IIO A. O. WEESE. 



power of the air also varies, and thus we get an evaporation 

 gradient plus a temperature gradient, so that the gradient under 

 these conditions is a double one. The optimum temperature is 

 in the neighborhood of 30 when the humidity and rate of flow of 

 the air is such as to produce an evaporation of 3 c.c. per hour, 

 as measured by a standard atmometer. 



3. Air humidity, air temperature and air current gradients are 

 all essentially evaporation gradients, and the optimum in each 

 case is a function of the evaporating power of the air. 



4. The most definite and clear cut reaction in a gradient shown 

 in this series of experiments is that in the substratum temperature 

 gradient. This reaction is, to a great degree, at least, inde- 

 pendent of accompanying differences in air temperature and 

 humidity, or at least, the effect of these variations is overshadowed 

 by the response to the gradient in substratum temperature. The 

 optimum substratum temperature is rather definite between 



36 and 40, and at the upper limit a very definite reaction (bur- 

 rowing) takes place. This optimum is correlated with the other 

 optima given above, in that the substratum temperature, under 

 the conditions given in paragraph I, is usually from five to ten 

 degrees above the temperature of the air. Soil temperature 

 (surface) is evidently of very great importance in determining 

 details of seasonal and daily life, having as well its accompany- 

 ing effect on the distribution of the species. 



4. It is dangerous to ascribe to any one factor or group of 

 factors the supreme r61e in determining the seasonal or general 

 distribution of a species. The factors are certainly not the same 

 for all species even in the same environment and before definite 

 conclusions can be drawn a careful analysis of the habitat must 

 be made, and experimental data must be obtained as to the 

 reaction of the animals in gradients involving the factors capable 

 of variation. 



IV. ACKNOWLEDGMENTS AND BIBLIOGRAPHY. 

 The writer is deeply indebted to Dr. V. E. Shelford, of the 

 University of Illinois, for help and direction in the prosecution 

 of the work and for assistance in the preparation of the manu- 

 script. He is also indebted to Dr. M. M. Wells, of the University 



