The Mental Life of the Monkeys 195 



when the door was left open only after great delay. The 

 time-curves for the experiments performed will be found 

 on page 186 among the others. The figures beside each pair 

 represent the number of days without practice. 



The records show a decided superiority to those of the 

 cats and dogs. Although the number of trials in the original 

 tests were in general fewer in the case of the monkeys, the 

 retention of the association is complete in 6 cases out of 8 

 and is practically so in one case where the interval was 

 8 months. 



EXPERIMENTS ON THE DISCRIMINATION OF 



SIGNALS 



My experiments on discrimination were of the following 

 general type: I got the animal into the habit of reacting 

 to a certain signal (a sound, movement, posture, visual 

 presentation or what not) by some well-defined act. In 

 the cases to be described this act was to come down from 

 his customary positions about the top of the cage, to a place 

 at the bottom. I then would give him a bit of food. When 

 this habit was wholly or partly formed, I would begin to 

 mix with that signal another signal enough like it so that 

 the animal would respond in the same manner. In the 

 cases where I gave this signal I would not feed him. I could 

 then determine whether the animal did discriminate or not, 

 and his progress toward perfect discrimination in case he did. 

 If an animal responds indiscriminately to both signals (that 

 is, does not learn to disregard the 'no food' signal) it is 

 well to test him by using two somewhat similar signals, 

 after one of which you feed him at one place and after the 

 other of which you feed him at a different place. 



If the animal profits by his training by acquiring ideas of 



