464 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Biedermann ('92, p. 487), who experimented on the chromatophores 

 of frogs and tree-toads, fully confirmed the earlier observations of Hering 

 ('69, p. 53) to the effect that in cold water the pigment of the melano- 

 phores passed out into the cell processes, produciug a dark skin, and in 

 warm water it passed back into the body of the cell, leaving the skin 

 light. Similar results have been more recently obtained on larval sala- 

 manders by Fischel ('96), and these have been confirmed by Flemming 

 ('97% 97"). 



Thus all recorded cases support the generalization that a high tem- 

 perature induces a migration of the pigment into the body of the cell, 

 and a low one the reverse. Wiedersheim (see Hoffmann, '90, p. 1353), 

 therefore, was not without reason in suspecting that the dark skin of 

 Phrynosoma on cool, cloudy days, and its pale skin on sunny days might 

 be due to temperature ditiferences, and Keller's ('95, p. 132) criticism of 

 this explanation is at least premature. It must, however, be admitted 

 that, as Thilenius ('97, p. 536) has pointed out, light is usually more 

 effective than heat as a chromatophore stimulus, though to deny heat 

 any part in the determination of color changes is, as our results show, 

 certainly erroneous. 



. The relations of heat to light in these respects are not without interest. 

 Thus while heat seems in all cases to follow the simple rule that high 

 temperature induces a central migration and low temperature a peripheral 

 migration of the melanophore pigment, light sometimes follows this rule 

 and at other times does not. For in Veranus, Uromastix, and Agama 

 (Thilenius, '97), a strong light acts like a high temperature and inducing 

 central migration, and a dim light or no light acts like a low tem- 

 perature and inducing peripheral migration. On the other hand, in the 

 African chameleon (Briicke, '52; Keller, '95, etc.), and in Auolis 

 (Carlton, :03) a strong light acts like a low temperature, inducing a 

 peripheral migration, and a dim light or no light acts like a high tem- 

 perature, inducing a central migration. In other words, in the first 

 group of animals mentioned light and heat act together ; in the second, 

 they are antagonistic. Whether there are animals which react to heat 

 in the reverse way, — high temperature causing in them a peripheral 

 migration of pigment, — as some do to light, remains still to be made 

 out, but from our present knowledge it seems probable that this is not 

 true, and furthermore that animals are much more differentiated toward 

 light than toward heat, even though from a physical standpoint these 

 two stimuli have much in common. 



