1911] COWLES—VEGETATIVE CYCLES 179 
ment of a mesophytic forest in a canyon is due in large part to 
the increasing shade which is cast by the walls as the canyon 
deepens. However, the predominating influence of shade certainly 
is in connection with forest development, and hence it is not unfair 
to group it with biotic influences. 
D. PLANT INVASION 
A further biotic influence is that of plant invasion. In the long 
period of geologic history, plant migrations from one region to 
another must have played a tremendous part in the changing 
aspect of vegetation. There is reason to believe, however, that 
such changes, apart from those due to human influence, have been 
Wrought almost as slowly as those due to climatic change. So 
imperceptibly do these migrations take place that we know of no 
profound change that has been wrought by this means in natural 
floras within historic time. 
E. MAN 
The last of the biotic influences to be considered is that of man. 
Most of the factors hitherto considered, especially increasing 
shade and accumulating humus with its varied kinds of influence, 
Cooperate to transform originally hydrophytic and xerophytic plant 
formations into those that are more mesophytic; that is, they 
Institute progressive successions. The influence of man, however, 
almost without exception, is retrogressive. Human culture reaches 
its highest expression in mesophytic climates or on mesophytic 
Soils; the xerophytic soils of rocky crags and of sand barrens are 
unfavorable places for human exploitation, and the desert is for 
man an unprofitable waste, except where he finds an oasis or makes 
a district mesophytic through irrigation. Similarly, the waters 
are of value chiefly as avenues of transportation and as a source 
of food, not as a habitation; and swampy tracts are considered 
valueless, unless made mesophytic by drainage. Man, therefore, 
- seeking a place of abode, in clearing land for agriculture, and in 
his Search for timber, has destroyed chiefly mesophytic vegeta- 
ton, in other words, the very vegetation which, in most areas 
Sccupied by human culture, has been seen to be the culminating 
Plant formation. 
