauna of the plateau of Mexico is a separate subject. 

 This fauna is in part of marine origin and antedates, in a large measure, 

 the mongrel fauna of the Mexican lowlands. 



^ Amazonian species form six or seven per cent, of the fauna of the Motagua, five of the Pacific 

 slope fauna, over 30 per cent, of the Magdalena fauna, over 42 per cent, of the Trinidad fauna, over 

 50 per cent, of the fauna of the Guianas, over 40 per cent, of that of the San Francisco, about 30 

 per cent, of the fauna of the coastwise streams east of the San Francisco, and over 50 per cent, of 

 that of the Paraguay. 



