380 
These factors, combined with changes in production in the 
river, vary the relation which these lakes bear to production in 
channel waters. In general, in times of low water and domi- 
nance of vegetation the outflow from these lakes isa diluent of 
channel plankton, but during the run-off of impounded back- 
waters or in years of higher levels and less vegetation it serves 
to enrich channel waters. 
The river and the two lakes exhibit in common a very 
marked similarity in the seasonal movement in production. 
The recurrent pulses, which may be traced whenever collections 
are of sufficient frequency, coincide closely in their location 
but exhibit considerable local differences in their amplitude. 
This similarity is greatest when local environmental factors, 
such as vegetation and tributary waters in the lakes and sewage 
contamination and recent flood water in the river, are least op- 
erative, and is diminished or obscured as these factors come 
more into action. The diversity, as shown in the differing am- 
plitudes of the pulses of production and in the divergences and 
interruptions in their rise and decline, can generally be traced 
to the preponderance of some local factor or factors above 
named. 
The similarity in the seasonal movement of production is 
allthe more marked when the striking differences of the three 
localities in question are considered and the general instability 
of the whole environment is borne in mind. The changes in 
the plankton content of the river,—turbid and fouled by sewage, 
traversed continuously by a considerable current, and scoured 
repeatedly by flood,—of Quiver Lake,—with gentle current,clear 
spring-fed waters, and greater or less, but always considerable, 
vegetation,—and of Dogfish Lake,—with tranquil, almost cur- 
rentless waters, without access of tributary contributions, and 
also with considerable vegetation,—all exhibit a harmony that 
compels us to admit the potency of those general factors of the 
environment common to all—their climatic and geographical 
surroundings, which determine the amount and distribution of 
the light and heat, and the chemical constituents of the medi- 
