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1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 181 
areas are features of a young typography and they are soon filled 
up by accumulating peat. Algae and other hydrophytes charac- 
terize the lake or pond stages, and largely by their partial decay 
the water becomes shallow enough to support the vegetation of a 
marginal swamp, particularly rushes and sedges. After these forms 
there appear the most remarkable plant societies of the entire 
series, characterized chiefly by Cassandra and other ericads with 
xerophytic structures. Following the shrub stage there is the 
tree stage in which the tamarack often dominates, though pines 
appear later and ultimately mesophytes. In these swamps one 
finds the most perfect examples of the regular succession of 
plant societies, and hence of zonal arrangement. These peat bog 
societies contain a most striking collection of northern plants. 
There are various diverging types of undrained swamps, some 
with shallow soil and a vegetation without extreme xerophytic 
structures, others in which an imperfect drainage may account 
for the facts observed. The marginal vegetation is the same in 
all cases and is remarkably characteristic; this flora is some- 
times seen after the lake and swamp floras have gone. Half- 
drained areas are characterized by luxuriance of the vegetation 
in the lake stage. The lake is followed in order by the bulrush, 
sedge, and grass stages, the latter being denominated the prairie 
stage. Whether this latter type passes into the forest is not 
certain; in any event, this mesophytic stage is not final, for the 
region must subsequently pass through the stages of the river 
series. 
Uplands, as well as swamps, have an interesting history 
before they are attacked by stream erosion. Limestone hills 
and outcrops show first a lichen vegetation, followed by mosses 
and crevice herbs and later by shrubs and trees. The stages 
on clay hills pass far more rapidly, indeed early stages are 
hard to detect, and one almost uniformly finds a mesophytic 
forest of oaks and hickories in these habitats. Where the 
forest conditions are disturbed, there is a rapid return through a 
series of herb and shrub stages to the same tree types. It is 
somewhat probable that the oaks will be followed by sugar 
