Article II. — Plankton Studies. IV.^ The Plankton of the 

 Illinois River, 1S94-1899, with Introductori/ Notes upon the Hydrog- 

 raphji of the Illinois Biver and its Basin. Part I. Quantitative 

 Investigations and General Besults. By C, A. Kofoid. 



Introduction. 



When the work of the Illinois Biological Station was be- 

 gun in 1894, it seemed to the Director desirable to determine 

 as far as possible the normal routine of aquatic life as a 

 necessary basis for the detection of problems for investiga- 

 tion and experiment, and as an indispensable background for 

 their adequate solution. Such an investigation demands not 

 only the discovery and specific determination of the biological 

 population, but involves also the study of life histories, seasonal 

 changes, and mutual dependencies of the assembled organisms 

 by quantitative and statistical methods, together with a study 

 of the environment and an analysis of its factors. The plank- 

 ton presented itself as the most available and concrete assem- 

 blage of organisms to which this method of vstudy could be ap- 

 plied, and it afforded, moreover, a problem not only of prime 

 scientific interest, but also of some important practical relation 

 to fishculture. A presentation of the most general results of 

 this investigation of the free microscopic fauna and flora, or 

 plankton, of this typical stream of the Mississippi Valley is the 

 object of the present paper. 



Inasmuch as this is the first of a series of reports upon the 

 plankton of the Illinois River system, it has seemed advisable 



I. The three preceding numbers of this series, all by the present writer, have 

 been published as articles in the Bulletin of the Illinois State Laboratory of Natural 

 History, Volume V., as follows : — 



Article I. Plankton Studies. I. Methods and Apparatus in Use in Plankton In- 

 vestigations at the Biological Experiment Station of the University of Illinois. 



Article V. Plankton vStudies. II. On Pleodorina illinoisensis, a New Species 

 from the Plankton of the Illinois River. 



Article IX. Plankton Studies. III. On /'/cJi/j'c/i^/'z';/^, a New Genus of the Family 

 VolvocidcF, from the Plankton of the Illinois River. 



