
1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 163 
oak forests have been seen with a pronounced undergrowth 
of beech. It would seem that one of the chief factors in deter- 
mining the order of succession of forests is the light need of 
the various tree species, the members of the culminating forest 
type being those whose seedlings can grow in the densest forest 
shade. There are evidences that the oak forests about Chicago 
are being succeeded by the beech or maple. The best instance 
of this which the author has seen is on the low moraines along 
the Desplaines river west of Deerfield. The sugar maple (Acer 
saccharinum) has already been mentioned as a character plant of 
the temporary mesophytic forests of ravines. Here we see it in 
the more permanent forest of the morainic hills. The beech 
(Fagus ferruginea) is much rarer than the sugar maple, though it 
is a rather important constituent of the mesophytic forests about 
Chesterton. Why the beech-maple forest has lagged so far 
behind in the region about Chicago is a question not yet settled. 
If these forests elsewhere have had an oak stage it indicates 
that the development here is very slow. 
Though the forests just described, whether of the oak-hick- 
ory or the maple-beech type, are of a high degree of permanence, 
it can be seen that this permanence is but relative. Sooner or 
later stream action will enter these districts and base leveling 
processes will begin on a more rapid scale. But for these activi- 
ties the lowering of hills would be very slow indeed, so slow as 
hardly to interfere at any point with a luxuriant development of 
the vegetation. The destruction of these morainic forests by 
stream erosion is well shown near the shore north of Evanston 
and also along Thorn creek. ig. 78 shows a morainic island 
in a flood plain, the sole remnant of an extensive stretch of 
upland mesophytic forest. We must therefore regard upland 
- forests as temporary also, though they endure for a much 
longer time than do the temporary mesophytic forests of the 
ravines. 
C. The sand hill—A third type of upland is found in the 
sand hills, but since most of these in our district are of dune 
origin, their treatment will be deferred until later. 
