52 



THE AMERICAN NATURALIST [Vol. LIII 



low, however, considering the character of the normal 

 food. The excretion of water is reduced to a minimum 

 by the character of the nitrogenous excreta, which are 

 almost exclusively in the form of insoluble uric acid. 



3. An important factor in the distribution of these 

 animals is the texture of the soil, which must be suitable 

 for burrowing, as this is the reaction of the animal to 

 unfavorable conditions generally, and specifically to tem- 

 peratures inducing hibernation and estivation. The soil 

 texture is affected adversely by increases in moisture con- 

 tent, and by increases in the amount of vegetation present. 

 The color of the soil is probably important from the stand- 

 point of invisibility and it is probable that there is some 

 degree of approximation of the color of the animal to that 

 of the substratum. It is difficult to see how this fact 

 could be of much use to the animals, especially in the case 

 of such profusely armored species as Phrynosoma 

 cornutum. 



4. The role of light in the daily and seasonal life of the 

 homed lizards has not been shown, although they are posi- 

 tively phototactic and avoid extremes in a color gradient. 

 The optimum in this gradient lies in the green and in the 

 yellow. This may be correlated with the predominant 

 colors of soil and vegetation in the natural habitat. 



V. Acknowledgments and Bibliography 

 It is with great pleasure that I express my gratitude to 

 Professor A^ictor E. Shelford, of the University of Illi- 

 nois, for his constant inspiration and ready assistance 

 rendered in the performance of this work. 



LITERATURE CITED 



1912. Metabolie Water: Its Production and Role in Vital Phenomena. 

 Wisconsin Besearch Bulletin, 22: 87-181. 

 Bachmetjew, P. 



1901. Experimentelle entomologische Studien. Leipzig. 

 Bailey, Vernon. 



1905. Biological Survey of Texas. No. Am. Fauna, 25. 



