RETINAL PIGMENT OF PLANORBIS 383 



of optical isolation, have likewise associated a retreat of the pig- 

 ment in dim light with the presence of a more diffuse and weaker 

 photic stimulation, but, withal, a stimulation which, under the 

 circumstances, thereby rises to its highest efficiency. An assump- 

 tion, however, is involved in this reasoning, for it must be shown 

 that the pigment really does retreat in dim light, as, in truth, 

 it does in darkness. This essential point, however, has too 

 often been ignored. 



Furthermore, if the distrilnition of pigment in twilight were 

 essentially smiilar to that in bright light, it follows that ex- 

 planations which attempt to solve the meaning of the position 

 assumed by the pigment in light, only answer half of the ques- 

 tions involved, for it may well be asked — Why should there 

 be an extensive movement in total darkness, or why, since a . 

 permanently expanded condition would seem to be all that is 

 required, any mo\'ement at all? These queries merely show that 

 easily devised ex])lanations, as for examj^le those based upon 

 the theory of optical isolation, may not touch the ]irimaiy reason 

 at all — such oln-ious relations may have a secondary ini])ortance 

 or may even be ])urely fortuitous. 



Returning to the case under ct)nsiderati()ii, the movements 

 of the retinal pigment of Planorbis, through the influence of light, 

 are very limited in comparison with those in many animals. 

 The dispersion in the conqiacted ]iigment mass, and the forma- 

 tion of spai'se gramUar ]irocesses, do not visibly alter the den- 

 sity of the main pigmented zone. It is difficult to see (assuming 

 that a proxunal migration does occur in dim light as well as in 

 darkness) how these changes would be of any great value to 

 the animal in the ways in which the pigment commonly has been 

 supposed to act. Even the vague assumptions of a nutritional 

 relation of the pigment to adjacent sensory cells is discre]iant 

 with certain ]ihases of the ]ihoto- and thermo-mechanical changes. 



In the body chromatojihores of certain animals, e.g., lizards, 

 the response of the pigment to light and temperature may be 

 of use in regulating the body temperature of the animal. Thus 

 (quoting from Parker, '06, p. 411): "The dark color of the 

 lizard's skin in modei'ate illumination at a moderate temperature 



