CHAPTER XVII 

 COMMUNITIES IN RUNNING WATERS 



Inland waters may be classified primarily into flowing and stand- 

 ing waters. The former are sometimes called lotic, the latter lentic, 

 environments. It is desirable to separate the standing waters into the 

 fresh-water lakes and ponds proper and, on the other hand, the salt 

 lakes or pools which contain in solution large amounts of such sub- 

 stances as sodium chloride, magnesium sulphate, and humus. It is also 

 useful to give separate treatment to subterranean waters whether 

 found as ground water or in caves or wells. 



There are certain regularly recurring differences between the in- 

 habitants of running and standing waters which necessitate a separa- 

 tion of the two. Differences in area, movements of the water, the dif- 

 fering relationships of depths, and the differences in temperature bring 

 about divergences which in extreme cases become very great. These 

 two types of water environment are united by very gradual transitions ; 

 a river of the plains in which the current is hardly noticeable (as in 

 many of the steppe rivers of south Russia in summer) , a lake through 

 which a river flows, an ox-bow lake through which there is a current 

 only during high water, offer examples of situations where a difference 

 between the faunae of running and standing waters is hardly per- 

 ceptible. Furthermore, the animal communities of the wave-washed 

 rocky eroding shores of lakes are closely similar to those of neighboring 

 rocky rapids in streams. 



Departing from common custom, we shall consider the running 

 waters first because of their relatively long geological duration, their 

 more extensive continuity, and their usual connection with the ocean. 

 They furnish the routes for the active migration of living forms from 

 the ocean into fresh water, which is still going on. Not only fish, snails, 

 mussels and larger crustaceans, which are capable of considerable 

 movement, but also less active animals such as leeches and other an- 

 nelids, turbellarians, and the lower crustaceans, have thus reached 

 rivers and lakes. 



The composition of the deep-water communities of the lower alpine 

 lakes, for instance, makes it very probable that at least a part of their 

 inhabitants came in at the end of the glacial period by way of the large 



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