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THE AMERICAN NATURALIST [Vol. LIII 



that the animal burrowed, first at a temperature of about 

 38° and later at a slightly lower temperature (indicated 

 by the circles in the first and sixth minutes of the record). 

 This burrowing reaction was found to take place very 

 often, throughout the whole series, usually at the upper 

 limit of the optimum temperature range. This agrees 

 with the phenomena observed in the field, of burrowing 

 as the air temperature rises in the course of the day. 



Plirynosoma douglassii, as shown in the graphic records 

 of Experiments 82 and 85 (PI. I), seems to choose a some- 

 what lower temperature, between 30° and 35°, although 

 there is a considerable amount of individual variation. 



Plirynosoma cornutum, the behavior of which in the 

 gradient is illustrated by the records of Experiments 78 

 and 81 (PL I), appears to show a preference for a tem- 

 perature slightly higher than that shown by the other 

 species. 



2. Substratum Temperature Gradients.— Yoy the pur- 

 pose of establishing this gradient the cage was placed in 

 a water bath so arranged that hot water flowed into the 

 latter at one end and cold water at the other, the water 

 being directed backward and forward beneath the cage, 

 and running out near the center, in such a manner as to 

 produce a gradient in the temperature of the cage bottom. 

 The temperature of the substratum was taken at intervals 

 along the edge of the cage by thermometers whose bulbs 

 were just covered by the sand in the bottom. 



The statistical records of Experiments 43, 126 and 127 

 (Table II) show an optimum substratum temperature for 

 Phrynosoma modestum of about 40°, or about 5° higher 

 than the optimum air temperature for the same species. 

 In this species the response to changes of temperature of 

 the substratum is very definite, and by varying the tem- 

 peratures of the gradient, the animals can be driven re- 

 peatedly from one end of the cage to the other as the 

 temperature is raised or lowered. The lizards often bur- 

 rowed at or near the upper limit of the optimum tempera- . 

 ture, and, less often, at the temperatures below the 



