such use should prove desirable, the
depth throughout was made 6 feet, and it was therefore possible for the
experimenter to walk about erect in it.

The experimental procedure was briefly as follows: A small quantity of
food having been placed in each of the food cups and covered by the
metal flanges on the exit doors, the experimenter raised door 11 of
figure 17 and then opened door 10 and the door of the cage in which the
desired subject was confined. After the latter, in search of food, had
entered the runway D, the experimenter lowered door 11 to keep it in
this runway, and immediately proceeded to set the reaction-mechanisms
for an experiment (trial). Let us suppose that the first setting to be
tried involved all of the nine boxes. Each of the entrance doors would
therefore be raised. Let us further suppose that the right door is
defined as the middle one of the group. With the apparatus properly set,
the experimenter next raises door 12, thus admitting the animal to the
response-compartment E. Any one of the nine boxes may now be entered by
it. But if any except number 5, the middle member of the group, be
entered, the entrance door is immediately lowered and both the exit and
entrance doors locked in position so that the animal is forced to remain
in the box for a stated period, say thirty seconds. At the expiration of
this time the entrance door is raised and the animal allowed to retrace
its steps and make another choice. When the middle box is chosen, the
entrance door is lowered and the exit door immediately raised, thus
uncovering the food, which the animal eats. As a rule, by my monkeys and
ape the reward was eaten in the alleyway G instead of in the
multiple-choice box. As soon as the food has been eaten, the exit door
is lowered by the experimenter, and the animal returns by way of G and H
to runway D, where it awaits its next trial.

As rewards, bananas and peanuts were found very satisfactory, and
although occasionally other foods were supplied in small quantities,
they were on the whole less constantly desired than the former.

Four problems which had previously been presented to other organisms
were in precisely the same form presented to the three primates. These
problems may be described, briefly, by definition of the right reaction
mechanism, thus: problem 1, the first mechanism at the subject's left;
problem 2, the second mechanism at the subject's right (that is, from
the end of the series at