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THE AMERICAN NATURALIST [Vol. LIII 



the approximation of tlie color of the animal to that of the 

 substratum is, according to Redfield, subordinate to the 

 daily rhythm of color change occasioned by changes in 

 light and temperature, and to changes brought about by 

 the emotional condition of the animal. 



4. Food.— An adequate study of this factor would re- 

 quire much more extended observation than has been pos- 

 sible. Some suggestions as to the character of food re- 

 quired have been made above. 



5. Light.— An estimation of the effect of light of vary- 

 ing intensity and quality in the natural habitat would be 

 very difficult, but it is probable that the relations of light 

 in the life of such animals have been greatly underesti- 

 mated. Experiments with a gradient of the color of light 

 are included here. 



III. Experimental Besults 



1. Air Temperature Gradients.— T^o series of experi- 

 ments were performed in which air varying in tempera- 

 ture was passed across the experimental cage previously 

 (1917a) described. In the first series the air passing 

 across one third of the cage was heated to a temperature 

 of about 38° by being passed through coils immersed in 

 hot water, that passing across the next third was heated 

 to about 33°, while the remaining third was supplied with 

 air at about 29°. The air was unmodified except as to 

 temperature and the rate of flow was the same in each 

 case. Typical results of this series {Phrynosoma mo- 

 destum only) are shown statistically in Table I. 



In the second series the air for the hottest third was 

 heated to a temperature in the neighborhood of 50°, 

 which is about the maximum soil surface temperature on 

 unprotected sand exposed to the direct rays of the sun. 

 This temperature was obtained by passing the air through 

 heated iron pipes. A medium temperature was obtained 

 by passing the air through coils immersed in hot water, 

 as above, while the lowest temperature was that of the 



