44 



THE BEHAVIOR OF LOWER ORGANISMS. 



directed away from the source of light, and swims in the direction so 

 determined. The reaction to light is of essentially the same character 

 as the reaction to other usual stimuli, and takes place by what we may 

 call the method of trial and error. When the animal comes to the 

 boundary of a lighted area, or when the anterior end is illuminated, 

 this constitutes error ; the animal tries some other direction, and repeats 

 the trial till the condition constituting error disappears. 



Are these results in agreement with all the observed facts.? The 

 only point on which perhaps question might arise is in regard to the 

 production of a clearly marked orientation such as we find shown by 

 Stentor when the light falls upon it from one side. In this case, as 



Fig. 17.* 



we have seen, Stentor swims directly away from the source of light, 

 and shows thus a typical orientation. As we have had the dictum 

 that a motor reaction, such as I have described, " cannot account for 

 an orientation" (Garrey, 1900, p. 313), it will be well to examine this 

 matter a little farther. In a previous paper (Jennings, 1900, a) I have 

 shown how orientation could be produced through a motor reaction ; 

 the case of Stentor exactly realizes the possibility there set forth. If 



*FiG. 17. — Diagram to illustrate the difference between the method of orienta- 

 tion to light required by the tropism schema and that which actually takes 

 place. To light coming from the direction shown by the straight arrows the 

 tropism schema requires that an organism in the position x-y should attain the 

 position y-z by turning in the direction indicated by the (broken) arrow rt-3. 

 The position is actually attained by turning in the direction indicated by the 

 long arrow c-d. 



