ATMOSPHERE 161 



definite positions with reference to wind (anemotaxis) (128a), as for 

 example some flies hover in the air in one position with the head toward 

 the wind. Some animals, such as the land salamanders, frogs, toads, 

 millipedes, spiders, and insects turn away from currents of air because 

 of increased evaporation. 



7. ATMOSPHERIC ELECTRICITY (125) 



The effect of atmospheric electricity upon organisms is little known. 

 It varies with variations in other conditions of the atmosphere. It will 

 probably be found to be important in the life of animals. 



IV. Combinations or Complexes of Factors 



As has already been pointed out (55), the animal environment is a 

 combination of moisture, temperature, light, pressure, materials for abode 

 and food, all of which factors taken together constitute a complex of 

 interdependences. These various factors are so 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 environ- 

 ment which shall include a number of the most important environ- 

 mental 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 botanists and 

 plant ecologists on account of much evidence, both experimental 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 practically no evidence of an experimental 

 sort for the application of such a scheme to animals. Relative humidity 

 has been suggested as an important index (128) but does not properly 

 express the influence of atmospheric humidity upon the animal body 

 (125, p. 53). The saturation deficit has also been suggested but does 

 not take temperature into account. 



