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 inclination 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 meantime, it had sometimes been evident that Sobke was near to the solution of his problem.  He would often make correct choices in three or four trials in succession and then apparently 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 further found desirable to give Sobke all of his food in the apparatus.  When the rewards obtained in the several trials did not satisfy his hunger, additional food was presented, on the completion 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 observation, his deliberateness and care in choice increased correspondingly.  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 problem was near at hand.  This fact the experimenter recorded in his daily notes, and sure enough, on the following day Sobke chose correctly throughout the series of ten trials.  The time for this series was only ten minutes.  The choices were made deliberately and readily.

An analysis of the data of table 5 reveals five methods or reactive tendencies which appeared more or less definitely in the following order:  (a) Choice of first box at the left, because of experience in problem 1.  This tendency was very quickly suppressed by the requirements in connection with problem 2.  Indeed one of the most significant differences which I have discovered between the behavior of the primates