30 IOWA STUDIES IN NATURAL HISTORY 
forest from which it has been derived, minus shade and with a 
decreased humidity and humus, if such statement can be ac- 
cepted. Such areas are represented on the township maps by 
v-shaped marks. 
PRAIRIE-F'OREST MARGINAL ZONE 
The zone lying at the juncture of the prairie and the forest 
has already been discussed and needs but little elaboration here. 
It is a narrow strip of intermingling prairie and forest plants 
which is consequently populated by individuals of the animal 
life from each of the meeting habitats but is never composed 
purely of forms from either. 
AQUATIC HABITATS 
The aquatic habitats of Johnson County are naturally of much 
more limited extent than the terrestrial ones. They permit of 
logical and natural division into two groups, those of running 
water and those of standing water. Each will be found in turn 
to be composed of various stages in an ecological series. 
RUNNING WATER SERIES OF HABITATS 
It is generally recognized that in the reduction of any land 
elevation to its base level the establishment of the streams fol- 
lows a perfectly definite and regular series of steps. First, the 
actual run-off after rainfall succeeds in producing a ravine, 
which, attaining a sufficient depth, acquires ground water and 
becomes a brook. The brook increases its length and broadens 
its valley by further cutting until it becomes a creek, and the 
creek by like processes grows into a mature river. Each of these 
stages presents a separate and definite series of animal habitats 
which go hand-in-hand with a regular, well defined, ecological 
succession of animal associations. 
(1) Brook habitats. The brook represents the first stage in 
the establishment of a permanent stream—the ravine has just 
captured ground water. The main physical features of the 
brook are its V-shaped valley without a flood plain and its alter-. 
nate rapids and pools. It has a low oxygen content, due to the 
fact that the water has been but a short time exposed to the air, 
contains but little vegetable debris or organic matter, due to the 
