MOVEMENTS IN THE VISUAL CELLS 181 



have the significance that he beheved it possessed, when he 

 attempted to furnish phj^siological proof that in the frog: "Jeden- 

 falls aber laufen .... auch retinomotorische Fasern von 

 den grossen Nervencentren aus durch den Sehnerv zuvi Auge" 

 ('85, p. 506). 



4. DISCUSSION 



We have so accustomed ourselves to view the phenomena 

 exhibited by living organisms from the evolutionary standpoint 

 that an 'explanation' which will reveal the adaptiveness of an 

 organism to its environment is demanded whenever a system 

 of relations involving constant responses to definite stimulating 

 agents is discovered. 



To make dogmatic assertions regarding the presence or ab- 

 sence of adaptation in a set of responses is, obviously, a matter 

 of exceeding danger, j-et if the phenomena exhibited by a series 

 of representati\e animals to definite stimulating agents are 

 shown to be variable, it is at least evident that a single inclusive 

 explanation will not be forthcoming. 



The writer has attempted to show elsewhere (Arey '15) that 

 the discontinuous occurrence of photomechanical responses in 

 the visual cells and retinal pigment, both in the various verte- 

 brate classes and among different representatives of certain 

 individual classes, renders it extremely difficult from the adapta- 

 tional standpoint to devise a satisfactory ox])lanation for the 

 meaning of these movements. The majority of theories which 

 attempt to link the known responses of the retinal ele- 

 ments with the mechanism of light perception are, without 

 doubt, highly speculative, and since for the most part they lack 

 an experiment^al basis of any kind, they must remain of inter- 

 est only as ingenious and interesting possibilities. In the case 

 of the retinal pigment at least, it is possible to compare the 

 responses to light and darkness with those exhibited by melano- 

 phores in general (Parker '06, p. 413), and from our present 

 knowledge we are unable to see either in the reactions of the 

 retinal pigment or in those of the rods and cones anything more 



