SUCCESSION OF COMMUNITIES 113 



ecologically similar to the present fish community 5. This is repre- 

 sented in the lower waters of Bull Creek stage F. 



Ecological succession is one of the few biological fields in which pre- 

 diction is possible. We may carry this discussion a little farther. We 

 have noted that the developing streams continue to erode their beds, 

 grow larger, and bring down the surface of the land. These processes 

 have not stopped in Bull Creek; it will become larger, contain a larger 

 volume of water at the locality 5, and the fish community of locality 5 



\ X X X \ 



FIG. 69. This figure is based on Fig. 68. The profiles of the streams shown 

 here are 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 stage; H, 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 represented by a stream 

 cross-section at the right. The direction of reading in succession is indicated by the 

 vertical line with the arrowheads 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 occupied by similar fish communities. 



will be succeeded by a fish community ecologically similar to that now 

 at locality 6. This stage has been designated as hypothetical stage G 

 in the diagram. With a further continuation of the process, the fish 

 community of stage G, locality 6, will be succeeded by a fish community 

 ecologically similar to that now found at the locality 7 (Dead River) 

 stage H. The communities of every stream have some such history as 

 we have reconstructed, but the details may be modified by conditions. 

 That branch of ecology which deals with such histories is called genetic 

 ecology. 



