184 HOMER E. CHENOWETH. 



of the fact that most theories of distribution are based largely 

 upon mammals we know of no studies of mammalian behavior 

 bearing on this question. However if our knowledge of climate 

 were complete, and if it were possible to bring representatives of 

 all the land animals into the laboratory and test their reactions 

 to physical factors at intervals through their life history a dis- 

 tribution map could probably be made fairly accurate for each 

 species. 



II. HABITAT AND DISTRIBUTION. 



One of the commonest mammals of any woodland areas of 

 Illinois is the white-footed wood mouse. The habitat of this 

 subspecies is practically coextensive with the woodland. This 

 mouse is sometimes found in the fields when the food supply is 

 short in the forest or it may be tempted to a nearby cornfield. 

 They may migrate to the forest edge or to open cleared 

 places a mile or so from the woods during the summer. They 

 seem to be able to adapt themselves to different moist environ- 

 ments and this is no doubt a great factor in determining their 

 abundance though it seems that houses and barns are seldom if 

 ever entered and it is certain that they never become a household 

 pest. In the woods they are at home under any kind of rubbish 

 and the roots of trees but they are not limited to the ground strata 

 as they are sometimes found living in the trees. The range of 

 the subspecies is from Novia Scotia to central Minnesota, thence 

 south through the humid parts of eastern Nebraska and Kansas 

 and eastward to the Atlantic. 



The main purpose of these experiments was to determine the 

 environmental factors that restrict the range of this mouse to 

 the woodland. So many factors make up the environment of 

 any animal and a change in one usually affects the others so that 

 a study of this kind is a very difficult one. 



Since the relative humidity is so much greater in the forest 

 than on the prairie (Shimek 'n) the writer shows in these ex- 

 periments that the mouse is sensitive to this difference and reacts 

 to any change in which the humidity is lower than that of the 

 woodland in which its habitat is restricted. Since changes in 

 the relative humidity directly affect the rate of evaporation which 

 in turn affects the physiological processes the conclusion that 



