




1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 177 
The future of the vegetation on the established dunes and 
beaches is somewhat problematical. From analogy with other 
plant societies in this region, and from established dunes in 
Michigan, we should expect a mesophytic forest, probably of 
the white oak-red oak-hickory type at first and then followed by 
a beech-maple forest. There are evidences that some such 
changes are now taking place. On many of the oak dunes, 
especially where protected from exposure, there is already a 
considerable accumulation of humus. Herbaceous ravine meso- 
phytes like Hepatica, Arisaema, and Trillium are already 
present, and with them mesophytic shrubs and trees, including 
the sugar maple itself, though the beech has not been found on 
the dunes of our area, as it has in Michigan. One might expect 
that the flora of the older Glenwood beach would have advanced 
more toward the mesophytic stage than has the flora of the 
younger Tolleston beach. Such, indeed, seems to be the case, 
especially at Glenwood, where the white oaks are more numerous, 
and the black oaks. much larger and more luxuriant. The humus 
is richer and most things look as if the age of this beach were 
notably greater than that of the Calumet or Tolleston beaches. 
This subject, however, needs much further investigation. In 
any event, one character of the sand hill stands out in bold 
relief, viz., its great resistance to physiographic change. Not 
only is its erosion slower than that of the clay hill, but the 
advance of its vegetation is vastly slower at all points along the 
line. The slowness of humus accumulation accounts for this, 
perhaps, more than all else. 
Ill. Summary and conclusion. 
In the present paper the author has endeavored to show the 
need for a classification of plant societies which shall form a 
logical and connected whole. Warming’s classification, based on 
the water content of the soil, is doubtless the best possible classi- 
fication, if but one factor is used. Graebner’s classification, based 
on soil characteristics, includes the advantages of Warming’s 
scheme, and adds desirable new features. 
