ArticLeE If.— Plankton Studies. IV. The Plankton of the 
Illinois River, 1894-1899, with Introductory Notes upon the Hydrog- 
raphy of the Illinois River and its Basin. Part I. Quantitative 
Investigations and General Results. By C. A. Kororp. 
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 study could be ap- 
plied, and it afforded, moreover, a problem not only of prime 
scientific interest, but also of some important practical relation 
to fisheulture. 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 Studies. II. On Péeodorina illinoisensis, a New Species 
from the Plankton of the Illinois River. 
Article IX, Plankton Studies. III. On P/atydorina, a New Genus of the Family 
Volvocide, from the Plankton of the Illinois River. 
