283 
In the light of these volumetric data the conclusion is pat- 
ent that plankton data from fluviatile environment contain on 
the average a distribution error which approximates that in 
plankton data from limnetic areas of similar extent selected 
with reference to unity of environment as determined by local 
factors. 
It should be noted in this connection that the conditions 
prevailing when this plankton traverse of the Illinois River was 
made, were most adverse to an equalized plankton in the fol- 
lowing particulars. It was at a time of rapid seasonal change 
in plankton during the decline of the vernal pulse, and it was at 
a time of intercalation of flood water of local and recent ori- 
gin, whose poverty in plankton is brought into contrast with 
the larger content of the run-off of impounded backwaters else- 
where. Finally, the river stage, which was 9 feet at Kamps- 
ville and 6.9 at La Grange, was such that the equalizing effect 
of general overflow on plankton content in impounded back- 
waters had ceased and local differences were emphasized, while 
at the same time their discharge continued in considerable 
volume. All of these factors, the last two of which are more impor- 
tant in the river than in the lake, tend to diversify the plankton 
content in the river at this season. It is reasonable to suppose 
that under other conditions—such as general overflow, the 
more stable features which attend falling levels above or below 
9-7 feet, or in prolonged low water—we should find the uni- 
formity of distribution of the plankton more pronounced than 
it was on May 18-21, 1899, barring, however, the effect caused 
by sewage contamination, which at all stages and seasons is 
the most potent factor in the environment of the plankton of 
the Illinois River. 
TRANSVERSE DISTRIBUTION AND RELATION OF SHORE TO PLANKTON. 
The shore is a factor of great importance in the aquatic 
environment. It is here that land and water come into most 
intimate relation; seepage and drainage waters enter here; 
vegetation gains its foothold, affects the gaseous contents of 
