48 



THE AMERICAN NATURALIST [Vol. LIII 



In summing up the results of the air temperature and 

 substratum temperature experiments (over one hundred) 

 in relation to those of the evaporation gradient previously 

 reported (1917a), it is found that the animals choose con- 

 ditions which are very near the normal conditions in the 

 usual habitat at the time of the greatest activity. These 

 conditions represent the optimum for the animals. For 

 example, as reported in a previous paper, the evapora- 

 tion optimum for Phrynosoma modestum appears to be 

 near 3 c.c. per hour, as measured by the standard atmom- 

 eter, which is very near the average outdoor evaporation 

 as observed in the natural habitat of the animal at the 

 season and at the time of day when the animal is most 

 active. If the temperature under such conditions be ob- 

 served, it will be found that the average atmospheric 

 temperature, 1 cm. from the surface of the soil, in the 

 sun, is in the neighborhood of 35°, and that of the surface 

 layer of the soil about 40°. These temperatures vary 

 greatly, of course, with other features of the weather, 

 such as air movements, clouds, etc., but the above figures 

 represent a normal condition. Of the variables men- 

 tioned here, substratum temperature has much the 

 greatest effect on the behavior of the animal. 



3. Moisture of Substratum Gradieiit.— Although, it was 

 impossible to esta])lish and observe an effective gradient 

 in general soil conditions, several experiments were per- 

 formed on the direct effect of a soil moisture gradient. 

 The gradient in water content of the substratum was ob- 

 tained by placing a layer of torpedo sand saturated with 

 water on the bottom of one third of the cage, a mixture 

 of saturated sand and dry sand in the adjoining third, 

 and dry sand in the romainder of the cage. In none of 

 the s])('ci('s ohxTNcil was ;uiy marked preference for any 

 l'<>''^i<>'^ *>t" t!u' ca.uc ('xhii)it(Ml. Soil moisture, as such, 



although, in the natiu-nl lialiitat. Ilic hiuli cxaiM.i-atiiig 

 power of the air pro(hi.-c> a .Mwi-idcrablc (Icuivc of tem- 

 perature difference between dry soil and wet soil l)y the 



