42 
volumetric (animal) pulse may therefore in a measure present the 
wavering tendency to establish an equilibrium between these two 
elements of the plankton. The presence of an abundant animal 
plankton may therefore be a cause of some of the minimum periods 
between diatom pulses. Other causes, such as decline of food ele- 
ments, may also arise, but in our waters the nitrates at least rarely 
ever reach a level where an unutilized margin capable of support- 
ing a large diatom population is not still present. Data concerning 
other food elements are not at hand, but their paucity in water 
derived from such varied sources and so liberally fertilized by 
organic wastes seems improbable. There is also the further possi- 
bility—and, indeed, from the data in hand the probability—of the 
existence among diatoms of reproductive cycles, interrupted by 
resting periods. The available data do not, however, throw any 
light upon the nature of this internal factor or the cause for the 
running down of the energy of reproduction, and but little upon the 
operation of environmental factors which stimulate anew the 
process of reproduction. 
The seasonal distribution of the diatoms as a whole, and that of 
individual species also, offer repeated instances of recurrent pulses 
at intervals approximating four weeks—the lunar month. In 1898 
thirteen such pulses can be detected. These often correspond 
roughly to minor flood intervals, but not always so, for occasionally 
two pulses occur on the decline of a single flood. Similar appear- 
ances may be traced in other years, when collections were frequent 
enough to exhibit minor pulses. They are, however, in all cases 
quite irregular, and exceptions are frequent. 
That cosmic factors may indirectly, through ammedtately environ- 
ing factors, affect the reproductive phenomena of pelagic organisms 
has been suggested by the work of Kramer (97), Mayer (00), and 
Friedlander (’01) in the case of the “ Palolo’’ worm, a coral-reef 
annelid whose seasonal swarming for reproductive purposes occurs 
at somewhat definite lunar intervals. 
While the data concerning the seasonal distribution of diatoms 
in the Illinois River may serve to suggest the operation of an enig- 
matic cosmic factor, I wish distinctly to state that in my opinion 
they are wholly inadequate to establish either its presence or its 
potency. It is much more probable that we have to deal merely 
with some matter of food relations between the plants and animals 
