rgtr] COWLES—VEGETATIVE CYCLES 173 
depositing phases of present streams and shores, and on the other 
hand is the pre-erosion topography (as it may be termed) which is 
characteristic of those areas that have not as yet been invaded by 
erosive forces. In our northern states the areas characterized by 
the presence of a pre-erosion topography often greatly exceed in 
extent the areas which are characterized by an erosion topography. 
South of the glaciated region, however, the areas characterized by 
the presence of an erosion topography often greatly dominate. 
But the influence of biotic agencies is not confined to areas that are 
characterized by a pre-erosion topography. For example, in our 
eastern forested region the development of a ravine, which furnishes 
a characteristic illustration of rapid topographic dynamics, exhibits 
only here and there actual erosion or deposition; the ravine slopes 
as a whole are covered with a mesophytic vegetation, because at a 
given spot the interval between periods of active erosion often is 
sufficiently long to permit the development of an entire biotic 
cycle. Perhaps in no other way could there be brought out more 
strikingly the durational contrast between topographic and vege- 
tative cycles; a ravine is an index of extreme topographic youth, 
and yet in its development there is ample time for the complete 
development of many vegetative cycles. Quite as in ravines, the 
cliffs of streams and shores often exhibit temporary exemption from 
erosion, whereupon there is at once instituted a biotic cycle, which 
often has sufficient time for complete development before erosion 
again becomes active. 
B. THE HUMUS COMPLEX 
4. Water.—It is now time to consider the varying aspects of the 
biotic agencies which institute succession. Of these the first to be 
Mentioned, because of its unquestioned supremacy, is the accumula- 
hon of humus. There are a number of different ways in which the 
‘ccumulation of humus affects the trend of succession. It can 
scarcely be doubted that the most important of these humus 
Influences, and perhaps the most important of all influences, 
inheres in the change which the humus brings about in the water 
telation of the soil. Speaking generally, humus accumulation 
Ccasions an increase in soil moisture on uplands and a decrease in 
