303 
environmental factors might be treated under this head. But 
this has not been my method nor is it now my purpose to 
adopt it. 
The phenomena of growth and reproduction of the con- 
stituent organisms of the plankton, on the other hand, owing 
to our ignorance of their controlling factors, can at present be 
treated only under this head. The volumetric data in them- 
selves contain little evidence bearing directly upon the prob- 
lem, but in the light of the statistical results the fluctuations 
in the plankton become dependent upon fluctuations in the 
rate of growth, and especially in that of the reproduction of its 
constituent organisms. These fluctuations are often concur- 
rent, or, at most, shortly consequent, in many species at the 
same time and in several different localities, and give rise to 
the coincident volumetric pulses to which attention has so 
often been called in the preceding pages. Somewhat regular 
alternations of growth and rest, of fission and spore formation, 
or of parthenogenesis and sexual reproduction, are funda- 
mentally the basis of the cyclic movement in production. The 
amplitudes, and to some extent the location and duration of the 
pulses, are plainly affected by the various factors of the envi- 
ronment discussed in preceding pages—by light, temperature, 
vegetation, tributary water, various hydrographic factors, and 
by food supply, and, possibly, also, by chemical conditions not 
directly concerned in nutrition, but the available data fail com- 
pletely to afford any satisfactory environmental factor or group 
of factors which stands in correlation, even remotely obvious, 
with this cyclic movement in production. J therefore class 
this periodic growth, these serual cycles which cause volumetric 
pulses, under the head of internal factors. The element of 
periodicity in itself does not seem to be consequent upon any 
known external factor. 
NORMAL REGIMEN OF PLANKTON PRODUCTION. 
The records of plankton production in the Illinois River, 
its tributaries, and backwaters, contained in this paper raise 
