On the morning of May 13, work was interrupted in the ninth trial by what seemed at the moment a peculiarly unfortunate accident, but in the light of later developments, an incident most fruitful of valuable results.

Skirrl, in trial 9, directly entered box 1.  Since this was not the right box, he was punished by being confined in it for ten seconds.  While in the box he howled and when the entrance door was raised for him to retrace his steps, he came out with a rush, showing extreme excitement and either rage or fear, I could not be sure which.  At intervals he uttered loud cries, which I am now able to identify as cries of alarm.  Repeatedly he went to the open door of box 1 and peered in, or peered down through the hole in the floor which received the staple on the door.  He refused to enter any one of the open boxes and continued, at intervals of every half minute or so, his cries.  For thirty minutes I waited, hoping to be able to induce him to complete the series of trials, but in vain.  Although it was obvious that he was eager to escape from the apparatus, he would not enter any of the boxes even when the exit doors were raised.  Instead, he gnawed at the door (12 in fig. 17) to the alleyway D and attempted to force his way through, instead of taking the easy and clear route to the alleys, through one of the boxes.  His behavior was most surprising and puzzling.  Finally, I gave up the attempt to complete the series and returned him to his cage by way of the entrance door to the response-compartment E.

I then entered the apparatus to seek some explanation of the animal’s behavior, and my search was rewarded by the finding of two sharp pointed nails which protruded for an inch or more in the middle of the floor of box 1.  My assistant, who had been charged with the task of installing the locks for the several doors, had used nails instead of screws for attaching staples underneath the floor and had neglected to clinch the nails.  Skirrl, in the dim light of the box, doubtless stepped upon one of the nails and inflicted a painful, although not serious, injury upon himself.  It was impossible for him to see clearly the source of his injury.  He was greatly frightened and expressed the emotion most vigorously.  His behavior strongly suggested a superstitious dread of some unseen danger.  It may be that the instinctive fear of snakes, so strong in monkeys, was partly responsible for his response.

The first result of this accident was that more than two weeks were lost, for it was impossible, during the next few days, to induce the animal to enter any of the multiple-choice boxes voluntarily.  From May 14 to May 24, I labored daily to overcome his newly acquired fear.  The usual procedure was to coax him through one box after another by standing at the exit door with some tempting morsel of food.  After several days of this treatment, he again trusted himself to the boxes, although very circumspectly and only when both entrance and exit doors were raised.  Not until May 24 was it possible to resume regular experimentation, and on that day it was found necessary to indicate the right box by raising the exit door slightly and then immediately lowering it.  Trials in which this form of aid was given are indicated in table 2 by a star following the last choice.