VICTOR E. SHELFORD. 



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FIG. 4. This figure is based on Fig. i. The profiles of the streams shown there 

 are here separated vertically at the mouth. The curved lines represent seven 

 stream stages as follows: B, Glencoe Brook; C, hypothetical stage; D, hypothetical 

 stage; E, County Line Creek; F, Pettibone Creek; G, hypothetical stajir; //, Bull 

 Creek-Dead River. The hypothetical stages could, no doubt, be found along the 

 shore of Lake Michigan, the difficulty arises from the introduction of sewage into 

 so many streams. 



The comparative size of the mouth of each stream stage is representdl by a 

 stream cross-section at the right. 



The direction of reading in succession is indicated by the vertical line with the 

 arrow heads pointing downward. The oblique lines marked i-i, 2-2, 3-3, etc., 

 pass through points in the stream profiles which are in the same physiographic 

 condition, and which are occupied by similar fish communities. 



When Bull Creek was at a stage represented by the first stage 

 in our diagram (which is now represented by the present Glencoe 

 Brook) its fish, if any were present, were ecologically similar 

 to those now in Glencoe Brook in their relations to all factors 

 except climate. This ecological type is represented by the horned 

 dace alone. As Bull Creek eroded its bed and became hypo- 

 thetical stage C of the diagram, the fish community of stage B 

 was succeeded by a fish community ecologically similar to the 

 fish communities at the localities marked 2 in Figs. 2 and 4. 

 The fish now ecologically representing this community are the 

 horned d.-Mv and the red-bellied dace. The community <>f the 

 single species, the horned dace, had at such a period moved 

 inland to the point where line i-i crosses the curved line repre- 

 -enting the profile of hypothetical stage C. 



As erosion continued tin- l'\^\\ community ecologically ivpre- 

 -i-utrd by the horned d.u . and red-bellied dace mo\vd gradually 

 inland and was succeeded by a fish community occup\ing the 



