ECOLOGICAL SUCCESSION. 77 



dependent upon one another that any change in one usually 

 affects several others. This property of environmental complexes 

 is what makes ecology one of the most complex of sciences, and 

 experimentation in which the environment is kept normal except 

 for one factor, an ideal rarely realized in practice, even under the 

 best conditions. 



The efforts of ecologists, geographers, and climatologists have 

 long been directed toward the finding of a method, of measuring 

 the environment, which shall include a number of the most 

 important environmental factors. De Candolle undertook to 

 base the efficiency of a climate, for supporting plants, upon the 

 mean daily temperatures above 6 C., this temperature being 

 taken as the starting point of plant activity. Merriam has 

 followed this lead and calculated total temperatures for many 

 places in North America and made maps and zones based upon 

 such totals. This system however, has been rejected by botan- 

 ists and plant ecologists on account of much evidence both experi- 

 mental and observational, which is quite out of accord with this 

 view. The scheme has not been generally accepted by zoologists 

 outside of the United States Biological Survey. There is prac- 

 tically no evidence of an experimental sort, for the application 

 of such a scheme to animals. Relative humidity has been sug- 

 gested as an important index (Walker, '03) but does not properly 

 express the influence of atmospheric humidity upon the animal 

 body (Hann, '03, p. 53). The saturation deficit has also been 

 suggested but does not take temperature into account. 



i. Evaporation. 



"The total effect of air temperature, pressure, relative humid- 

 ity, and average wind velocity upon a free water surface in the 

 shade or in the sun, is expressed by the amount of water evapo- 

 rated" (Hann, p. 72). Since temperature in the season without 

 frost is directly due to the sun's rays, light is in part included. 

 In our latitude, clouds in summer slightly decrease the air tem- 

 perature (Hann, p. 72). In winter however the temperature 

 of cloudy days is higher. The strongest light is usually 

 associated with the greatest evaporation. Yapp ('09) found 

 that the rate of evaporation was directly correlated with tern- 



