he finds himself in a passage which
leads to a box of food; if he enters the other door he gets into a
blind alley, which he explores, and then, coming out, continues his
explorations till he reaches the food box and is rewarded. After this
first trial is thus completed, place him back at the starting point,
and he is very apt to go straight to the door that previously led to
the food, for he learns simple locations very quickly. But meanwhile
the experimenter may have shifted the yellow sign to the other door,
connected the passage behind the marked door with the food box, and
closed off the other passage; for the yellow disc in this experiment
always marks the way to the food, and the other door always leads to a
blind alley. The sign is shifted irregularly from one door to the
other. Whenever the rat finds himself in a blind alley, he comes out
and enters the other door, so finally getting his reward on every
trial. But for a long time he seems incapable of responding to the
yellow signal. However, the experimenter is patient; he gives the rat
twenty trials a day, keeping count of the number of correct responses,
and finds the number to increase little by little, till after some
thirty days every response is correct and unhesitating. The rat has
learned the trick.

He learns the trick somewhat more rapidly if punishment for incorrect
responses is added to reward for correct responses. Place wires along
the floor of the two passages, and switch an electric current into the
blind alley, behind {305} the door that has no yellow circle on it.
When the rat enters the blind alley and gets a shock, he makes a
prompt avoiding reaction, scampering back to the starting point and
cowering there for some time; eventually he makes a fresh start,
avoids the door that led to the shock and therefore enters the other
door, though apparently without paying any attention to the yellow
sign, since when, on the next trial, the sign is moved, he avoids the
_place_ where he got the shock, without reference to the sign. But in
a series of trials he learns to follow the sign.

Learning to respond to a signal might be classified under the head of
substitute stimulus, since the rat learns to respond to a stimulus,
the yellow disk, that at first left him unmoved. But more careful
consideration shows this to be, rather, a case of substitute response.
The natural reaction of a rat to a door is to enter it, not to look at
its surface, but the e