214 S. O. MAST 



mg that an earthworm becomes oriented after making five 

 preliminary trial movements, he says, p. 463: "In this total 

 act the significant part so far as ultimate orientation is concerned 

 is not the making of the five trial and error movements but the 

 final ' selection ' of the best one as a line to be followed in the 

 real locomotor act. Now it is difficult for the reviewer to see 

 how this ' selection ' can be accomplished without involving 

 just that feature of symmetrical stimulation which is the essential 

 part of the tropism idea. The worm presumably moves finally 

 in the direction of that trial which through the stimulation of 

 symmetrical points on its body has been found to be most 

 favorable for orientation. Without complicating this problem 

 by raising the question as to the means by which the worm 

 could retain impressions of the five trial and error movements 

 preparatory to following up one of them, it must be evident that 

 these movements in their effect on orientation are quite subor- 

 dinate to symmetrical stimulation, in fact, not essentially 

 involved in orientation at all." 



What necessity is there for assuming " symmetrical stimula- 

 tion " and " retention of impressions " of preceding movements 

 in the process of orientation by trial? Contrary to Parker's 

 implication I did not emphasise trial as a factor of exceptional 

 importance in the orientation of the earthworm and I referred 

 to it only very briefly. But I did discuss this question rather 

 fully in my description of the process of orientation in fly larvae, 

 pp. 189-196. Careful reading of this description will convince 

 almost any one that there is no necessity whatever of assuming 

 " symmetrical stimulation " or " retention of impressions " to 

 account for orientation by trial reactions. Not only is this true 

 in the case of fly larvae but there is good experimental evidence 

 for opposing especially the former assumption. This evidence is 

 found on pages 194 and 195 with the following conclusion: "The 

 orienting reactions [of fly larvae] could readily be explained by 

 assuming the area sensitive to light to be restricted to a small 

 mass of substance located in the middle of the very tip of the 

 anterior end." 



But my reviewer is not the only one who seems to have diffi- 

 culty in understanding how orientation by trial could occur 

 without the activity of psychic factors, to say nothing about 

 mystical factors of various sorts. Witness e. g. the statements 



