﻿4 



Indiana University Studies 



just completed. They are also treated in the series of articles 

 going thru the press of various journals. 



Many of the species rocognizid were new to science and are 

 for the most part figured in ths forthcoming volume. It was 

 found that the ancestors of the present fauna came in small part 

 from Central America, in small part from the ocean, in large part 

 it had a common origin with the fauna of the present Orinoco 

 and Amazon basins. The ancient fauna of South America, ex- 

 tending from ocean to ocean, was divided by the formation of 

 the Andes which arose as a screen, dividing the ancient -fauna 

 into cis-Andean and trans -Andean sections. Since the Andes 

 have become an effective barrier against the cis-Andean and 

 trans-Andean migrations, the parts of the ancient fauna have 

 undergone an independent evolution resulting in many genera 

 and species peculiar to the various rivers. 1 A study of the mi- 

 grations and interrelations of the different river faunas shows 

 that the fauna of the Guayas and the region south was separated 

 from the Amazon, the fauna of the Magdalena from the Orinoco, 

 and that the Guayas and Magdalena have had little or no inter- 

 migration. 2 The present fauna of western Peru north of the 

 Rimac is a relict of the Guayas fauna; south of the Rimac, in 

 part at least, of the Chilenean fauna. The Chagres fauna has 

 come in part from the north and in part from the south (the At- 

 rato via the Tuyra and Chepo), chiefly during the lifetime of its 

 present species. The Atrato-San Juan valley has been used as a 

 highway between the Atlantic and Pacific drainage, but to a lim- 

 ited extent. The fishes of the San Juan and Atrato, separated 

 b} r a very narrow divide, a little over 300 feet above sea level, 

 differ from each other more than the faunas of the Paraguay and 

 Amazon. 



These and other questions are considered in detail in the 

 special articles of which the present study forms a part. 



1 It is probable that at least the genera peculiar to the west and of wide distribu- 

 tion north and south antedate the formation of the Andes. 



2 The present Magdalena fishes show a much closer relationship to the tauna east 

 of the Andes than does the fauna of western Ecuador. Many species are still identical 

 on the two sides of the Cordillera of Bogota, making it seem certain that the specific 

 markings of these species are older than the Andes, unless a road around the north 

 remained open after the Andes became an effective barrier. 



