302 
the plankton, or to the interaction of environmental and internal 
factors. 
That there is a periodicity in the reproductive processes of 
organisms, of both plants and animals, is generally apparent. We 
see it in the flowering and fruiting seasons of the phanerogams, and 
in the breeding seasons of many invertebrates, of mollusks and 
insects, and of the vertebrates generally,—of fishes, amphibians, 
reptiles, birds, and most mammals. Fluctuations in environmental 
conditions, notably in food and temperature, influence these re- 
productive processes. The phenomenon of rise and decline of the 
microscopic population in laboratory aquaria is likewise an illustra- 
tion of the periodicity of organisms, but usually within a briefer 
interval than that of the organisms above mentioned. The studies 
of Maupas (’88) and Calkins (’02) have shown that even in the 
seemingly uniform conditions of the laboratory, the reproduction 
of the ciliate Protozoa is essentially periodic. 
On a priort grounds it seems highly improbable that in the case 
of the organisms of the plankton, internal factors should determine 
the coincidence of the periods of growth and reproduction in several 
hundred species. While it is not impossible, or indeed improbable, 
that these species of the plankton if bred in pure cultures or uniform 
environment would still exhibit a periodic reproduction, it seems 
highly improbable that so diverse an assemblage of alge, diatoms, 
flagellates, protozoans, rotifers, and entomostracans as is found in 
the Illinois River, ‘would exhibit in laboratory cultures under 
uniform conditions any such coincidence in the location and duration 
of their pulses as is found in the waters of the stream. Whatever 
the internal factors involved in the growth and reproduction of 
plankton organisms may be, it is patent that we must look for some 
environmental factor or factors lying at the foundation of the 
coincidence of seasons of growth and reproduction of plankton 
organisms, which results in the phenomenon of recurrent pulses in 
species, groups, and volumetric plankton. 
We may simplify the problem somewhat by recognizing at the 
outset the importance of nutrition in supplying the basis for the 
periodic growth of any organism. The rotifers and entomostracans, 
at least the limnetic types, depend in large measure, either directly 
or indirectly, upon the synthetic planktonts, such as the alge, dia- 
toms, and flagellates, for their food. Since the pulses of these animal 
