


1901 } PHYSIOGRAPHIC ECOLOGY OF CHICAGO 169 
physiographic sense is the river bluff. When a stream has 
banks of clay the conditions seem decidedly similar and yet the 
flora is not the same. A comparison of the lake bluffs at 
Glencoe with the bluffs along Thorn creek shows that some 
species are common, notoriously Ostrya, Rhus, Quercus, Popu- 
lus. Yet the differences are still more striking, for the bluffs 
along Thorn creek do not show Sahx glaucophylla nor Shep- 
herdia; most striking of all, however, is the entire absence of 
conifers. When we compare the lake bluffs with the rock 
bluffs of the Illinois river we find that the resemblances are 
greater than the differences, since the river bluffs have conifers, 
though even here some of the lake bluff forms are absent. 
When, however, we compare the Glencoe bluffs with the dunes, 
we find that all of the dominant shrubs and trees of the bluff are 
found also on the dunes ; not only this, the dominant bluff forms 
are dominant on the dunes also. 
The facts of the preceding paragraph are pregnant with sig- 
nificance. One obvious corollary is that given similar soils but 
dissimilar conditions of atmospheric exposure, as at Glencoe and 
Thorn creek, the vegetation is unlike. Another and more strik- 
ing corollary is that given the most dissimilar soils possible, viz., 
the Glencoe clay and the dune sand, we still have similar vege- 
tation, because the atmospheric conditions are the same in the 
two cases. The evidence of the Illinois river bluffs is less clear; 
they are more xerophytic than the bluffs along Thorn creek, but 
whether this is chiefly due to rock as against clay or to greater 
exposure is not certain. At all events these facts show that it is 
not enough to know about chemical or physical conditions in 
the soil. We cannot divide plants into those of clay, rock, and 
sand, but must take into account that most plants have a wide 
range of life so far as soil is concerned, provided the atmos- 
pheric conditions are congenial. The chief exception to this 
statement seems to be found not in the original soils but in the 
Superimposed humus. There are many plants that require 
humus for their occurrence in nature, but it makes no difference 
whether the subsoil is rock, sand, or clay, provided alone that 
