126 COMPARATIVE PHYSIOLOGY 



toads which have been kept upon a background of dark cinders, 

 are transferred to one of white sand, they become noticeably 

 paler after one day and reach within five days a condition of 

 maximum pallor. But when the eyes are bhndfolded, exposure 

 to the same surroundings for several weeks does not result 

 in the disappearance of the dark condition ; and the results 

 of carefully controlled experiments showed that this is not 

 due to the mechanical influence of the bandage, but can only 

 be interpreted on the assumption that response to the nature 

 of the background arises through stimuli received in the first 

 place through the organs of vision. In considering the 

 possibilities of pigmentary control through the eyes, it is of 

 interest to contrast the relatively slow and accumulative nature 

 of the response to background with the more rapid primary 

 reaction to incident light. In the case of the horned toad, 

 the intimate nature of the background response was not 

 investigated by Redfield, who devoted his investigation of 

 the co-ordinating mechanism in reptilian colour-response 

 to the peculiarly characteristic phenomenon of excitement- 

 pallor, and by an ingenious and painstaking series of experi- 

 ments has arrived at the conclusion that here too hormonic 

 regulation plays an important part in the process. 



The method adopted to induce pallor by nocuous stimula- 

 tion was the application of a faradic current to excitable areas, 

 such as the cloaca or roof of the mouth, a procedure which, 

 as we have seen, results in general contraction of the melano- 

 phores throughout the skin of the whole body. That such 

 treatment results in uniform melanophore contraction in dark 

 animals even after the denervation of definite areas of the skin, 

 such as can be achieved by severing all the nervous connections 

 of a limb, suggests at once that the melanophores are accessible 

 to stimuli received through the circulatory system. Several 

 lines of experimental evidence converge to this conclusion ; 

 in particular, the possibility of evoking pallor by transfusion 

 of blood from an excited animal. It has been known for some 

 time that extracts of the suprarenal medulla induce melanophore 

 contraction in fishes and amphibia. In the horned toad 

 destruction of the cord between the eighth and thirteenth 



