136 Studies in Animal Behavior 



of a chain reflex; when one part of the structure 

 concerned is excited it tends to increase the tonus 

 of the associated parts, and thus reinforce the origi- 

 nal response. I have found that in the crayfish stim- 

 ulation of the antennules, which are important or- 

 gans of smell, sets up chewing movements of the 

 mouth parts and grasping movements of the small 

 chels. Similarly stimulating the small chelae evokes 

 chewing movements of the mouth parts and twitch- 

 ing of the antennules, while stimulating the mouth 

 parts directly may cause movements in both the 

 other sets of organs. We have here as a matter 

 of fact a number of reflexes which mutually rein- 

 force one another. Suppose that in the chick the 

 sight-pecking response and the taste-swallowing re- 

 sponse are related as the feeding reflexes demon- 

 strably are in the crayfish ; the second response would 

 thus tend to reinforce the first, and if this tendency 

 persisted we would have a case of learning by ex- 

 perience. 



Animals in the course of their instinctive responses 

 encounter stimuli which bring about other responses. 

 These become associated. According to the nature 

 of the nervous pathways involved, there may be re- 

 inforcement of, or interference with the original re- 

 action. Experience brings about an extension of the 

 range of adaptations by the assimilation of con- 

 gruent reactions and the elimination of acts whose 

 secondary consequences are in the nature of an- 

 tagonistic and thereby inhibitory responses. Such 



