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 



