CONDITIONS OF EXISTENCE 23 



ciated with aquatic environments, size and texture of surface ma- 

 terials, direction of prevailing winds, etc. 



In streams the strength of the current is a function of volume of 

 water and slope of stream bed. The amount of sediment carried 

 and the size of the sediment particles is determined by the strength 

 of the current and by the character of the materials eroded. The 

 character of the stream floor, the ventilation of the environment, 

 and hence its gaseous content as well as turbidity, are determined 

 by the same factors. All these factors combined comprise impor- 

 tant conditions of existence which while they influence organisms are 

 often so difhcult to analyze into constituent controlling factors that 

 for ordinary purposes it is better to lump them together under the 

 head of physiographic conditions in streams. Fishes and mollusks 

 migrate upstream during floods and downstream during drought 

 periods. Thus different species of fishes in a number of streams 

 about equally accessible to Lake Michigan but differing in size and 

 age as shown in Fig. 2 are very definitely related to the longitudinal 

 conditions in the various streams, each fish species penetrating up 

 stream to a point characterized by certain physiographic conditions, 

 regardless of the size of the stream as a whole (compare Table I 

 with Fig. 2). An analysis of the physical factors to which the 

 fishes respond and which thus determine the locality they occupy 

 would be a very intricate task but by a simple method of physio- 

 graphic analysis the differences in their ecological constitution is 

 clearly brought out. Thus important features of conditions of exist- 

 ence may be determined by physiographic analysis and the classifi- 

 cation of streams should be determined by physiographic age and 

 physiographic conditions. 



Conditions of existence in lakes and ponds are markedly influ- 

 enced by physiographic conditions. High surrounding country 

 broken into hills and valleys influences the action of winds on the 

 surface. Wind is important in determining circulation. The sur- 

 rounding topography determines the carrying power of streams and 

 thus the amount of sediment carried into lakes. The amount of 

 sediment determines the depth of light penetration. 



The depth of lakes and ponds is definitely related to physio- 

 graphic conditions. Coastal lakes are usually shallow with sandy 



