84 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



or more species of "lake herring," salmonids of the genus Leu- 



cichthys. 36 



Variation in the environmental conditions is in general less exten- 

 sive in flowing water, though some important variation in the com- 

 position of the water of brooks and rivers does occur and affects the 

 faunae concerned in the same way as in lakes. The black-water rivers 

 of South America, with their high content of dissolved humus mate- 

 rials, differ in the composition of their faunae from the neighboring 

 streams. The bog-fed brooks of Scandinavia lack the alpine turbel- 

 larians. These are exceptions, however. 



The effect of isolation in preventing interbreeding is more marked 

 in the rivers than in lakes. The different large drainage basins of 

 Germany have different species and varieties of mussels. 37 The well- 

 isolated rivers of Patagonia, flowing into the Atlantic and separated 

 by semi-arid plains, are inhabited by distinct faunae of fresh-water 

 mollusks. 38 The widespread fresh-water crustacean Caridina nilotica, 

 found in Africa, Bengal, and Celebes, has 8 different species besides 

 the one in the Nile. The fish genus Rhamdia (Pimelodus) has split 

 into at least 15 species in the rivers of Central and South America, 

 south of the La Plata, a species for each river system. 39 In North 

 Africa each river is inhabited by different species of Barbus, 40 and in 

 the Rocky Mountains each river system tends to have its own species 

 or subspecies of trout. 



In spite of the vastly increased opportunities for isolation in fresh 

 water, the great number of species that might be expected from the 

 abundance of isolated areas is not realized in fact. The modifications 

 of mollusks, crustaceans, and fishes cited above are in part transitory, 

 arising from the direct influence of the given external conditions, and 

 not genetically permanent species characters. Many invertebrate fresh- 

 water animals have a world-wide distribution. The animal species of 

 fresh-water plankton are to a large extent cosmopolitan (copepods, 

 for example, but with the exception of the Centropagidae, and clado- 

 cerans). 41 Of bottom-dwelling forms, Protozoa and rotifers are very 

 widespread. Thirty-six species and varieties of rhizopods from the 

 region of Lake Tanganyika 42 contained only three new forms, and of 

 213 rotifers from the same territory only 21 are endemic. 43 The fresh- 

 water sponge Ephydatia fluviatilis occurs in Europe, North America, 

 South Africa, Australia, and in the Malay Archipelago, and most 

 sponge genera have a very wide range. All the fresh-water coelenterates 

 and sponges of Australia belong to European genera. Two species of 

 Hydra and three of Spongilla are specifically identical with the Euro- 



