50 



THE AMERICAN NATURALIST 



[Vol. LIII 



4. Gradient in the Color of Light {Wave-Length) .- 

 Although it would be difficult to estimate the efifect of the 

 various light components in the natural habitat, a series 

 of light experiments has been included. For use as a 

 color gradient the cage used in the other experiments was 

 covered with an accessory lid composed of a series of six 

 equal strips of gelatine ray filter in the principal colors 

 (violet, blue, green, yellow, orange, red). Three forty- 

 watt electric lamps were placed above the cage within the 

 observation hood, so that the light was approximately 

 equally distributed throughout the cage, each sixth being 

 illuminated principally by rays of a narrow range of 

 wave-length. 



Experiment 112 illustrates the movements of Phryno- 

 soma modestum in such a gradient. The longest rays 

 were avoided, as well as the shortest, although the animal 

 remained for greater lengths of time in the red section 

 than in the violet. The optimum seems to lie in the green 

 and the yellow. 



Phrynosoma cornuttim (Experiments 116 and 118, PI. 

 II) avoided both red and violet, with an optimum near 

 the middle of the spectrum. Phrynosoma douglassii did 

 not respond regularly and seemed little affected. 



The color reactions are probably not as significant as 

 those involving some of the other factors here considered. 

 Direct sunlight in the arid regions contains a rather 

 larger amount of the light of the shorter wave-lengths 

 than elsewhere, and it is possible that the avoidance of 

 violet light as shown in these experiments is of signifi- 

 cance in explaining the avoidance of sunlight under cer- 

 tain conditions, but it is more probable that temperature 

 is the dominating factor in this reaction. 



IV. Summary and Conclltsions 

 1. Of the temperature conditions capable of being 

 tested in the gradient, the temperature of the substratum 

 calls forth the most definite response. In addition to the 



