THE REACTIONS OF CERTAIN MOIST FOREST MAM- 

 MALS TO AIR CONDITIONS AND ITS BEARING ON 

 PROBLEMS OF MAMMALIAN DISTRIBUTION. 1 



HOMER E. CHENOWETH. 



PAGE. 



I. Introduction 183 



II. Habitat and Distribution 184 



III. Experimental Stady of the Relation of Environmental Factors to Dis- 



tribution 185 



1. Method of Establishing Evaporation Gradients 186 



2. General Plan of Experiments 190 



3. Material 191 



4. Experimental Results 191 



5. General Discussion 197 



IV. Summary 199 



V. Acknowledgments and Bibliography 200 



I. INTRODUCTION. 



The reactions of animals to environmental factors in experi- 

 ments indicate conditions suitable for them in nature. If such 

 experiments are carried out carefully and at different periods of 

 the life history, the reason for the presence of any animal in a 

 particular environment may be explained in terms of measurable 

 physical factors which it selects and avoids. It has been shown 

 that insects, spiders and amphibians from moist and dry habitats 

 respectively were sensitive to variations of the evaporating powers 

 of the air. The physical factors that determine the rate of 

 evaporation are humidity, pressure, wind and temperature, and 

 these are measured in combination by instruments measuring 

 evaporation. It is the main purpose of this paper to show that 

 representatives of the mammalian group, wood mouse (Pero- 

 myscus leucopsus noveboracensis Fischer), react to variations in 

 the evaporating power of air in a similar manner regardless of 

 the cause of the variation. The local distribution of an animal 

 is determined by its reactions to environmental factors and its 

 geographic distribution is a function of the distribution of its local 

 habitat, or of conditions indicated by its local habitat. In spite 



1 Contribution from the Zoological Laboratory, University of Illinois, No. 82. 



183 



