No. 624] REACTIONS OF PHRYNOSOMA 



39 



gradients involving all environmental conditions, and in 

 such cases we must rely on careful observation and 

 analysis. Such a review as has just been given of the 

 habitat and habits of the horned lizards may indicate to 

 us the probable external conditions variations of which 

 are of importance in the daily and seasonal life of the 

 individual and of the species. The following are the most 

 apparent of such external conditions : • 



1. Temperature. * 



(a) Air. 

 (h) Soil. 



(c) Maxima and minima. 



2. Water. 



(a) Eelative humidity and evaporating power of 

 air. 



{b) Soil moisture. 



(c) Food in relation to its water content. 



3. Soil. 



(a) Texture as influenced by 



1. Composition. 



2. Moisture content. 



3. Vegetation. 

 {h) Color. 



4. Food. 



(a) Character. 



(h) Abundance or scarcity. 



5. Light. 



(a) Quality. 

 (h) Quantity, 

 (c) Rhythm. 



In the natural habitat it is rare that one of the above 

 conditions varies without an accompanying variation in 

 one or more of the others ; for example, a variation in 

 temperature of the air is accompanied by a variation in 

 the relative humidity and in the evaporating power of the 

 air, and may be followed by an alteration of soil tempera- 

 ture and soil moisture, as well as soil texture. Thus it 

 is difficult to consider these conditions separately. 



