296 ANIMALS IN INLAND WATERS 



relative lengths of the horns, and that there is a marked difference 

 according to whether Ceratium is found in shallow (2-4 m.) or deep 

 (4-16 m.) ponds; he proved by experiment that this depends on the 

 direct influence of the environment. The snails and mussels vary in 

 Germany from pond to pond and from one river system to another; 

 similarly the lakes in Celebes have their own local forms of Melaniidae 

 and Corbiculidae, 13 and each of the Patagonian river systems, which 

 are especially effectively isolated, contains its own groups of mollusks 

 which are very distinctly different as to species or race from those in 

 adjacent rivers. 14 



The lower Crustacea give excellent examples of the endless varia- 

 tion of species in inland waters. The brine shrimp Artemia salina varies 

 so greatly that almost every body of salt water has its own race. 15 

 Numerous species of Cladocera of different genera are distinguished 

 by this great variation. Among Bosmina coregoni the end forms are 

 distinguished by a large number of characteristics but are united by 

 unbroken transitional chains from various localities. 16 In the genus 

 Daphnia, twenty-eight forms which had been described as different 

 species are now included in the species D. longispina; others are 

 similarly variable. 17 Weber 18 reports similar local variations for the 

 African fresh-water crab, Telphusa perlata. Just as the whitefish Core- 

 gonus displays peculiar characteristics from lake to lake, so the Euro- 

 pean brown trout (Salmo fario) and the European lake trout (S. lacus- 

 tris) also vary considerably, and these two are so closely connected by 

 transitional forms that they have even been placed in one species. 19 

 The American black-spotted trout (west of the Great Plains) also 

 have innumerable subspecies and local forms. 20 In Australia almost 

 every river has its own variety of the salmonoid Galaxias. 21 Such 

 examples among fishes might be greatly multiplied. 



Although many inhabitants of the inland waters are subject to 

 mutation and show exceptionally large numbers of varieties and local 

 forms within the species, the fauna of inland waters the world over 

 maintains a marked uniformity. Species of world-wide distribution 

 occur everywhere along with regionally distinct species and genera of 

 limited distribution; these are sometimes called universal, as compared 

 with the regional fresh-water fauna. In this surprisingly great number 

 of such very widely spread species, which has been marked by many 

 investigators, the fresh-water fauna stands in sharp contrast with the 

 fauna of the ocean as well as that of the land. The explanation sug- 

 gested above (p. 85 ff.) was that the short life of a given habitat in the 

 inland waters has conditioned the evolution of forms capable of ready 

 transfer by accidental or incidental migration. Local forms incapable 



