511 
The normal regimen is accordingly not delineated by a 
planktograph of marked definiteness in its course, but is one 
formed by a sequence of recurring pulses of approximately a 
month’s duration and varying amplitudes, low in winter floods, 
rising with the temperature to a vernal maximum of consider- 
able magnitude, often, but not always, declining during the 
summer months, frequently rising again in late summer or au- 
tumn, and in some localities and years to an extent exceeding 
that of the vernal season, and falling with autumn tempera- 
tures but increasing in stable winter conditions after the min- 
imum winter temperature is reached,—such is the general reg- 
imen of plankton production in the Illinois River and its back- 
waters. 
If this cyclic movement-in production be characteristic of 
the plankton generally, fresh water and even marine, it must 
follow that scattered and irregular collections, or those at in- 
tervals exceeding a week or at most a fortnight, may fail en- 
tirely to give an adequate representation of the course of pro- 
duction or relative fertility of a body of water. Chronological 
series throughout the whole seasonal range of climatic condi- 
tions and at close intervals—of one week or less—are neces- 
sary for any accurate delineation of production and fertility 
of water by the plankton method. 
SOURCE AND MAINTENANCE OF THE POTAMOPLANKTON. 
The existence of a very characteristic and abundant plank- 
ton in the Illinois River at once raised the question as to its 
source and maintenance. We find at Havana a stream which 
year in and year out carries by a burden of life, microscopic as 
to its individuals, to be sure, but in the aggregate a volume of 
surprising extent. This stream of life exhibits a routine of 
seasonal changes in constitution and quantity which neither 
flood, the ice of winter, nor the drouth of summer wholly inter- 
rupts. It recurs year after year with a regularity which stands 
in strong contrast with the fluctuations of the environment. 
Although the water in the stream is subject to continuous re- 
