GENERAL CONSIDERATIONS ON SEASONAL CHANGES.* “ 
It follows from the facts set forth in the preceding discussion that 
in general each month of the year, characterized by a certain range 
of hydrographic, thermal, and chemical conditions, and of illumi- 
nation, has a plankton characterized as follows :-— 
1.. There is a certain range of component species, some of 
which are occasional stragglers and others more or less uniformly 
present. 
2. There is a certain range of numbers of individuals, varying 
with the species and profoundly affected by fluctuations in the 
environmental factors, which change the proportions of the various 
species from year to year. These proportions vary also from month 
to month and constitute one of the main elements in the seasonal 
changes of the plankton. 
3. Transitions from month to month are most profound at 
seasons of greatest environmental change, as, for example, at the 
times of vernal increase and autumnal decline in temperatures. 
4. Seasonal changes in the plankton follow the environmental 
changes and not the calendar. Autumnal plankton is found when 
autumnal temperatures arrive. 
5. In the main, but two types of plankton are found in the 
Illinois River—the summer, and the winter assemblage. The vernal 
and autumnal types are only transitions between the two when 
organisms from both are present. The winter plankton is charac- 
terized by a small number of species peculiar to that season, and a 
number of perennial forms; the summer, by a larger number of 
summer organisms with the perennial types. 
LAKE VERSUS RIVER PLANKTON. 
Is the plankton of streams (potamoplankton) different from 
that of lakes (limnoplankton) and ponds (heleoplankton)? This 
terminology, introduced by Zacharias (’98 and ’98a), seems to imply 
a distinction which lies not only in the differences in the configura- 
* The detailed discussion of seasonal changes in the plankton is deferred to a 
later paper. 
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