
78 BOTANICAL GAZETTE [FEBRUARY 
whereas the soil influences (soil heat, soil air, soil water, soil 
chemistry and physics) are of predominant edaphic importance, 
though of little account when distribution over wide areas is 
considered. We may say then that atmospheric or climatic 
factors determine distribution in the large, while local differences 
are produced by changes in the edaphic or soil factors. 
The soil conditions are chiefly determined by the surface 
geology and the topography. The original character of the soil, 
whether rock, sand, clay, or marl, depends upon the geological 
relations. From the vegetation standpoint the topographic rela- 
tions are commonly much more important, since they condition 
the presence or absence of drainage, and hence cause striking 
variations in air content and humus. Doubtless the charac- 
teristic features of peat bog vegetation are due to the absence 
of drainage and consequent poor aeration and accumulation 
of organic products. Moreover, in so far as the atmospheric 
factors have an influence on distribution locally, it is largely 
due to topographic diversities, such as angle and direction of 
slope. 
Having related the vegetation largely to topography, we must 
recognize that topography changes, not in a haphazard manner, 
but according to well-defined laws. The processes of erosion 
ultimately cause the wearing down of the hills and the filling up 
of the hollows. These two processes, denudation and deposi- 
tion, working in harmony produce planation; the inequalities are 
brought down to a base level. The chief agent in all these 
activities is water, and no fact is better established than the 
gradual eating back of the rivers into the land and the wearing 
away of coast lines; the material thus gathered fills up lakes, 
forms the alluvium of flood plains, or is taken to the sea. Veg- 
etation plays a part in all these processes, the peat deposits add- 
ing greatly to the rapidity with which lakes and swamps are 
filled, while the plant covering of the hills, on the contrary, 
greatly retards the erosive processes. Thus the hollows are 
filled more rapidly than the hills are worn away. As a conse- 
quence of all these changes, the slopes and soils must change ; 



