282 ALFRED C. REDFIELD 



Light and heat thus act in opposite ways and their effects need 

 not be confused. The expansion of the melanophore pigment in 

 the light cannot be due to heat, as this stimulus causes a con- 

 traction of the pigment. 



In certain individuals light dominates over temperature in 

 determining the state of the melanophores through a larger range 

 of temperatures than that indicated above. Some horned toads 

 maintain a contracted condition of the pigment in the dark at 

 temperatures as low as 5°C. The pigment of others may expand 

 upon illumination when confined at temperatures as high as 

 37°C. 



These conclusions are in very good agreement with those which 

 Parker ('06) drew from experiments upon Phrynosoma blainvillei. 

 By their means may be explained the rhythmic changes in the 

 color of horned toads which are correlated with the cycle of day 

 and night. The expansion of the pigment in the morning is due 

 to the stimulation of light. The heat of midday causes a con- 

 traction in spite of the light, but as the air cools in the afternoon 

 the light effect again dominates and the pigment expands. WTien 

 the light fails, at night, the pigment becomes contracted as a 

 result. 



2. The color of the substratum 



It has been pointed out on a preceding page that the color of 

 the environment has a marked effect upon the condition of the 

 melanophores. A dark substratum produces an expansion of the 

 pigment in horned toads which live upon it; a light colored sub- 

 stratum has the reverse effect. The color of the substratum must 

 consequently be considered the stimulus which initiates the 

 adaptive changes in the color of this animal. 



This stimulus must not be confused with the effect of illumina- 

 tion upon the melanophore. The reaction which it produces is 

 quite a different phenomenon, as the following considerations in- 

 dicate. The results of the two forms of stimuli are diametrically 

 opposed; light causes an expansion, a light background a contrac- 

 tion of the pigment. The photo-receptors concerned with the 

 two sources of stimulation are quite distinct, as will be shown in 

 another place. 



