356 Aquatic Societies 



IV 



The population of stream beds — If we distinguish 

 between lenitic and lotic societies by presence or 

 absence of growths of vascular plants, then the greater 

 part of stream beds shelter lenitic societies. The 

 greater part has not a current of sufficient swiftness to 

 prevent the growth of such plants. And indeed it is 

 only in restricted portions of any stream that we 

 find the animals specially adapted to meet conditions 

 imposed by currents. 



Where the stream bed forms a basin, there the condi- 

 tions of life, for the larger organisms at least, approxi- 

 mate those of a lake. Hence we find in those places 

 in large streams where the water is deep and still, there 

 occur many forms like those in lakes. The sturgeon 

 belongs in both, and so do the big mussels and the 

 operculate snails, the big burrowing mayflies, the big 

 tube dwelling midge larvae, etc. The basins of creeks 

 offer conditions like those in ponds; the basins of 

 brooks, conditions like those of pools. And the largest 

 species are restricted to such of the larger basins as can 

 afford them adequate pasturage and suitable places for 

 rearing their young. To be sure, in all those basins, 

 the water is constantly passing on down stream and 

 the plancton of the basin, while in part developing there, 

 is in a large part constantly lost below and constantly 

 renewed from above. Kofoid (08) states that 'The 

 plancton of the Illinois River is the result of the 

 mingling of small contributions by tributary streams, 

 largely of littoral organisms and the quickly growing 

 algae and flagellates, and of the rich and varied plancton 

 of tributary backwaters, present in an unusual degree 

 in the Illinois because of its slightly developed flood- 

 plain, and from which it is never entirely cut off, even 

 at lowest water. * * * * To these elements is added 

 such further development of the contributed or indigen- 



