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 is a 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 outof 25 instances. In like manner 

 months of high water, such as August, when local differences are 

 to some extent submerged, exhibit greater agreement than 

 months of low water, when they are emphasized. Thus in August 

 (average river gage, 7.42 ft.) 92 percent, of the changes in produc- 

 tion are in agreement, 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 

 results that 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 



