CONDITIONS OF EXISTENCE 



25 



est in the swiftest current and the finest in the most sluggish cur- 

 rent. In the curves of streams the current is usually swiftest on 

 the outside and most sluggish on the inside. Dift'erent animals 



JjAKE MICHIGAN 



Fig. 2. 

 Diagrammatic arrangement of four streams flowing into Lake Michigan. The streams are mapped to 

 a scale of one mile to the inch, and the maps are placed as closely together as possible in the diagram. 

 The intermediate shore lines are shown in broken lines which bear no relation to the shore lines which 

 exist in nature. Toward the top of the diagram is west. Each number on the diagram refers to the pool 

 nearest the source of the stream which contains fish, as follows: i, homed dace (Semotilus alromaculatiis) ; 

 2, red-bellied dace {Chrosomus erythrogaster) \ 3, black-nosed dace {Rhinichthys atronasus); 4, the suckers 

 and minnows; 5, the pickerel and blunt-nosed minnow; 6, sunfish and bass; 7, pike, chub sucker, etc. 

 The bluff referred to is about 60 feet high. The stippled area is a plain just above the level of the lake. 

 (After Shelford.) 



tend to occupy the different kinds of bottom materials (Fig. 3). 

 Thus the differentiation of bottom constitutes an important differ- 

 entiation of conditions of existence. 



The bottom of a swift stream eroding sandy soil is very unstable 

 and the fauna very sparse. Such streams are essentially aquatic 

 deserts and only a few burrowers are able to live in them. Sandy 

 bottomed streams with sluggish current have a luxuriant fauna 

 of burrowers and flora of rooted vegetation. Rocky and stony 

 streams have rich faunas of clinging and hiding animals. 



In lakes and ponds the importance of bottom is determined by 

 the strength of wave action and the amount of current. The 

 fine bottom materials around the margin of a large lake are con- 

 stantly moved about; the particles grind upon one another mak- 

 ing the presence of bottom organisms impossible. Thus the sandy 



