The Mental Life of the Monkeys 195 
when the door was left open only after great delay. The 
time-curves for the experiments performed will be found 
on page 186 among the others. The figures beside each pair 
represent the number of days without practice. 
~The records show a decided superiority to those of the 
cats and dogs... Although the number of trials in the original 
tests were in general fewer in the case of the monkeys, . the 
retention of the association is complete in 6 cases out ra 8 
and is practically so in one case where the interval was 
8 months. 
EXPERIMENTS ON THE DISCRIMINATION OF 
SIGNALS 
My experiments on discrimination were of the following 
general type: I got the animal into the habit of reacting 
to a certain signal (a sound, movement, posture, visual 
presentation or what not) by some well-defined act. In 
the cases to be described this act was to come down from 
his customary positions about the top of the cage, to a place 
at the bottom. I then would give him a bit of food. When 
this habit was wholly or partly formed, I would begin to 
mix with that signal another signal enough like it so that 
the animal would respond in the same manner. In the 
cases where I gave this signal I would not feed him. I could 
then determine whether the animal did discriminate or not, 
and his progress toward perfect discrimination in case he did. 
If an animal responds indiscriminately to both signals (that 
is, does not learn to disregard the ‘no food’ signal) it is 
well to test him by using two somewhat similar signals, 
after one of which you feed him at one place and after the 
other of which you feed him at a different place. 
If the animal profits by his training by acquiring ideas of 
