CHAPTER XIV 



TROPISMS AND THE LOCAL ACTION THEORY OF TROPISMS 



A large share of the behavior of lower as well as of higher animals j 

 consists of movements either toward or away from certain objects or 

 sources of stimulation. Behavior can thus be largely classified into 

 two great classes: "positive and negative" reactions; movements of 

 "attraction and repulsion," of approach and retreat. To account 

 in a general way for these directed movements certain theories have 

 been proposed, and one of these has become widely accepted. This is 

 the so-called "tropism theory." The word "tropism" has been used in 

 several different senses by different authors, and not always as imply- 

 ing a definite theory (see page 274). But there is a certain theory 

 which is usually implied when tropisms are mentioned; it has become 

 so generally accepted that it is often spoken of as the tropism theory. 

 It will perhaps be more accurate to speak of it as the local action^ theory 

 of tropisms. "Tropisms" has become the key- word for the behavior 

 of lower organisms, and the theory mentioned is supposed to furnish 

 explanation of most of the puzzles found in this field. A theory so 

 generally accepted demands separate special treatment. What is this 

 tropism theory as usually understood in discussions of animal behavior, 

 and how far does it go in helping us to understand the behavior of lower 

 organisms ? 



According to this tropism theory the primary feature in the directed 

 movements of lower organisms is the position or orientation of the body 

 with respect to the source of stimulation, and this orientation is brought 

 about by the direct local action of the stimulating agent on that part of 

 the body on which it impinges. The essential points in this theory are 

 then two: first, orientation; second, the production of orientation by 

 local action. These points we may consider separately. 



(1) By this tropism theory a stimulus is considered to force the 

 animal to take a certain position with respect to the direction from which 

 the stimulus comes; in this position it is said to be oriented. Usually 

 the organism becomes oriented with anterior end either toward or away 

 from the source of stimulation. This is the essential feature in the action 

 of the stimulus. "The essential point in all directive stimulation is 



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