536 ADAPTATIONS TO PHOTIC QUALITY 



Others on the local Rana pipiens indicate that in this frog only one pitu- 

 itary hormone is involved, not two. Blood serum from dark pipiens, in- 

 jected into pale ones, will darken the latter; but interestingly enough 

 pale-frog serum fails to blanch dark frogs. Adrenalin injections, or the 

 removal of the pituitary, will produce a more complete paling than will 

 the bright illumination of a normal frog in white surroundings. Frogs 

 whose pituitaries have been removed will, in time, lose much of their pig- 

 ment — a 'morphological' change. Experiments similar to those of Szep- 

 senwol (p. 530), designed to test whether the eye itself secretes skin-con- 

 trolling hormones, have yielded conflicting evidence in amphibians. 



In amphibians the color-changes are less widespread and conspicuous, 

 as well as less rapid, than in teleosts. Few have any greater repertoire 

 than the brown-green-cream series of phases in the common tree-frog 

 Hyld yersicolor. Not only is the control largely (sometimes, wholly) 

 hormonal instead of predominantly nervous (except in a few tree-frogs) , 

 but the authority of the eye has begun to dwindle, approaching the situ- 

 ation in lizards. In some forms, as for instance the common newt 

 {Tritums riridescens) , the pattern of the adult shows no measurable 

 changes toward photic stimuli, though darkening will occur at low tem- 

 peratures and paling can be induced by pituitrin. Response to back- 

 ground does occur in many species, but only when temperature and hum- 

 idity conditions permit — a conjunction which is so far from being the 

 rule that it is almost an accident. When a frog is caught in the daytime 

 amid green grass, and happens to be green in color, it is in a sense a coin- 

 cidence. At a lower temperature, the frog would have been brown. So 

 also, if the grass had happened to be wetter. The frog can blend with its 

 environment only when three factors are just right: light, temperature, 

 and moisture. The eye can aid the response to only one of these factors, 

 /. €., light. In teleosts, the eye is able to control the skin largely because 

 temperature and humidity do not control it. But at least amphibians are 

 able to attain the pale phase (appropriate to bright light and backgrounds 

 of high albedo) at moderate temperatures — in contrast to the situation 

 in most lizards, which can take on their pale phases only at relatively 

 high temperatures. 



Even tactile stimuli may have an influence. The European tree-frog, 

 ceteris paribus, will turn brown on a rough surface and green on a 

 smooth one. In a roundabout way, such changes are perhaps adaptive to 

 background, for the brown bark of a tree is rough, and the green leaves 

 are smooth. 



