Te2 ANIMAL COMMUNITIES OF STREAMS 
bed and became hypothetical stage C of the diagram, the fish community 
of stage 1 was succeeded by a fish community ecologically similar to the 
fish communities at the localities marked 2 in Fig. 67. The fish now eco- 
logically representing this community are the horned dace and the red- 
bellied dace. The community of the single species, the horned dace, 
had at such a period moved inland to the point where line 1-1 (Fig. 69) 
crosses the curved line representing the profile of hypothetical stage C. 
As erosion continued, the fish community ecologically represented by 
the horned dace and red-bellied dace moved gradually inland and was 
succeeded by a fish community occupying the mouth of hypothetical 
1 Ge ne ee ES AE 
wey 
Fic. 68.—A diagram showing the successive stages in the profile (general shape 
of the bottom) of a very young stream, curved lines, A~B, A-C, A-D, A-E, A-F, 
A-G, A-H representing the successive profiles. The uppermost horizontal line 
represents the surface of the land into which the stream is eroding. The horizontal 
line with the arrowheads indicates the migration of the source of the stream and 
accordingly of similar stream conditions. The vertical line with arrowheads when 
followed downward passes through a succession of stream conditions and represents 
physiographic succession at the locality B. The point A is the mouth of the stream. 
Opposite this are shown three successive sizes of the stream, and therefore succession 
at that point. 
stage D, ecologically similar to that now found at the point 3. This is 
represented by the three daces and the Johnny darter. 
As the hypothetical stage D eroded its bed and became stage E, 
which is represented by County Line Creek, fish community 3 was then 
succeeded by a fish community ecologically similar to the fish community 
now present at point 4. This is ecologically represented by the three 
daces, the Johnny darter, and the young of the common sucker. The 
fish communities designated as 1, 2, 3 have meanwhile moved inland and 
are arranged in the order which their ecological constitution requires. 
The continuation of the process resulted in displacing a fish com- 
munity ecologically similar to the fish community 4 by a fish community 
