1gt1] COWLES—VEGETATIVE CYCLES by gt ¢ 
tation, or at least retrogressive (i.e., tending to cause departure from 
the mesophytic), while the influence of deposition is constructive 
or progressive (i.e., tending to cause an approach toward the 
mesophytic). Progressive successions are well illustrated in the. 
development of flood plains along rivers, and in the growth of sandy 
shores; retrogressive successions are associated with the eroding 
activities of streams and of receding shores. 
Sometimes erosion may not have a retrogressive influence and 
sometimes the effect of deposition is not progressive. For example, 
on a somewhat rapidly eroding clay cliff of Lake Michigan, there 
often occur certain xerophytic annuals, which develop during the 
comparatively stable summer period, and a few perennials, such 
as the sumac and Eguisetum, which have underground organs that 
enable them to migrate landward as fast as the cliff recedes; here 
we have a remarkable instance of rapid topographic change with- 
out a corresponding plant succession, either progressive or retro- 
gressive. A marked increase in erosive intensity would destroy 
all vegetation, and a marked decrease in erosive intensity might 
institute a progressive vegetative succession. Deposition unaccom- 
panied by progressive changes may be illustrated by an instance 
from the Lake Michigan sand dunes. Frequently a growing dune 
is inhabited by xerophytic annuals and by a few shrubs or trees 
(as various willows and the cottonwood); such a place illustrates 
the extreme of topographic dynamics, but often the vegetation is 
Static. A great increase in depositional intensity results in the 
destruction of all the plants, while a decrease in depositional inten- 
sity results in progressive succession. Retrogression or a static 
Condition of vegetation is to be seen also along rapid streams, 
Where there is a considerable deposition of coarse material. A 
striking illustration of retrogression associated with deposition 
8 afforded by lava flows. . 
6. Biotic successions 
A. GENERAL FEATURES 
Of less interest, perhaps, to the physiographer than are the 
Vegetative changes hitherto considered, but of far greater import 
to the plant geographer, are the vegetative changes that are due to 
