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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 alake. 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 yetat 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 
