r might be raised to release him,
the lock-bar, sliding under the floor, made a slight grating noise, and
the instant the entrance door was opened, he jumped out excitedly. _He
made no outcry, but as soon as he was out of the box, sat down, and
taking up his right hind foot, examined it for a few seconds._ Having
apparently assured himself that nothing serious had happened, he went on
unconcernedly about his task. The presumption is that the sound of the
lock-bar, associated as it was with his painful experience in box 1,
revived the strongly affective experience of stepping on the nail.
Psychologically described, the sound induced an imaginal complex
equivalent to the earlier painful experience. The behavior seems to the
writer a most important bit of evidence of imagery in the monkey.
Finally, on August 9, after ten hundred and seventy trials, Skirrl
succeeded in choosing correctly in the ten trials of a series, and he
was therefore considered to have solved the problem of the second door
from the right end of the group.

On the following day, he was given a control series with the settings
which are presented on page 19 and also at the bottom of table 2. In
this series he chose correctly five times,--in other words, as often
correctly as incorrectly. An analysis of the choices indicates, however,
that two of the five correct choices were made in box 8, which, as it
happened, had proved a peculiarly easy one for him throughout the
training, since from the first he tended to avoid door 9. Consequently,
it is only fair to conclude, from the results for this control series
and for those given on August 11 and 12, that the animal chose not on
the basis of anything remotely resembling a general idea of secondness
from the right end, but instead on the basis of gradually acquired modes
of reaction to the particular settings. This conclusion is strengthened
by the fact that he had failed to learn to react appropriately and
readily to most of the settings of the regular series.

The curve which represents the course of the learning process in this
problem is presented in figure 19. For this and all other curves which
involve more than a single series of observations a day, the method of
construction was as follows: The first series for each day of training
is indicated on the curve by a dot, while the second or third series on
a given day, although space is allowed for them, are not so indicated.
Consequently, the form of th