COLOR CHANGES IN AMPHIBIANS 535 



lieved that the threshold of difference was not a constant percentage in- 

 crement, but rather that it varied as the logarithm of the magnitude of 

 the stimulus. 



In psychological phenomena, Fechner's (or Weber's) law breaks down 

 with both high and low values of the stimulus, but holds fairly well for 

 a long intermediate range of values. So, Sumner finds, does the log- 

 arithmic relationship of increased pigmentation and albedo. Whether or 

 not this phenomenon is an instance of the operation of Fechner's law, it 

 is difficult to say. But when the control of the adaptive alteration of the 

 protective colorations of vertebrates was originally delegated, logically 

 enough, to the eye, it was also fairly logical that the eye should proceed 

 to administer this particular physiological territory in accordance with 

 the laws governing its own operation as the receptor of the visual sense 

 — even though visual consciousness plays no part in the processes of 

 color-change control. 



Color Changes in Amphibians — Lister's pioneer work suggested that 

 the dermal changes of the Amphibia are nervous reflexes. This idea was 

 supported by Babak in 1910-1913, and in fact was quite generally accept- 

 ed up to about 1924. The physiological changes of a frog between its 

 pale and dark phases may take hours, or a day or more, to accomplish. 

 In some tree-frogs, a few seconds may suffice. Morphological changes 

 can be induced by experimental illuminations, and are particularly sus- 

 ceptible to dietary manipulation. These changes are of course a matter 

 of weeks or months, as in teleosts. Such slow actions hardly look like per- 

 formances of the nervous system. Yet if the eyes are removed, or the op- 

 tic nerve cut, the changes in response to illumination are largely inhibited. 



In 1898, Corona and Moroni found that injections of adrenal extracts 

 would blanch a frog. Lieben rediscovered this reaction eight years later; 

 and from 1922 on, Hogben and his colleagues argued for an almost 

 strictly hormonal intermediation between the eye and the 'pigmentary 

 effector system' of amphibians. As long as adrenalin was the only endo- 

 crine substance known to affect the phenomena, there was still room for 

 the nervous system as the centerpiece of the picture; for the association 

 of the adrenals with the sympathetic nervous system was well established. 



But Hogben found that extracts of the intermediate lobe of the pitu- 

 itary would darken frogs; and he came to believe, from further experi- 

 ments, that a blanching hormone was produced by the pars tuberalis of 

 the same gland. Much of this work was done on the primitive African 

 clawed frog, Xenopus Icevis. In this country the studies of Parker and 



