540 



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. Sports- 

 men long ago discovered their advantages and club-houses and places of 

 summer resort are numerous on the borders of the most attractive and 

 easily accessible. They offer also an unusually rich field to the natur- 

 alist, and their zoology 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 characterized by an isolation, independence, and uniform- 

 ity which can be found nowhere 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 Illinois, and in 

 Geneva Lake, Wisconsin, just across the line. Upon one of these 

 Illinois lakes I spent a week in October, and an assistant, Prof. H. Gar- 

 man, now of the University, spent two more, making as thorough a physi- 

 cal an4 zoological 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 physical characters and the fauna of these lakes, and of the 

 relations of the one to the other ; to compare, in a general way, the ani- 

 mal 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 Illinois ; to make some similar 

 comparisons with the lakes of Europe; and, finally, 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 stable 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 we worked ; and will then do what I can to 

 furnish you the materials for a picture of the life that swims and creeps 

 and crawls and burrows and climbs through the water, in and on the 

 bottom, and among the feathery water-plants with which large areas 

 of these lakes are filled. 



Fox Lake, in the western border of Lake county, lies in the form 

 of a broad irregular crescent, truncate at the ends, and with the con- 

 cavity of the crescent to the northwest. The northern end is broad- 

 est and communicates with Petite Lake. Two points projecting inward 

 from the southern shore form three broad bays. The western end 

 opens into Nippisink Lake, Crab Island separating the two. Fox River 



