IIO ANIMAL COMMUNITIES OF STREAMS 
deeper water at their sources (due to springs) and in which the graded 
series of conditions found in the North Shore streams is wanting. 
d) The swift-water fishes begin markedly at the head of erosion in 
Hickory Creek. 
e) The fish communities differ as to species where the conditions 
are very similar, for example, in Thorn-Butterfield and Hickory creeks. 
The general habits of the fishes are the same. 
f) Larger fishes are found in the larger water course and in the down- 
stream portions of the smaller streams. 
g) Fish, when entering a stream, go upstream to a point suited to 
their physiological constitution, regardless of its physiographic mode of 
origin. 
4. GENETIC ECOLOGY OF STREAMS 
Several years ago Adams (103) pointed out that the dispersal of 
aquatic animals is determined by the shifting backward of the head- 
waters and other conditions in streams as erosion proceeds. The forms 
that are in the young streams are moved back as the headwaters are 
moved back and as the river system spreads out into the usual fan shape, 
the animals that belonged in or near the headwaters move backward as 
the conditions migrate backward. In a broad geographic way this is 
unquestioned but details may be studied in the small streams of the 
bluff between Glencoe and the Wisconsin state line. 
Fish are the only strictly aquatic forms in these streams that might 
not have entered by some other method than through the mouth of the 
stream. We have made a study of the fish of these streams for the pur- 
pose of determining whether the fish in the headwaters of the large 
streams are the same as the fish that are found in streams that are just 
large enough to have a single fish species, and the relation of the animals 
to stream development. The changes in animal communities which 
take place at one point are called succession. 
a) Ecological succession.—Ecological succession is the succession 
of ecological types (physiological types, modes of life) over a given point 
or locality, due to changes of environmental conditions at that point. 
From this point of view we have nothing to do with species, except that names 
are necessary. However, we may speak of the succession in terms of 
species whenever their life habits (mores) are not easily modifiable. 
Succession always involves all the animals of a community but it is 
often easier to discuss the changes which take place with respect to one 
group, such as the fishes. It is always to be understood that with changes 
in the fish communities there are similar changes in the communities of 
