402 
due solely and unequivocally to the diluent action of the in- 
vading waters, though their share in the phenomenon seems 
probable. 
I have previously called attention to the similarity in the 
movement in production in the several localities wherever col- 
lections were of frequency sufficient to permit the tracing of 
the fluctuations in production. The course of production in 
Thompson’s Lake in 1896 forms no exception to this similarity, 
though the parallelism is less precise than itis in some other in- 
stances. Thus the plankton content rises or falls together in 
Thompson’s Lake and the Illinois River in 18 out of 26 instances 
of coincident or approximate collections ; in Thompson’s and 
Dogfish lakes in 18 out of 26 instances ; in Thompson’s and Flag 
lakes in 16 out of 25 cases; and in Thompson’s and Quiver lakes 
in 12 out of 25. The direction of the change thus agrees in a total 
of 64 out of 104 possible instances in the data. This isa some- 
what greater proportion of instances in agreement than chance 
would demand, and its significance is enhanced by the fact that 
the agreement with Thompson's: Lake is greatest (64 and 69 
per cent.) in the case of Flag and Dogfish lakes—impounding 
bodies similar to Thompson’s Lake—and of the river (also 64 per 
cent.), which is in a measure and especially in this year a sum- 
mation of impounded backwaters. Quiver Lake, on the other 
hand, where tributary waters increase the local differentiation, 
has an agreement in only 12 out of 25 instances. In hke manner 
months of high water, such as August, when local differences are 
to some extent submerged, exhibit greater agreement than 
months of low water, whenthey are emphasized. Thusin August 
(average river gage, 7.42 ft.) 92 per cent. of the changes in produc- 
tion are inagreement, while in July (average river gage, 4.55 ft. ) 
only 58 per cent. exhibit this relation. Again, since the above 
comparisons are based on coincidence of changes in production it 
resultsthat slight chronological dislocations of otherwise similar 
movements in production indicate a greater disagreement than 
really exists. This is especially true of the vernal pulses of April— 
June, where as a whole only 58 per cent. of the coincident or 
