48 ROBERT M. YERKES 



was deliberate, quiet, and extremely business-like. The time 

 for the series was thirty-one minutes. 



The period of punishment was increased on May 12 to thirty 

 seconds. Previously, for the greater number of the trials, it 

 had been ten to fifteen seconds. This increase apparently did 

 not disturb the monkey, for he continued to work perfectly 

 throughout the series, although making many mistakes in spite of 

 deliberate choices and the refusal of certain boxes in each trial. 



An interesting and significant incident occurred on May 13 

 when at the conclusion of trial 5, Doctor Hamilton came into 

 the experiment room for a few minutes. Sobke immediately 

 stopped working, and he could not be induced to make any 

 choices until Doctor Hamilton had left the room. This well 

 indicates his sensitiveness to his surroundings, and his inclina- 

 tion to timidity or nervousness even in the presence of conditions 

 not in themselves startling. 



Work was continued thus steadily until May 28 when, because 

 of the failure of the animal to improve, it seemed wise to increase 

 the period of confinement as punishment to sixty seconds. In 

 the meantirae, it had sometimes been evident that Sobke was 

 near to the solution of his problem. He would often make cor- 

 rect choices in three or four trials in succession and then appar- 

 ently lose his cue and fail utterly for a number of trials. 



After June 1, in order to hasten the solution of the problem, 

 two series per day were given. In some instances the second 

 series was given almost immediately after the first, while in 

 others an interval of an hour or more intervened. It was fur- 

 ther found desirable to give Sobke all of his food in the appa- 

 ratus. When the rewards obtained in the several trials did not 

 satisfy his hunger, additional food was presented, on the com- 

 pletion of the series of experiments, in one or more of the food 

 cups. On days marked by unwillingness or refusal to work, 

 very little food was given. Thus, the eagerness of the monkey 

 to locate the right box was increased and, as a matter of obser- 

 vation, his deliberateness and care in choice increased corre- 

 spondingly. Sixty seconds punishment was found satisfactory, 

 and it was therefore continued throughout the work on this 

 problem. 



It was evident, on June 9, from the behavior of the monkey 

 as well as from the score, that the perfect solution of the prob- 



