COMMUNITIES IN RUNNING WATERS 307 



middle of its course, while its upper course may correspond to the carp 

 region and may contain carp, tench, and pike. Wherever the classifica- 

 tion can generally be applied, there is a decrease in number of species 

 of fish from the mouth to the source of the river. In the Rhine, for 

 instance, one may find 41 species of fish in Holland, 22 of them being 

 Cyprinidae, 4 salmon, and 9 species which have migrated from the 

 sea. 9 In the upper Rhine, below the falls, there are 33 species, and 

 above the falls 28, or better, after the subtraction of 3 whitefish 

 (Coregonus) which are confined to Lake Constance, 25; 11 species go 

 as high as 700 m. above sea level, 5 as high as 1100 m., and only 3 

 above 1900 m. (trout, minnow, miller's thumb). 10 The distribution of 

 fish according to height above sea level in Colorado is similar; 11 below 



Fig. 83. — The brook planarians of central Europe. Top to bottom : Planaria alpina ; 

 Polycelis cornuta; Planaria gonocephala. After Voigt. 



1500 m. there are 44, between 1500 and 2100, 47; and this number 

 decreases to 24 between 2100 and 2750 m.; only 13 species go beyond 

 2750., 7 of them being salmonids. Almost all those which occur higher 

 than 2750 m. are also found as low as 2100, and those which are not 

 salmonids are found in the still lower regions; this gives a clear illus- 

 tration of selection through severe factors of environment. 



The physiographic history of many streams includes a transition 

 from the swift-water and rock-bottom stage to the slow-moving, mud- 

 bottom type as an end stage, so that the present distribution of fishes 

 in a stream with varied types of environment from mouth to source 

 gives us ar insight into the general phenomena of ecological succession 

 in streams. 8, 12 



Many other animals are restricted, like the fish, by velocity of 

 current, substratum, and temperature to certain definite regions in 

 flowing waters. The stratifying influence of temperature is especially 

 marked among European brook planarians. The three species of flat- 

 worms characteristic of running water in Germany, Planaria gono- 



