1038 FRESH-WATER BIOLOGY 



skipped in a tantalizing way, from mountain stream to mountain 

 stream, appearing wherever conditions are favorable. Here, as 

 elsewhere, the mystery will probably dissolve when all the facts 

 are in. The catfishes and darters have not been able to cross to 

 the Pacific slope in the United States, but in Mexico they have 

 accomplished this feat. A tilting of the land, or change in relative 

 rainfall, or some other reason has enabled some of the Pacific 

 slope streams to capture some of the former tributaries of the Rio 

 Grande. With the tributary went the darters, the catfishes, and 

 other fishes it contained. A freshet or a cave-stream may some- 

 times be responsible for an apparently mysterious distribution. 

 Salt water is sometimes a barrier to the migration of fresh-water 

 fishes. Jordan ^ says of the streams of San Luis Obispo County, 

 California, of which the San Luis Creek mentioned before is one: 



"The county of San Luis Obispo lies along the coast of Califor- 

 nia, midway between Monterey and Santa Barbara. It is com- 

 posed of two or three isolated valleys opening out to the sea, and 

 surrounded on all sides by high and barren mountains. These 

 mountains have served as a barrier, shutting off all access of fishes 

 to the streams of the region from the larger basins of the north and 

 east. The valleys of San Luis Obispo are traversed by clear, 

 swift, cold streams rising in mountain springs. Lti these streams 

 very few species of fishes are found, and these few, except in one 

 case {Agosia nubila), are species which have come into the fresh 

 waters by way of the sea. None of the characteristic types of 

 the San Joaquin and Sacramento valleys are found in San Luis 

 Obispo County. This is evidently not due to any character of the 

 waters, but simply to the fact that these fishes cannot reach San 

 Luis Obispo except by descent to the sea." 



But there is also evidence that the ocean is not invariably a 

 barrier. To quote again from Jordan:^ 



"The passage of species from stream to stream along the Atlantic 

 slope deserves a moment's notice. It is under present conditions 

 impossible for any mountain or upland fish, as the trout or the 

 miller's thumb, to cross from the Potomac River to the James, or 



' Bull. U. S. Fish Com. for 1894, p. 141. 



* "Gu'de to the Study of Fishes," pp. 312 and 313. 



