104 A. O. WEESE. 



groups in this or in other experiments. In both cases the driest 

 air was avoided, and the greatest number were found in the cen- 

 tral portion of the cage, where the evaporation was .84 during a 

 twenty-minute period. Many other similar experiments gave 

 similar results. 



Experiment 75. This experiment did not show as much differ- 

 ence between the extremes but showed a great percentage in 

 favor of the central section of the gradient cage in which the 

 evaporation was much lower than in the preceding experiment (.5). 



Summary of Air Humidity Gradient Experiments. 



The animals ordinarily avoid air of excessive evaporating 

 power, or, if they remain in such air, are stimulated in such a way 

 that attempts to escape are made, or are stimulated to burrowing 

 or digging activities. The net result of either reaction, if success- 

 ful, is an escape from the unfavorable environment. As nearly 

 as could be determined from the data at hand the optimum 

 evaporation for the average animal lies between .5 c.c. and i.o c.c. 

 expressed as the evaporimeter reading for a twenty-minute ex- 

 perimental period. There is a considerable variation in individ- 

 ual behavior and that of any individual will vary from time to 

 time, due, presumably to changes in physiological condition. 

 The normal evaporating power of the air in the natural habitat 

 during the months of April and May varies from .15 c.c. twenty- 

 four hour average on a rainy and cloudy day, to i.o c.c. ; on a 

 bright clear day when the maximum temperature is in the 

 neighborhood of 30. Although day and night observations 

 have not been made separately it is evident that the average 

 evaporation for the daylight hours would be somewhat higher 

 than the figures just given. 



Animals kept for a long time in abnormally dry atmospheric 

 conditions evidence a marked degree of uneasiness, indulge in 

 many spasmodic movements, and finally either burrow or apply 

 the body closely to the substratum. They are, however, able to 

 endure long exposure to dry air without death. A 2.5 gram 

 animal was exposed to a constant current of air with an evapora- 

 tion of 2.4 c.c per twenty-minute period for a week before death 

 took place. Death in this case was also probably largely due to 



