FACTORS 43 
in and among these are various local complexes. The history to follow 
applies particularly to the local complexes. The analysis into factors 
applies to both local and climatic. 
II. Tue Ivportant FAcTors AND THEIR CONTROL IN NATURE 
Little experimentation has been conducted with a view to determin- 
ing the relative importance of different factors in the control of animals 
within an environmental complex. It is known, however, that moisture 
(evaporating power of the air), light, and materials for abode are factors 
important in the life of land animals; carbon dioxide, oxygen, materials 
for abode (including bottom), and current, in the life of aquatic animals. 
The evidence for these statements cannot be presented here, but will be 
given in appropriate places throughout the discussion which follows. 
I. THE CONTROL OF FACTORS 
This is related to physiography, surface geology, and vegetation. 
a) Physiography.—In streams, current and oxygen content are 
determined very largely by physiographic conditions. Current is a 
function of volume of water and slope of stream bed. Oxygen content 
is largely determined by the rate of flow, and therefore is influenced by 
physiography. In lakes, oxygen content is determined by the depth, 
the temperature, and winds—physiographic factors are again important. 
On land, moisture and light are in a measure controlled by physiographic 
features. Slope and direction of facing profoundly affect vegetation, 
moisture, and light. 
b) Surface materials and vegetation.—Materials for abode are largely 
the surface soil or rock or the vegetation. Surface soil or rock influences 
the moisture. Both moisture and surface materials influence the kind 
and amount of vegetation. All are interdependent (35a). 
Physiographic features change with time. Erosion changes the 
gradient of streams, the width of valleys, the steepness of valley walls 
and cliffs, the ground-water level, etc. The weathering of rock is a 
process familiar to all. It is the aggregate of processes by which the 
coarse and hard or massive materials are reduced to clay and soil. This 
requires time. ; 
The fact that vegetation grows upon the so-called sterile, coarse, 
rough-surface materials, usually scattered or ephemeral at first, but 
increasing in denseness with each generation, is also familiar (58). 
Plants add organic matter to the soil. This organic matter holds the 
water so that moisture increases and plants may increase. With such 
