150 Herbert Eugene Walter 



fections in the worm's nervous circuit, assuming that planarian 

 reactions are due to indirect rather than to direct stimulation of 

 the motor organs. In fact, repeated evidence of the failure of a 

 constant and perfectly invariable orientation on the part of pla- 

 narians has been given in the preceding pages. Such failure, 

 moreover, is quite as likely to occur in the application of the trop- 

 ism theory to behavior as it is in the case of the trial and error 

 theory, since stereotyped reactions and forced movements, as 

 Holmes ('05a, p. 112) has emphasized, are no more characteristic 

 of tropisms, w^hich depend upon a differentiated stimulation and 

 response, than they are of trial and error movements, resulting 

 from a single motor reflex given in response to all kinds of stimu- 

 lation. 



Furthermore, it has been urged that tropism indicates a simpler 

 form of reaction than trial and error for the reason that it in- 

 volves only a local part of an organism while the motor reflex of 

 trial and error requires that the organism act as a whole. Conse- 

 quently, since motor reflex has been indisputably demonstrated 

 as the method of infusorian phototaxis, Jennings ('04a, p. 95) asks, 

 "Should we conclude that the reactions in the higher metazoa are 

 simpler and less unified than in the protozoa V 



That the motor reflex, which occurs with machine-like uni- 

 formity, regardless of the point where the stimulation is received, 

 is more complex in character than the stimulation of an asym- 

 metrical part of an organism which may depend for its response 

 upon sense-organ, nervous transmission and motor apparatus 

 is an assumption difficult to sustain. It seems more reasonable 

 to agree with Harper ('05) in placing tropism higher in the evolu- 

 tionary scale than trial and error. 



The fallacy that "tropism leads nowhere; it is a fixed final thing 

 like a crystal" (Jennings, '04c, p. 251), while trial and error alone 

 ofi^ers possibilities of the higher evolution of phototaxis, has already 

 been answered by Holmes, who points out that trial and error, 

 at least that phase of trial and error depending upon motor reflex, 

 is even more fixed and stereotyped than the reactions occurring 

 in accordance with the tropism schema. To quote: "The end 

 result of both methods is the same, /. e., to get the organism away 



