262 The Animal Mind 



ful ones dropped off, we have a case where the dropping 

 off is not simply of useless but of harmful movements (since 

 the unsuccessful ones all result in a repetition of the harm- 

 ful stimulus) ; the final state is not one of no movement, 

 but of victory for the successful negative response. A 

 very interesting illustration of this type of learning was 

 obtained from Paramecium by Stevenson Smith (688) 

 and by Day and Bentley (178). The method used by 

 these experimenters was fundamentally the same. A 

 glass tube was drawn out until it was so fine that not more 

 than one Paramecium could get through it. This tube 

 was filled with water up to a certain point, and a single 

 Paramecium, carefully isolated for identification through- 

 out the experiment, was allowed to swim up the tube until 

 the surface film was reached. The animal behaved to- 

 wards the film as to any mechanical stimulus, darting 

 backward, rolling over towards the side away from the 

 mouth and swimming forward again. Since the tube was 

 so narrow, this method, which ordinarily succeeds in avoid- 

 ing obstacles, brought the animal against the surface film 

 again. After repeatedly going through the same perform- 

 ance, the Paramecium varied its response and succeeded in 

 turning completely around in the tube by bending its 

 body double. On being put again into the same predica- 

 ment, it gradually diminished the number of trials of the 

 unsuccessful negative response, and arrived at the point 

 where it almost immediately doubled over on striking the 

 surface film. These observations established the existence 

 of a relatively high type of learning in the simplest group 

 of animals. 



In this case the movements that are dropped off are 

 themselves negative reactions. In other cases they may 

 be feeding reactions or other responses whose vital impor- 



