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viduals only forty-eight. This rate of increase in individuals 
is theoretically possible only for those organisms which repro- 
duce by fission. In sexual and parthenogenetic reproduction 
other factors enter to reduce the rate of increase and to render 
the problem more complex, while life cycles, conditions of 
the physical environment, competitors, and enemies further 
modify and limit the increase in numbers. Thus, in every in- 
stance the struggle for existence sooner or later so checks the 
rate of multiplication that the mathematical possibilities of in- 
crease are never fully realized. In spite of these various modi- 
fying conditions the fact is patent that the current is a very 
important element in determining not only the amount of the 
plankton, but also the relative numbers of its constituent or- 
ganisms. 
To a less degree the current curtails the development of 
the plankton of the backwaters at times of overflow. in general 
it is not so strong in the overflowed territory as it is in the main 
channel, though local conditions in these regions sometimes pro- 
duce quite as rapid a flow in limited areas. The slackened 
current affording a longer time for breeding, the shallow water, 
higher temperatures, and the larger amount of organic de- 
bris combine to favor the development of the plankton in these 
impounding areas, which, in turn, drain into the main channel 
with the run-off of the flood. 
The current in the Great-Lake system in many places 
equals or exceeds that of the Illinois River. For example, the 
St. Clair River at Port Huron moves at the rate of four miles per 
hour; the Detroit River, at a rate of one to three miles per 
hour; and the St. Mary’s, at a rate of three quarters of a mile 
to seven miles per hour. It is well known that currents pre- 
vail in the open lakes, but there are no recorded measurements 
of their flow. Thus, in certain aspects of its current the [lli- 
nois River does not markedly differ from the Great Lakes. 
The impounded backwaters and the main stream at low- 
water stages have but a slight flow, probably not in excess of 
that in the open Lakes, while in the main channel at high 
