April, '14] SHELFORD: MEASURE OF EVAPORATION 231 



the character of the life mechanism in question and upon the kind 

 and intensity of the stimulation. 



Reactions to conditions in experiment, the condition selected and 

 avoided, indicate the conditions suitable for the animals in nature. 

 If such tests are made with reference to sufficient number and combi- 

 nations of conditions and at a number of periods in the life history, 

 much of a general nature concerning the relations of animals to envi- 

 ronmental factors can be determined. 



The reasons for the necessity of determining evaporation in connec- 

 tion with the effects of temperature, moisture, wind movement and 

 insolation, may be summarized as follows: 



1. The total effect of the air temperature, pressure, relative humid- 

 ity, and average wind velocity upon a free water surface is expressed 

 by the amount of water evaporated (Hann^). 



2. The same factors have been shown to determine the amount 

 of evaporation from the bodies of organisms (Reinhard^). 



3. Metabolism results in heat and the temperatures of the bodies 

 of animals both warm and cold blooded, is nearly always higher 

 than the surrounding medium, at least during activity^. The sur- 

 rounding conditions may be stated as usually acting on metabolism, 

 etc., as follows: (a) A moist cold atmosphere (very low evaporation) 

 causes body temperature to fall more rapidly than a dry cold one at 

 the same temperature, because of the more rapid conduction of heat. 

 Such a fall in temperature decreases metabolism of cold blooded ani- 

 mals, and increases metabolism of warm blooded animals within their 

 capacity for heat regulation. In a dry cold atmosphere the heat loss 

 is less pronounced because of the less rapid conduction of heat.* (b) 

 In a dry warm atmosphere (high evaporation) rapid evaporation 

 keeps down the peripheral temperature, and prevents death from 

 overheating and destructive metabolism in cold blooded animals, 

 and makes possible body temperature regulation and thus prevents 

 heat stroke and death in warm blooded animals. In a moist warm 

 atmosphere, death and heat stroke occur because of lack of evapo- 

 ration and lack of peripheral cooling in the case of warm blooded 

 animals even when the surrounding temperature is at or below the nor- 

 mal body temperature.^ (c) Wind movement (which increases evap- 

 oration) increases radiation of body heat and of heat due to insola- 

 tion^. It increases evaporation and further cools the body, thus within 



» Climatology, p. 72. » 



2 Zeit fiir Biol., Bd. 5, p. 28. 



' Schaeffer's Physiology, Vol. I, p. 793. 



" Hill's '" Recent Advances in Physiology and Biochemistry" ('06j, p. 256. 



^ Hill's "Recent Advances in Physiology and Biochemistry " ('06), p. 256. 



