No. 624] BEACTIONS OF PIIRYNOSOMA 



41 



points to a corresponding importance of the texture of 

 the soil. Evidently this must be such as to render the 

 success of the burrowing reaction comparatively easy, a 

 condition which is met only in soils of a low moisture 

 content, and little humus, containing a considerable 

 amount of loosely aggregated particles of sand or fine 

 gravel. In a heavy clay or loam it would be impossible 

 for the animal to burrow deep enough to get below the 

 zone of killing temperatures during hibernation. This 

 would also be impossible in a compact sod. Unfortu- 

 nately, the problem of the soil relation involves an ex- 

 tensive seasonal study which, so far, it has been impos- 

 sible to carry out. 



While the color and markings of the animals vary with 

 the individual and the species, and the color of the indi- 

 vidual changes from time to time, it may be said in gen- 

 eral that the color of the horned lizard is very similar to 

 that of the soil of its normal habitat. Experiments of the 

 author and others have shown that high temperature, 

 darkness or high evaporating power of the air causes a 

 centripetal movement of the melanophoric pigment, while 

 the opposite conditions cause a darkening. Thus, in gen- 

 eral, individuals observed after a rain are darker in color 

 than at other times. The soil is also darker when wet, 

 which might lead the observer to suppose that the change 

 had taken place as a direct adjustment to the color change 

 of the substratum, while the actual cause is the change in 

 the evaporating power of the air. Within the limits of 

 the conditions of the habitat, variations in the evapora- 

 ting power of the air are the most potent factors in the 

 production of color changes. No direct connection be- 

 tween the color of tlu^ animal and that of the substratum 

 has been verified ex}uTiiiu'iitally by the author. Eedfield 

 (1917), in a recently iMih!i>li('(l ].apor on tlio color changes 

 in Phrynosoma coruutinn, lias >tatiMl that th(M-e is a direct 

 approximation of the i-olor ot' the animal to tliat of the 

 substratum, and that the liulit i-ays iT'acliiii.i;- the retina 

 form the stimulus for such changes. The mechanism for 



