Spatially Determined Reactions 149 



that is not, indeed, ideally satisfactory, but may serve our 

 purpose, divides them into five groups : 



1. Reactions adapted to the position of a single stimulus 

 acting at a definite point on the body. 



2. Reactions to a continuous stimulus, which involve the 

 assumption of a certain position of the whole body with refer- 

 ence to the stimulus : orienting reactions. 



3. Reactions to a stimulus that moves, i.e. that affects 

 several neighboring points on the body successively. 



4. Reactions adapted to the relative position of several 

 stimuli acting simultaneously. 



5. Reactions adapted to the distance of an object from the 

 body. 



These forms of behavior will be successively discussed. 



54. Class I: Reactions to a Single Localized Stimulus 

 Responses to stimulation that are adapted to the point 

 of application of the stirmqliis are to be found among very 

 simpleanimals. They may be subdivided into three groups : 

 first, cases where the part of the animal that reacts is the part 

 directly affected by the stimulus; second, cases where the 

 whole animal reacts by a movement in the appropriate direc- 

 tion; and third, cases where a part of the body not directly 

 affected by the stimulus moves toward the point stimulated. 



i. Amoeba furnishes an example of the first class. Its 

 negative reaction occurs by the checking of protoplasmic 

 flow at the point where a strong mechanical stimulus affects 

 the body; its positive reaction by a flowing forward of the 

 protoplasm at the point where a weak stimulus acts, and its 

 food-taking reaction by an enveloping flow on both sides of 

 the point stimulated. This would seem to be the most 

 primitive way of adapting response to the location of a stimu- 

 lus: the effect is produced just where the force acts, as it 



