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. The Important 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 



