126 Studies in Animal Behavior 



Repetition of a stimulus may call forth not only 

 quantitative differences of response, but it may evoke 

 responses of very different character. Animals are 

 frequently provided with several modes of reacting 

 to a given stimulus which may be called into play 

 one after the other. Jennings has shown that if 

 a Stentor is subjected to a light mechanical stimulus 

 by causing fine particles of India ink to fall upon 

 its disk from a capillary pipette it usually reacts 

 first by bending a little to one side. If the parti- 

 cles continue to fall on the disk the beat of the 

 cilia covering the body may suddenly be reversed, 

 thus creating a current tending to carry the offend- 

 ing particles away. If in spite of this the particles 

 still impinge upon the disk the Stentor may con- 

 tract one or more times. Finally, if all these re- 

 actions are tried in vain the infusorian may give a 

 number of violent contractions, break loose from 

 its place of attachment, and swim away. 



It would be an error to interpret the varied be- 

 havior of this unicellular organism as a manifesta- 

 tion of intelligence, although it is not unlike what 

 the behavior of an intelligent creature might be 

 under the circumstances. No power of learning by 

 experience has ever been discovered in Stentor, or 

 indeed in any other protozoan. The organism is 

 provided with a number of different modes of re- 

 sponse, and which one Is set in action depends upon 

 internal factors which are influenced by the crea- 

 ture's previous activity. The organism which has 



