COLOR CHANGES IN REPTILES 539 



in true chameleons) it is due to individual variation. Thus particular 

 chameleons of a single species may vary, in their pale and dark phases, 

 between green and dark brown, yellow and olive, buff and black, etc. 



The melanophores react to temperature as in the amphibians, expand- 

 ing at low temperatures and contracting in high ones. In general, while 

 contraction will occur in amphibians at average outdoor temperatures, in 

 lizards the temperature must be rather higher before paling ensues even 

 in bright light. If the lizard happens to be a desert form and, after paling 

 in the heat of the sun, blends fairly well with the sand, we may call this 

 adaptation to background if we stretch a point. Actually however, Max 

 Weber's idea seems pretty sound where lizards are concerned. By paling 

 at high temperatures (which ordinarily means in the sun) , and darken- 

 ing at lower ones, they can reflect light (and heat) when they are already 

 well warmed, and absorb a larger proportion of it when the absolute 

 amount available is less. No useful purpose, in connection with heat-con- 

 servation or anything else, seems to be served by the paling which takes 

 place in darkness, however. Here, probably the ancient proclivity of 

 melanophores to contract in the dark is only asserting itself, and we need 

 not seek any ulterior explanation. 



The lizard's responses to high and low temperatures are direct re- 

 actions to temperature. But we may, if we like, take the attitude that 

 they are biologically intended, so to say, as responses to the accom- 

 panying light and darkness : bright light, in the environment of a lizard, 

 means high temperature; dim light or darkness connotes the cooling of 

 twilight and nightfall. Here we have an analogy for the effect of sub- 

 strate texture upon the European tree-frog (r. s.). 



Light and darkness, as such, are effective only within a restricted 

 range of temperature. Within this range, the similarity of the behavior 

 of lacertilian and teleostean melanophores is striking, as Sand pointed 

 out a few years ago. They contract on white backgrounds and expand 

 on black; they contract in darkness in both normal and eyeless animals; 

 they expand in eyeless animals upon illumination of the body; and they 

 expand in any denervated area of the skin. Blindfolded animals, unlike 

 eyeless ones, remain dark upon lighted backgrounds, showing some in- 

 hibitory influence of the eye ; but this influence is readily masked by that 

 of temperature or excitement. 



The response to denervation is but one of a number of indications 

 that the eye exerts its control through the nervous system. The responses 

 to temperature, however, are of doubtful mediation. Like those to light, 



