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water the rate in the river does not differ greatly from that in 

 the contracted portions of Great-Lake systems. In the rate of 

 its current the conditions in the Illinois River thus approach 

 those of a lake. In most lakes, however, in which currents may 

 be found whose velocity equals or exceeds that of the Illinois, 

 the movement of the water does not involve to any like degree 

 its replacement by tributary waters. Thus, in the Illinois, even 

 at lowest water, the rate in the channel would necessitate a 

 renewal of the water every twenty-three days — a rapidity of 

 change which few lakes attain. This replacement of the 

 water becomes a most important phenomenon in the environ- 

 ment of the plankton of a river as contrasted with that of a 

 lake, contributing to its fluctuations, complicating the problem 

 of its maintenance, and ever tending to sweep it out of ex- 

 istence. 



Biologically considered, the fundamental distinction be- 

 tween fluviatile and lacustrine waters lies in the more rapid 

 replacement and more recent origin, from springs and rain, of 

 the water of the stream as compared with that of the lake. 



The attempt has been made by Schroeder ('97) to give to 

 this relation of the plankton to the current a mathematical ex- 

 pression, a formulation which has been called Schroeder's law. 

 As a result of his plankton investigation upon the River Oder 

 and elsewhere the conclusion is reached that the volume of 

 plankton present in any stream is inversely proportional to the 

 rate of the current. It may well be that a comparison of the 

 plankton of certain slow and rapid streams, or of the same 

 stream under different conditions of discharge, will show con- 

 formity to this law of Schroeder's, though no such conformity or 

 data for such comparison are as yet at hand. That any extended 

 investigation of the subject will afford the basis for an expression 

 so precise as to be couched in mathematical terms seems improb- 

 able in view of the many complex factors environing the plank- 

 ton. Furthermore, as will be shown later in the discussion of 

 the plankton of tributary streams of the Illinois River, the 

 biological significance of the current as related to the plankton 



