DISTRIBUTION OF FRESH-WATER FISHES. 289 



nose sturgeons (Scaphirliynchus), and the paddle-flshes (Polyodon- 

 tidffl), both of them restricted to some two or three species, are con- 

 fined respectively to the river systems of Central Asia and the Mis- 

 sissippi, and the Mississippi and the Yangtse-Kiang ; the American 

 suckers (Oatostomus) have an outlying representative in Siberia; 

 while the East Indian genus Symbranchus, after skipping Africa, 

 reappears with a single species (S. marmoratus) in the waters of 

 South America. 



In what precise manner these equivalent t}'pes have found their 

 way to such widely removed portions of the earth's surface it has 

 been thus far impossible to determine. That some transference 

 was efEected by way of northern waters over land-surfaces now no 

 longer existing is very nearly certain ; hence, the occurrence of 

 identical or representative specific forms in the streams of Northern 

 Eurasia and North America is not very surprising. But that a 

 similar transference was eSected over the broader or equatorial 

 parts of the oceanic basins, a hypothesis necessitating the assump- 

 tion of the submergence of vast continental land-masses where no 

 traces of their former existence are visible, is more than doubtful. 

 At any rate, it is very unlikely that any such alternation in the 

 relative positions of land and water took place at a time so recent 

 as satisfactorily to explain such anomalies of distribution as are 

 presented by the genera Lates and Symbranchus. already men- 

 tioned, and by the genus Pimelodus (Africa and South America) 

 among the cat-fishes. It would appear at first sight far more 

 natural to assume that these fishes were originally of a marine 

 type, spread over the oceanic expanses, and that at a later period 

 they accommodated themselves to fresh-water conditions, and grad- 

 ually restricted their habitats to regions where we now find them. 

 This is not unlikely, seeing that some of these (Lafes, Symbranchus) 

 freely enter brackish water. TJiat such has been the recent origin 

 of many forms of fresh-water fishes is placed beyond question. 

 Several permanent species of the Northern Baltic, where, through 

 an excessive indraught of fresh water from the surrounding streams, 

 the water has lost most of its salinity, are identical with marine 

 types inhabiting the Arctic seas to the north. Yet the accommo- 

 dation to new conditions of existence was effected since the closing 

 off of the Baltic from the northern ocean, or since the Glacial 

 period. The gobies, blennies, and atherines of the northern lakes 

 20 



