INTRODUCTION 5 



They vary in the chemical character of the soil in the lake basin 

 as well as in their banks and bed, in the degree of exposure to 

 wind and sunshine, in the relative inflow and outflow in ratio to 

 their volume, in their altitude as well as in geographic location. 

 All of these and many other factors modify and control the types of 

 living things and their abundance in the waters. Lake, pond, 

 and swamp are successive stages in change from the water-filled 

 hollow to the terrestrial plain that ultimately occupies the same 

 location. Along the margin of the lake, especially at the points 

 where tributary streams empty into it, the inflowing water brings 

 detritus of all sorts that builds out the shore and forms a shelf on 

 which the littoral vegetation gains a foothold. As the lake grows 

 old this region increases at the expense of the pelagic and bottom 

 areas, until the latter disappears and the former persists only in 

 reduced amount. Finally the entire area is conquered by deposits 

 of silt and growth of vegetation. The swamp comes and is made 

 over into dry land traversed in winding channels by the stream 

 system that is responsible for these changes. In other cases the 

 outflowing stream cuts down the level and ultimately drains the 

 lake. 



Lakes are thus in a geologic sense only temporary features of 

 the river system to which they belong. Similar influences direct 

 the evolution of the stream from the violent instability of its 

 youth to the sluggish stability of its age. During this process of 

 evolution the life in the waters undergoes parallel changes. At 

 first the fauna is scanty but increases in numbers and variety as 

 new habitats are created. Unstable and intermittent conditions 

 indicate paucity of life; but when the aquatic environment be- 

 comes more permanent organisms more easily invade the territory 

 successfully and its life grows increasingly complex as time goes on. 



Lakes influence noticeably the life of a stream system in that they 

 act as filters or settling basins for inflowing waters and also regulate 

 the volume of the discharge; thus the outflowing stream is free 

 from sediment and approaches constancy in level. This greater 

 permanence militates against the development of certain types of 

 life but favors others. The continued dilution of the stream by 

 the addition of water free from life and the removal of such organ- 



