539 



must act as a check upon the one preceding it. The development of ani- 

 malcules is arrested and soon sent back below its highest point by the 

 consequent development of Entomostraca ; the latter, again, are met, 

 checked, and reduced in number by the innumerable shoals of fishes 

 with which the water speedily swarms. In this way a general ad- 

 justment of numbers to the new conditions would finally be reached 

 spontaneously; but long before any such settled balance can be es- 

 tablished, often of course before the full effect of this upward in- 

 fluence has been exhibited, a new cause of disturbance intervenes 

 in the disappearance of the overfloiv. As the waters retire, the 

 lakes are again defined; the teeming life which they contain is restricted 

 within daily narrower bounds, and a fearful slaughter follows; the lower 

 and more defenceless animals are penned up more and more closely with 

 their predaceous enemies, and these thrive for a time to an extraordinary 

 degree. To trace the further consequences of this oscillation would take 

 me too far. Enough has been said to illustrate the general idea that the 

 life of waters subject to periodical expansions of considerable duration, 

 is peculiarly unstable and nuctiiat.ng ; mat eacn species swings, pendulum- 

 like but irregularly, between a highest and a lowest point, and that this 

 fluctuation affects the different classes 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 accumulating 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 head- 

 waters of the Illinois and Wabash systems; and an area of water-shed 

 lakes occurs in Lake and McHenry counties, in northern Illinois. 



The latter region is everywhere broken by low, irregular ridges of 

 glacial drift, with no rock but boulders anywhere in sight. The inter- 

 vening 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 bor- 

 ders Lakes Michigan and Superior on the west and south, extending 

 through eastern and northern Wisconsin and northwestern Minnesota, 

 and occupying the plateau which separates the headwaters of the St. 

 Lawrence from those of the Mississippi. 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 



*A11 now drained and brought under cultivation. 



