The Reactions of Planarians to Light 153 



Summary. Orientation may occur without phototaxis. 



Two theories have been advanced to explain orientation and 

 phototaxis in lower organisms, namely, the trial and error theory 

 and that of the tropisms. The former may be based upon "motor 

 reflexes" or upon "random movements" according to the sym- 

 metry of the animal. 



The tropism theory rests upon asymmetrical response to asym- 

 metrical stimulation. It does not necessarily depend upon the 

 direct stimulation of the motor organs, nor is it essentially stereo- 

 typed in its character any more than are trial and error responses 

 by motor reflex or random movements. 



The tropic form of response may, and probably does, require a 

 more complex mechanism than that which causes the motor reflex, 

 consequently it is the form of response to be logically expected 

 among planarians, since the motor reflex has been proven to be 

 the form utilized by the protozoa. 



Tropisms, as well as trial and error movements, provide, through 

 the modifying control of an evolving central nervous system, suffi- 

 cient latitude of variation for natural selection to work upon in the 

 evolution of higher forms of behavior. 



Asymmetrical response may, in certain cases, result from sym- 

 metrical stimulation, but ordinarily its cause is asymmetrical 

 stimulation. 



Wigwag movements are occasioned most frequently by changes 

 in intensity, and they may result in orientation and phototaxis 

 by assisting an organism to secure asymmetrical stimulation. 



Latency of reaction accounts for some of the failures in orien- 

 tation which often occur even when asymmetrical stimulation is 

 acting upon an organism. 



Finally, the orientation and phototaxis of planarians is more 

 consistently explained by the theory of tropisms than by the theory 

 of trial and error. 



3 ADAPTATION. 



It remains, finally, to inquire how far the reactions of planarians 

 to light are adaptive; that is, how far the response to light is "of 



