


Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 77 
During 1896 and 1897 the author of this paper, in company 
with his students, endeavored to classify the vegetation about 
Chicago in accord with Warming’s principles. In 1898 a similar 
and more careful study of this kind was made in northern 
Michigan. It was of course found to be possible to classify the 
plant societies by the amount of water in the soil, but it was 
found that such a classification put together plant societies 
radically different in their character, and separated plant societies 
that were obviously closely related. The best instances of these 
difficulties were seen in the case of heaths and moors. Not 
only were heaths and moors found to have closely similar species 
and vegetative adaptations, but these plant societies were often 
found grading into each other. In water content these societies 
were very different, the peat moor or bog being hydrophytic and 
the heath xerophytic. Thus some factor other than water con- 
tent is responsible for both. In that same year (1898) Nilsson 
and Schimper published their views on the causes of the xero- 
phytic character of moor vegetation, as outlined above. Further- 
more the vegetation of peat bogs is radically different from that 
of river swamps which have the same water content. 
Further field study but added to the difficulties of the situa- 
tion, and the need of another classification was keenly felt. It 
Was seen at once that no one factor could take the place of the 
water content of the soil, since that is obviously the most impor- 
tant of all direct factors in distribution, as Warming so ably 
shows. An attempt was therefore made to relate the facts of 
distribution to combinations of factors, with the following 
results. The classification which is about to follow is based in 
the main on two ideas, viz., that a classification to be true must 
be genetic and dynamic. In other words, an attempt is made to 
Sroup plant societies according to their relationship and their 
evolution. 
The influences which govern the distribution of plants reside 
in the air or soil (regarding water as soil, for the sake of con- 
venience). The atmospheric influences (light, heat, air) operate 
Over wide areas and have subordinate edaphic importance, 
