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1901} PHYSIOGRAPHIC ECOLOGY OF CHICAGO 83 
it remains true, nevertheless, that each region must be worked 
out by itself. The general principles that are involved in the 
dynamics of plant societies, however, ought to be essentially the 
same everywhere. Some instances will be given which will 
show the necessity for working out the life history of the plant 
societies in all regions. While the culminating type throughout 
the northern states east of the Mississippi river is probably a 
deciduous mesophytic forest, yet the elements of this forest dif- 
fer greatly in different localities. In central Michigan the 
maple, beech, and the evergreen hemlock appear to be the lead- 
ing character trees of the mesophytic forest. In Indiana and 
Illinois the hemlock is not one of the dominant trees of this 
forest. In the Alleghanies of Tennessee a large number of tree 
Species assume a place of almost equal importance in the meso- 
phytic forest. Again, in the Chicago region the tulip tree and 
buckeye are rare and confined to the flood plain forests, while 
in Tennessee these trees are found in many other plant societies. 
In the Chicago region the arbor vitae is confined to undrained 
swamps and xerophytic cliffs, while in northern Michigan it is 
found in many other habitats. We may perhaps summarize 
these data by saying that each species varies in habitat in dif- 
ferent regions, and that in general a species can grow in the 
largest number of plant societies at its center of distribution, since 
there the climatic conditions favor it most highly. In other 
regions, especially near its areal limits, it can grow only in those 
plant societies which resemble most closely in an edaphic way 
the climatic features at the distribution center. Thus the tulip 
and buckeye, which flourish best in the mesophytic forest climate 
of the Alleghanies, are found near Chicago only in the most pro-. 
nounced of our mesophytic societies, those of the flood plain. 
Again, the arbor vitae, and with it many conifers and heaths, 
Stow near Chicago only on the cliffs and dunes or in the 
undrained swamps, since these are the most pronounced of our 
xerophytic habitats and most closely resemble the xerophytic 
northern climates. 
A few words should be said’in the way of indicating the 
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