360 
relation in 22 out of 31 instances, and 4 of the 9 exceptions fall 
in the period of low water in July,during predominance of veg- 
etation in the lake. The monthly averages in the lake also 
exceed those in the stream in all months but July and Sep- 
tember. Higher levels, increased impounding function, and 
decrease in vegetation thus favor plankton production in Quiv- 
er Lake, and tend to raise it from a diluent to a source of imme- 
diate enrichment. 
In this connection it should be noted that the increased 
production of this year (2.59) still falls below that of the river 
in 1894 and 1897, and, as seen in the table on p. 292, below the 
general average of the river production (2.71); and also that the 
higher river levels of this year tend to lower the proportion 
which the tributary spring and creek waters form of the total 
volume of Quiver Lake. 
A second significant fact brought out by the comparison is 
rendered patent by the frequency in this year of the coilections 
in Quiver Lake. The weekly interval from April to Septem- 
ber (Table V.) makes it possible to trace somewhat fully the 
movement of production, and demonstrates in Quiver Lake a 
pulse-like movement in production similar to that previously de- 
scribed in the Illinois River, and one, moreover, which exhibits a 
very striking coincidence of developmental succession. A superpo- 
sition of Plate XXVII. upon Plate X. will make this demon- 
stration apparent. There are exceptions, but these, as shown 
in the preceding discussion, are in most, if not all, instances to 
be correlated with local environmental factors confined to one 
or the other body of water. The return to parallelism with 
the cessation of the peculiar factor incident to the interruption 
serves still further to emphasize the significance of this simi- 
larity. The key to the parallelism must lie in fundamental 
factors common to the plankton of both areas or to their envi- 
ronment, 
