534 ADAPTATIONS TO PHOTIC QUALITY 



develop chromatophores on the head if kept in the light for several 

 months. Kurz has found that larval flatfishes (Pleuronectes) , placed in 

 the dark, cease to form any more melanophores and never develop lipo- 

 phores at all. He also found that in these fishes (but not in the pike, 

 Esox) white and short-wave lights stimulated the development of all the 

 pigments while red, yellow, and green lights retarded them. 



Abramowitz has reported that in Fundulus majalis the number of 

 xanthophores increases within two to six weeks when the fish is kept over 

 yellow or black substrates, decreases when the animal is over blue or 

 white grounds. Sumner and Fox, however, found that in Girella the 

 amount of xanthophyll extractible was greater in black individuals from 

 black surroundings than from yellow fishes assayed after a sojourn on 

 yellow or gray. Actually, there had been no gain in xanthophyll in the 

 black fishes — only less of the loss which in Girella ordinarily occurs in 

 the laboratory anyway. However, some of Sumner's recent work has in- 

 dicated that fishes can deposit more xanthophyll in the skin than is 

 accounted for in the food supplied to them. This hint, that fishes can 

 convert carotene into xanthophyll, is borne out by the work of Lonnberg 

 on Swedish marine fishes. Lonnberg finds only xanthophylls, no caro- 

 tenes, in the skins of certain fishes which feed upon crustaceans lacking 

 xanthophyll (but possessing carotenes) in their own pigmentations. 



As with his demonstration of the response to albedo in physiological 

 adaptation to substrate tone, so also Sumner has found an analogy for a 

 visual phenomenon at work in the morphological changes of teleosts. In 

 Gillichthys, Gambusia, and Lebistes, counts of the number of melano- 

 phores per unit area, or determinations of the amount of melanin in the 

 skin, showed that the increase of pigmentation was inversely related to 

 the logarithm of the albedo of the substrate. It was not surprising that 

 the albedo should prove so important, and the intensity of the incident 

 light a minor consideration; but the mathematical character of the relat- 

 ionship was unexpected. 



Sumner has advanced the suggestion that this aspect of the morph- 

 ological changes is in line with Fechner's modification of 'Weber's law'. 

 The latter is a battered old psychological dictum to the effect that if, in 

 any sensory modality, two stimuli differ quantitatively just enough to be 

 perceived as different, their objective difference expressed in per cent is a 

 constant. For example, if a five-pound weight and a six-pound one can 

 just be told apart by heft, one is 20% heavier than the other, and any 

 two weights must then differ by 20% to be discriminable. Fechner be- 



