182 LESLIE B. AREY 



than the presence of constant protoplasmic responses to a definite 

 stimulating agent. 



When the effect of temperature is considered, a lack of uni- 

 formity at once becomes apparent. 



Among the fishes the efifect of temperature upon the retinal 

 pigment is in agreement with Parker's ('06) general conclusion 

 that in all melanophores, low temperature has the same effect as 

 light and high temperature the same effect as darkness. ^^ In 

 the retina of the living frog where this statement holds true only 

 between the temperatures of 0° and 19°C. in the dark, notwith- 

 standing the identical photic responses of the retinal pigment 

 in fishes and amphibians, it is probable that a nervous control 

 is responsible for the expanded condition both at low and at 

 high temperatures; hence the responses in this animal are not 

 comparable to those in fishes. Since only a limited temperature 

 reaction occurs among fishes, and this in darkness as well as 

 in light, it probably has no immediate adaptive significance, 

 but merely represents the survival of a tendency shown in 

 the responses of melanophores in general. ^^ In homothermous 

 animals temperature, of course, can play no part in the normal 

 activity of the retinal pigment, and even if the temperature 

 responses in poikilothermous animals have an adaptive signifi- 

 cance, it is evident that this particular advantage must be un- 

 available to warm blooded vertebrates, in which keenness of 

 sight is best developed. 



Moreover, the action of temperature upon the cone cells in 

 the dark is variable. In the fishes, the cone myoids greatly 

 elongate when warmed and shorten when cooled, while in the 

 frog the cones are maximally shortened at high temperatures 

 only and at all other temperatures remain unchanged. 



The inconsistent action of temperature in producing move- 

 ments of the visual cells apparently has no common adaptive 



IS It should be pointed out, however, that in the case of the melanophores of 

 the frog's skin, the reverse of this statement is true, since high temperature pro- 

 duces similar effects to light, and low temperature to darkness. 



1^ Below 38°C. temperature has practically no effect on the rate of decom- 

 position of the visual purple, as Kiihne ('79) showed, hence the changes of the 

 pigment through the action of temperature are not related to this phenomenon. 



