No. 624] 



REACTIONS OF PHRYNOSOMA 



51 



indication of an optimum by the movements of the animal, 

 definite motor responses of a specialized character (bur- 

 rowing) are made to certain temperature conditions just 

 above or just below this optimum. The temperature of 

 the air calls forth similar reactions but not as readily or 

 as definitely as that of the soil, the reaction to the former 

 being overshadowed by the response to the latter when a 

 difference exists. The temperature of the substratum is 

 evidently of very great importance in the daily move- 

 ments of the horned lizards, and probably plays an im- 

 portant role in the control of distribution. The tempera- 

 ture of the soil is probably also of great importance in 

 connection with the deposition and hatching of the eggs 

 in those species which are oviparous. The differences 

 between the optimum temperatures of the various species 

 considered are in the direction and of the magnitude of 

 the temperature differences normal to their respective 

 habitats. While the limits of temperature variation 

 favorable for the completion of the life cycle of the animal 

 could not be subjected to experiment of the type here 

 used, it is evident that at least the minimum is of great 

 importance in connection with the phenomena of hiber- 

 nation, and the maximum is probably of similar impor- 

 tance in relation to the estivation which takes place more 

 or less regularly. 



2. In the gradient of the evaporating power of air 

 definite responses were obtained only in the case of one 

 species {Phrynosoma mod estum), and here only when the 

 gradient was steep. The daily and seasonal variation 

 in this factor is very large in the natural habitat. The 

 reactions of the animals to temperature changes act in 

 such a way as to prevent the exposure of the organism to 

 excessive desiccation. The effect of soil moisture is felt 

 indirectly, through the alteration of the temperature and 

 the texture of the soil, the latter of which is important 

 in relation to the burrowing habit. It is probable that 

 there is a certain minimum water content of food, below 

 which tlie animal can not survive. This must be very 



