538 



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 fluviatile lakes, which are much the more numerous and important, 

 are appendages of the river systems of the state, being situated in the 

 river bottoms and connected with the adjacent streams by periodical 

 overflows. Their fauna is therefore substantially that of the rivers them- 

 selves, and the two should, of course, he studied together. 



They are probably in all cases either parts of former river channels, 

 which have been cut ofT and abandoned by the current as the river 

 changed its course, or else are tracts of the high-water beds of streams 

 over which, for one reason or another, the periodical deposit of sedi- 

 ment has gone on less rapidly than over the surrounding area, and which 

 have thus come to form depressions in the surface which retain the 

 waters of overflow longer than the higher lands adjacent. Most of the 

 numerous ''horseshoe lakes" belong to the first of these varieties, and 

 the "bluff-lakes," situated along the borders of the bottoms, are many of 

 them examples of the second. 



These fluviatile lakes are most important breeding grounds and res- 

 ervoirs of life, especially as they are protected from the filth and poison 

 of towns and manufactories by which the running waters of the state 

 are yearly more deeply defiled. 



The amount and variety of animal life contained in them as well as 

 in the streams related to them is extremely variable, depending chiefly 

 on the frequency, extent, and duration of the spring and summer 

 overflows. This is, in fact, the characteristic and peculiar feature of life 

 in these waters. There is perhaps no better illustration of the methods 

 by which the flexible system of organic life adapts itself, without injury, 

 to widely and rapifllv fluctuating conditions. Whenever the waters of 

 the river remain for a long time far beyond their banks, the breeding 

 grounds of fishes and other animals are immensely extended, and their 

 food supplies increased to a corresponding degree. The slow or stag- 

 nant backwaters of such an overflow afford the best situations possible 

 for the development of myriads of Entomostraca, which furnish, 

 in turn, abundant food for young fishes of all descriptions. There thus 

 results an outpouring of life — an extraordinary multiplication of nearly 

 every species, most prompt and rapid, generally speaking, in such 

 as have the highest reproductive rate, that is to say, in those which 

 produce the largest average number of eggs and young for each adult. 



The first to feel this tremendous impulse are the protophytes and 

 Protozoa, upon which most of the Entomostraca and certain minute 

 insect larvae depend for food. This sudden development of their food 

 resources causes, of course, a corresponding increase in the numbers 

 of the latter classes, and, through them, of all sorts of fishes. The first 

 fishes to feel the force of this tidal wave of life are the rapidly-breeding, 

 non-predaceous kinds ; and the last, the game fishes, which derive from 

 the others their principal food supplies. Evidently each of these classes 



