EIGENMANN : FRESH WATER FISHES. 353 



and found in the north only on the Atlantic slope. It also contains 

 Ameiiints pncei,ont of the three Pacific slope catfishes and Etheostoma 

 pottsi, the only Pacific slope Percoid. 



3. The third fauna is the Mexican of the Rio de Santiago. This is un- 

 doubtedly the relict of an old fauna reenforced by a few immigrants from 

 the north. It is here not a question of the origin of the fauna from an 

 eastern one, but of an autochthonous development that has, on its part, 

 contributed elements to the surrounding rivers. It passively contributed 

 to the Atlantic slope fauna by having one of its small rivers captured by 

 the Rio Panuco. 



4. Of more particular interest is the origin of the fauna of western 

 Peru and Ecuador and that of western Central America. Not enough is 

 known of the fauna of the western part of Central America to attempt 

 an explanation of its origin. 



To quote the words of Gilbert and Sparks, who considered the 

 marine fishes on opposite sides of Panama (The Fishes of Panama Bay, 

 205, 1904): "The ichthyological evidence is overwhelmingly in favor of 

 the existence of a former open communication between the two oceans, 

 which must have been closed at a period sufficiently remote from the 

 present to have permitted the specific differentiation of a very large ma- 

 jority of the forms involved." They found that ''Oi the 82 families of 

 fishes represented at Panama all but 3 [Cerdalidce, CirJiitidce and Nema- 

 testiidce) occur also on the Atlantic side of Central America; "while of the 

 218 genera of our Panama list, no fewer than 170 are common to both 

 oceans." * " Fifty-four out of a total of 374 or 14.4 per cent, of the Pacific 

 coast species are identical with Atlantic coast species." 



There seems to me to be just as conclusive evidence that the present 

 fresh-water fauna of the Pacific slope developed after the obliteration of 

 the waterway connecting the two oceans, if this waterway occupied the 

 Amazon valley, and that the Atlantic slope streams of the Cordilleras 

 became colonized by their present fauna after the obliteration of this 

 waterway. 



The similarity between the Amazonian and Pacific slope faunas of South 

 America is much greater than the similarity between the Atlantic and 

 Pacific slope faunas of North America. This similarity leaves the com- 

 munity of origin of the two faunas unquestioned. Is a continuous coast 

 line between the two slopes necessary to account for the similarity? Such 



