170 BOTANICAL GAZETTE [MARCH 
ditions that particularly favored the angiosperms, which as a 
group are much more mesophytic than are the gymnosperms. 
To summarize on regional successions, it would seem that secular 
changes in climate, that is, changes which are too slow to be attested 
in a human lifetime, and which, perhaps, are too slow to be attested 
in a dozen or a hundred lifetimes, are the dominating factors. 
It is possible that these changes sometimes are more rapid than at 
other times, and there are those who would have us believe that the 
climate now is growing warmer, as witness the rapid recession of 
many of our North American glaciers; there are others who are 
quite as sure that the climate is growing colder, as witness the 
southward retreat of the “timber line” in Scandinavia. Still 
others feel equally confident that the recession of glaciers is due to 
increasing aridity; this explanation has the advantage also of 
accounting for retreating “timber lines.’ And there yet remain 
some who believe that all such changes may be of short duration, 
as it were cycles within cycles, or feeble and short-lived oscilla- 
tions of great climatic waves. It is to be pointed out that great 
earth movements, either of elevation or subsidence, that is, the far- 
reaching and long-enduring epeirogenic movements, as contrasted 
with the oscillations of coast lines, must be considered in account- 
ing for regional successions; the elevation of the Permian and the 
base-leveling of the Cretaceous must have played a stupendous 
part in instituting vegetative change. 
5. Topographic successions 
In striking contrast to secular successions, which move 5° 
slowly that we are in doubt even as to their present trend, are 
those successions which are associated with the topographic changes 
which result from the activities of such agents as running water, 
wind, ice, gravity, and vulcanism. In general these agencies 
occasion erosion and deposition, which necessarily must have 4 
profound influence upon vegetation. I have considered in another 
place and in some detail (18, 19, 20) the influence of most of these 
agencies, and it will suffice in this place to summarize a few of the 
leading kinds of phenomena that are involved. As might be 
expected, the influence of erosion generally is destructive to ves® 
