;^S4 LESLIE B. AREY 



insur(>s, ])()ssil)ly, iunoii*;- otluM- tilings, a (-vriii'm dc^roo of waniith 

 wliich would be superfiuous, if not dangerous, at a higher teiu- 

 ])erature, and in consequence the skin })ecomes light-colored 

 in hot sunlight." 



The significance of temperature responses of the retinal pig- 

 nuMit of Planorbis is even more obscure than the meaning of 

 photic influences. It is easy to construct several groundless 

 hypotheses to explain why low temperature, acting similar to 

 light, and high temperature, acting similar to darkness, could 

 be of advantage to the snail in the regulation of its retinal econ- 

 omy. Such interpretations, how^ever, possess even less value 

 than the speculations concerning the role played by light. 



To devise an explanation from the adaptional standpoint 

 that will account for the temperature responses occurring in 

 total darkness would seem a hard task. It may be, however, 

 that the cell thus responds to temperature stimulation because 

 the change is of use in the light-adapted eye only, whereas in 

 darkness the similar response to similar stimulation is gone 

 through with perfunctorily, as it were, even though it is of no 

 use to the organism. 



The cautiousness of the following position, with the formula- 

 tion of which this discussion w411 be closed, may be its only com- 

 mendation; yet when our absolute ignorance of fundamental 

 facts is considered, caution may well qualify as a cardinal virtue. 

 An analysis of the conditions of pigment migration in the various 

 vertebrate classes has led the writer (Arey, '15; '16*^) to an iden- 

 tical conclusion for these animals as well. 



Recognizing, therefore, that the movements of the retinal 

 pigment of Planorbis, as well as of other animals, to light and 

 to temperature may have an adaptive significance, and, further- 

 more, realizing that (with the possible exception of arthropods) 

 we at present are quite unaware of the meaning of these move- 

 ments, it would seem that we are permitted to indulge only 

 in interpretations formulated in terms of protoplasmic re- 

 sponses to definite stimulating agents and that the questions of 

 utility thereby involved must await the establishment of a 

 more thorough knowledge of the co-existing factors. 



