569 
lation which should efficiently prevent pollution of the stream 
by deleterious industrial wastes, protect the most desirable 
food and game fishes from depletion, and, at the same time, per- 
mit the full utilization of the annual crop of matured and mar- 
ketable fish, there is no apparent reason why the Illinois River 
and its backwaters should not become an increasing source of 
wealth to the state, and the great waste which now occurs be 
utilized to a considerable extent in future development. 
ConcLUSIONS. 
The following are the more important conclusions arrived 
at from this examination of the plankton and its environment 
in the Illinois River and its backwaters, based upon the study 
of 645 collections made in 7 localities in 1594-1599. 
1. There is little correlation between the seasonal flux in 
chemical conditions (as shown in data of sanitary analyses) 
and the seasonal course of plankton production (as shown in 
the catches 6f the silk net). The nitrogenous matters are in- 
fluenced by the plankton pulses, especially when diatoms are 
multiplying rapidly, but the changes are not uniform or pro- 
portional. 
2. The plankton in the Illinois River is distributed with 
a uniformity approximately equal to that found in German 
lakes and in Lake St. Clair. The average departure from the 
mean in short distances (8 miles) probably falls within + 10 
per cent. Chronological catches in periods of 2 to 15 days in 
14 series yield an average departure of + 14.1 per cent. In the 
river, in 205 miles of the course the average departure in flood 
conditions was + 51 per cent., or + 43 per cent. if the river is 
divided into four sections, or + 29.7 per cent. if computations 
are based on total catch of the net. 
3. The average departure from the mean plankton con- 
tent in two tests in a cross-section of the river is + 27.2 or 
+ 23.2, or, omitting marginal collections, + 21.9 or + 12.1 per 
cent. 
4. The plankton method can be apphed to a stream as 
legitimately as to a lake. 
