Article IX. — The Lake as a Microcosm*. By Stephen A. Forbes. 



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 inter- 

 actions by which they influence and control each other has remained sub- 

 stantially unchanged 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 annihilated it would doubtless be long before the general 

 multitude of the inhabitants of the lake would feel the effects of this event 

 in any important way. It is an islet of older, lower life in the midst of the 

 higher, more recent life of the surounding region. It forms a little 

 world within itself — a microcosm within 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 

 sensibility of such an organic complex, expressed by the fact that 

 whatever affects any species belonging to it, must have its influence of 

 some sort upon the whole assemblage. He will thus be made to see 

 the impossibility of studying completely any form 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 hlacic bass, for example, he will 

 learn but little if he Hmits 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 sufificiently he will find that he has run through the whole compli- 

 cated mechanism of the aquatic life of the locality, both animal and vege- 

 table, 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 



•This paper, originally read February 25, 1887, to the Peoria Scientific Associa- 

 tion (now extinct), and published In their Bulletin, was reprinted many years ago 

 by the Illinois State Laboratory of Natural History In an edition which has long 

 been out of print. A single copy remaining in the library of the Natural History 

 Survey is usr-d every year by clas.S( s In the University of Illinois, and a profess"! 

 of zoology In a Canadian unlver.'^ity borrows a copy regularly from a Peoria library 

 for use in his own classes. In view of this long-continued demand and In 

 the hope that the paper may still tn- found useful elsewhere, it is again reprinted, 

 with trivial emendations, and with no attempt to supply Its deficiencies or to bring 

 It down to date. 



