THE LAKE AS A MICROCOSM. 

READ BEFORE THE SCIENTIFIC ASSOCIATION FEBRUARY 25, 1887, 
BY S. A. FORBES, A.M., A. A. A. 8., ETC., 
Professor of Zoélogy and Entomology in Lilinois State University, and State Entomologist. 

A lake is to the naturalist a chapter out of the history of a primeval time, 
for the conditions of life there are primitive,—the forms of life are, as a 
whole, relatively low and ancient, and the system of organic interactions by 
which they influence and control each other has remained substantially un- 
changed from a remote geological period. 
The animals of such a body of water are, as a whole, remarkably isolated,— 
closely related among themselves in all their interests, but so far independent 
of the land about them that if every terrestrial animal were suddenly anni- 
hilated, it would doubtless be long before the general multitude of the inhab- 
itants of the lake would feel the effects of this event in any important way. 
One finds in a single body of water a far more complete and independent 
equilibrium of organic life and activity than on any equal body of land. It 
is an islet of older, lower life in the midst of the higher more recent life of the 
surrounding region. It forms a little world within itself,— a microcosm with- 
in which all the elemental forces are at work and the play of life goes on in 
full, but on so small a scale as to bring it easily within the mental grasp. 
Nowhere can one see more clearly illustrated what may be called the sensi- 
bility of such an organic complex,—expressed by the fact that whatever affects 
any species belonging to it, must speedily have its influence of some sort upon 
the whole assemblage. He will thus be made to see the impossibility of study- 
ing any form completely, out of relation to the other forms,— the necessity for 
taking a comprehensive survey of the whole as a condition to a satisfactory 
understanding of any part. If one wishes to become acquainted with the 
black bass, for example, he will learn but little if he limits himself to that 
species. He must evidently study also the species upon which it depends for 
its existence, and the various conditions upon which these depend. He must 
likewise study the species with which it comes in competition, and the entire 
system of conditions affecting their prosperity, and by the time he has studied 
all these sufficiently he will find that he has run through the whole compli- 
cated mechanism of the aquatic life of the locality, both animal and vegeta- 
ble, of which his species forms but a single element. 
It is under the influence of these general ideas that I propose to examine 
briefly to-night the lacustrine life of Illinois, drawing my data from collections 
and observations made during recent years by myself and my assistants of the 
State Laboratory of Natural History. 
The lakes of Illinois are of two kinds, fluviatile and water-shed. The fluvia- 
tile lakes, which are much more numerous and important, are appendages of 
the river systems of the State, being situated in the river bottoms and con- 
nected with the adjacent streams by periodical overflows. Their fauna is 
therefore substantially that of the rivers themselves, and the two should, of 
course, be studied together. 
