198 HOMER E. CHENOWETH. 



avoidance resulted. The problems involved in the results of 

 these experiments are complex and the rate of evaporation plays 

 an important role in the distribution of animals. 



Why is the habitat of the white-footed mouse coextensive with 

 the woodland area or why is any animal restricted to any par- 

 ticular environment? If any one should attempt to answer 

 these questions he would encounter a very complex physiological 

 problem; however it is evident that this experiment work in- 

 cates that the evaporation is the best index of conditions affecting 

 warm-blooded animals. 



Change of environmental factors influence organisms in one 

 of three ways (Shelford '14): (a) They may produce death. 

 (6) They may modify structure or behavior, (c) They may 

 stimulate migrating animals and cause them to turn back when 

 an increase or decrease in the factor is noted. These factors 

 have been shown to modify form, color, size and the behavior of 

 animals under experimental conditions, Entemann ('09), (Allen 

 '74) and Sumner ('10). 



The integument or covering of an animal has a great deal to do 

 with the power to withstand air of high evaporation. The white- 

 footed mouse with a heavy coat of hair no doubt can withstand 

 these unfavorable conditions more readily than the salamander 

 or other soft-skinned amphibians, since there is no mechanism to 

 prevent loss of water. The shrew (Blarina brevicauda Say) has 

 thinner coverings of hair and they live in a much more restricted 

 habitat. It is almost impossible to keep them in the laboratory. 

 The essential factor for their existence seems to be plenty of 

 moisture and they will die if left in the traps over night without 

 water. 



When the mice encountered the air of high evaporation there 

 was first a period of stimulation or of heightened sensibility which 

 is indicated by the increasing avoidance of the air of high evapor- 

 ation after several entrances into it as shown in all the experi- 

 ments. It is impossible to give a solution which would account 

 for this heightened sensibility, but the work of past investigators 

 gives some interesting data on which to base some conclusions. 

 Waller ('09) showed that a small increase of CO 2 increased the 

 irritability of the nerves. Osborne ('10) showed that CO 2 



