119 



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 Illi- 

 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 



