388 ENTOMOLOGY 



simple observation, unaided by experimentation, one cannot disen- 

 tangle the effects of one factor from those of others acting with it. 

 The environment is a complex of many interdependent factors. 



The factors control one another, but those that are more immediate 

 in their operation are controlled by the larger influences of physiography 

 and vegetation. 



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 physio- 

 graphic features. Slope and direction of facing profoundly affect 

 vegetation, moisture, and light." (Shelford.) 



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 influ- 

 ence the kind and amount of vegetation. All are interdependent. 



" 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. Plants 

 add organic matter to the soil. This organic matter holds the water 

 so that moisture increases and plants may increase. With such changes 

 it is obvious that an area of sterile soil will support more animals as 

 time goes on, than at the outset, when the conditions were such that 

 only a few hardy species could live. Here again, then, time is the 

 important factor in determining the change of the area, so as to be 

 suitable for more species (because more species are adapted to live in 

 the resulting than in the initial conditions) . The length of time which 

 has elapsed since a given set of surface and physiographic conditions 

 became exposed to the atmosphere is very important in governing the 

 number, kind, and distribution of animals in a given area." (Shelford.) 



