422 
Flag (XX XIV.) lakes by 50 to 150 per cent.,—is to be attributed 
to the diluent effect of invading flood-waters from Spoon River, 
whose plankton content on Apr. 27 was only .05 em.’ per m.* 
The entrance of these flood waters, indicated by the check in 
the decline of the hydrograph (Pl. XLI.), was noticeable at the 
station on the day of collection, and is the cause of the increased 
turbidity if Phelps Lake on that day (Table IX.). 
The similarity in the movement of production in Phelps 
Lake in 1897 to that in the other bodies of water examined by 
us, is very close. In the case of the Illinois River, 8 out 9 pos- 
sible instances, or 89 per cent., are in agreement ; in Thomp- 
son’s Lake 7 out of 9, or 78 per cent.; in Flag Lake all in- 
stances are in agreement; in Quiver Lake 6 out of 9, or 67 per 
cent.; and in Dogfish Lake 4 out of 6, or 67 per cent. As a whole, 
32 out of 40, or 80 per cent., of the changes in the direction of 
production in Phelps Lake accompany similar changes in di- 
rection in these other localities. All of the 8 exceptions to this 
agreement occur at levels below 8 ft., when local environments 
are more potent, and 5 of the 8 are found in Quiver and Dogfish 
lakes, where vegetation and access of tributary waters become 
proportionately more or less potent as levels fall or rise in May 
to August, when the 5 exceptions occur. 
This unusual degree of agreement in 1897 must be attrib- 
uted in large part to the hydrographic conditions in the period 
of comparison. For almost 5 months of the year levels were 
above 8ft., when fluctuations have relatively but a shght effect 
on the various environments. Above this level the several lo- 
calities are more or less submerged in the general overflow, and 
all share alike in the wide stretches of open water in which some 
current exists, and the commingling to an increasing extent, 
as levels rise, equalizes and obliterates local differences in pro- 
duction. The first 4 collections of this year were made under 
such conditions, and agree without exception in the course of 
production. The remaining 5 were taken at stages below 8 ft., 
in the very midst of the season of local diversification, and the 
proportion of agreements falls from 100 to 60 per cent., and is 
