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.