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1gor | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 75 
and made variations in the water content of the soil a basis of 
classification. Plant societies were divided into hydrophytes, 
mesophytes, and xerophytes; further than this, however, little 
attempt was made by Warming to subdivide the plant socie- 
ties, except in the case of swamp and dune plants. In these two 
instances, as will appear later, Warming adopted the order of 
succession in his method of treatment. 
Since Warming’s great work appeared, ecologists have in gen- 
eral followed his ideas and have attempted to work them out. 
Noteworthy contributions have also been made which make a 
comprehensive view of the subject more possible. Schimper? 
has analyzed in a most thorough manner the conditions which 
determine the distribution of plants in the large, though he has 
discussed but briefly the purely local or habitat factors. We owe 
to him, however, the first clear statement of the distinction 
between edaphic and climatic factors and formations. The 
minuter treatment of the edaphic formations did not lie within 
his field, and he has attempted in no sense to give a classifica- 
tion, except in the case of climatic formations. 
_ Graebner*5 has published a classification of some of the Ger- 
man vegetation formations, which has not received the attention 
it deserves. This classification is based in the main on the 
chemical and physical characteristics of the soil. The primary 
divisions are chemical, depending on the richness or poverty of 
the soil in plant foods. The secondary divisions are based 
chiefly: on soil moisture.. 
N. H. Nilsson® in some preliminary notes on Swedish 
Swamps and their vegetation called attention to the striking 
difference between hydrophytic and xerophytic same ae and 
3 Op. cit. 
*GRAEBNER, P.: Gliederung der westpreussischen Vegetationsformationen. 
Schrift. Natale Ges. Danzig 9 : 43-74. 1898. See Bot. Centralb. 75: 277-279. 1898. 
EBNER, P.: Ueber die Bildung natiirlichen Vegetationsformationen im 
norddeutschen Flachlande. Archiv der Brandenburgia 4: 137-161. 1898. See Bot 
Centralb, 77 :212-214. 1899. 
° NItsson N. H.: Einiges tiber die Biologie der scicecotirer Sumpfpflanzen. 
Bot. Centralb. 76:9-14. 1898. 
