282 
we should find such irregularity in the (estimated) plankton. 
We now come to the section of the river dominated by the 
Peoria-Pekin pulse of sewage, including 70 miles of channel— 
from Holmes Landing to Peoria. The flood waters are still in 
evidence, but in reduced volume, and there is marked increase 
in the plankton content. The average departure from the mean 
plankton is +32 per cent., with arange of —64 to +48—a total of 
112 per cent. In the case of the total catches the average de- 
parture from the mean is +36 per cent., with a range of —60 
and +89—a total of 149 per cent. 
The upper section of the river, above Peoria, a stretch of 
40 miles, was less disturbed by flood conditions, there being 
only slight local invasions. This region is within the sphere 
of influence of Chicago sewage, and not receiving any large 
tributaries, we might expect but do not find conditions some- 
what equalized here. The average departure from the mean 
plankton is +76, with a range of —76 to +80 per cent.—a total of 
156 per cent. The average departure of the total catch is +34 
per cent., with arange of—27 to +66 per cent.—a total of 93 per 
cent. These departures will be much reduced if we break this 
section into an upper and lower region of two collections each, 
the percentages falling from +34 to +2 and +0 for plankton, 
and to +1 and +89 per cent. for the total catch for the two 
sections, each of which represents 20 miles approximately. 
The average departures from the mean plankton in the 
four sections are respectively +12,+51,+32, and +76 per cent., 
yielding a grand average of —43 per cent.; while the corre- 
sponding average departures for the total catches are +44, +5, 
+86, and +34, with agrand average of +29.7 per cent. These 
four subordinate units of environment represent longitudinal 
extensions of 20, 60, 70, and 40 miles. The area included in 
Reighard’s Lake St. Clair collections has a length of 32 miles 
and a maximum width of 54, and the average departure from 
their mean (computed by similar methods for all localities ) is + 
28.8 per cent. Similar methods of computation thus yield for 
Lake St. Clair and these sections of the Illinois River almost 
an identical + error of distribution. 
