The Lake as a Microcosm. 79 
ferent on successively, in the order of their dependence upon each other 
for food. 
Where a water-shed is a nearly level plateau with slight irregularities of the 
surface, many of these will probably be imperfectly drained, and the accumulat- 
ing waters will form either marshes or lakes, according to the depth of the 
depressions. Highland marshes of this character are seen in Ford, Livingston, 
and adjacent counties, between the headwaters of the Illinois and Wabash 
systems; and an area of water-shed lakes occurs in Lake and McHenry coun- 
ties, in northeastern Illinois. 
The latter region is everywhere broken by low, irregular ridges of glacial 
drift, with no rock but boulders anywhere in sight. The intervening hollows 
are of every variety, from mere sink-holes, either dry or occupied by ponds, to 
expanses of several square miles, forming marshes or lakes. 
This is, in fact, the southern end of a broad lake belt which borders Lakes 
Michigan and Superior on the west and south, extending through eastern and 
northern Wisconsin and northeastern Minnesota, and occupying the plateau 
which separates the headwaters of the St. Lawrence from those of the Missis- 
sippi. These lakes are of glacial origin, some filling beds excavated in the 
solid rock, and others collecting the surface waters in hollows of the drift. 
The latter class, to which all the Illinois lakes belong, may lie either parallel 
to the line of glacial action, occupying valleys between adjacent lateral 
moraines, or transverse to that line, and bounded by terminal moraines. 
Those of our own State all drain at present into the Illinois, through the Des 
Plaines and Fox; but, as the terraces around their borders indicate a former 
water level considerably higher than the present one, it is likely that some of 
them once emptied eastward into Lake Michigan. Several of these lakes are 
clear and beautiful sheets of water, with sandy or gravelly beaches, and shores 
bold and broken enough to relieve them from monotony. Sportsmen long ago 
discovered their advantages; and club-houses and places of summer resort are 
rapidly springing up on the borders of the most attractive and easily accessi- 
ble. They offer also an unusually rich field to the naturalist; and their zoélo- 
gy and botany should be better known. 
The conditions of aquatic life are here in marked contrast to those afforded 
by the fluviatile lakes already mentioned. Connected with each other or with 
adjacent streams only by slender rivulets; varying but little in level with the 
change of the season, and scarcely at all from year to year; they are character- 
ized by an isolation, independence, and uniformity which can be found no- 
where else within our limits. 
Among these Illinois lakes I did considerable work during October of two 
successive years, using the sounding line, deep sea thermometer, towing net, 
dredge, and trawl in six lakes of northern I[\linois, and in Geneva Lake, Wis- 
consin, just across the line. Upon one of these Illinois lakes I spent a week 
in October, and an assistant, Prof. Garman, now of the University, spent two 
more, making as thorough a physical and zodlogical survey of this lake as was 
possible at that season of the year. 
I now propose to give you in this paper a brief general account of the phy- 
sical characters and the fauna of these lakes, and of the relations of the one 
to the other; to compare, in a general way, the animal assemblages which they 
contain with those of Lake Michigan — where also I did some weeks of active 
aquatic work in 1881— and with those of the fluviatile lakes of central Illi- 
nois; to make some similar comparisons with the lakes of Europe; and, final- 
ly, to reach the subject which has given the title to this paper,— to study the 
system of natural interactions by which this mere collocation of plants and 
animals has been organized as a settled and prosperous community.— First let 
us endeavor to form the mental picture. To make this more graphic and true 
to the facts, I will describe to you some typical lakes among those in which- 
