542 ADAPTATIONS TO PHOTIC QUALITY 



These fascinating results help to explain why this lizard, which can 

 turn green and disappear when it is on a background of foliage in its 

 native haunts, does not always do so. Green light can stimulate the retina 

 to evoke, through the nervous system, a green body coloration; but any 

 light — even a green one — striking the body strongly, only leads to melan- 

 ophore expansion and the onset of the non-adaptive brown phase. If 

 there are any photoreceptors in the skin, they are unfortunately not 

 specifically responsive to green light. 



Wilson points out that when Anolis is among leaves and the eyes 

 receive light filtered by other green leaves overhead, the green phase 

 ensues. The animal is then adapted to its background — though not, as a 

 teleost would be, through the character of the light reflected from that 

 background. But let the lizard come out from cover, still standing on a 

 green leaf, and the reception of light on its skin quickly turns it brown 

 and causes it to stand forth like the proverbial sore thumb. Despite the 

 interesting demonstration of a specific response of the oculodermal mech- 

 anism to green light, it seems over-charitable to credit this one reptilian 

 species with an adaptability to background in the teleostean sense of the 

 expression. 



Color-changes have been reported in snakes from time to time; but, 

 apart from some authoritative-looking old claims by Leydig for Matrix 

 natrix, it is probable that all gross changes (particularly those in green 

 tree-snakes mentioned by Fuchs) are caused not by chromatophoral alter- 

 ations but by a spreading of the skin, revealing areas between the scales. 

 Dryophis, in the anterior part of the body, exhibits a startling change of 

 this character when excited. 



Very recently Rahn has demonstrated that the dermal and epidermal 

 melanophores of rattlesnakes, and the epidermal melanophores in three 

 colubrid genera, will contract permanently if the pars intermedia (or the 

 whole pituitary) is removed. Injections of 'intermedin' expand the 

 chromatophores once more. Whether light and temperature alter the 

 skin color in any of these snakes (by way of the eye or otherwise) is not 

 known; but it is unlikely, inasmuch as the superficial layers of the epi- 

 dermis — those due to be shed at the next ecdisis — contain a pattern of 

 motionless pigment which conceals the activity of the melanophores be- 

 neath. The paling of the body, produced by removal of the pituitary, 

 manifests itself only after the next subsequent moult. 



The only other reptile in which melanophore activity has been dem- 

 onstrated is the alligator. Kleinholz has found that the pigment in scat- 



