e spider made the defensive reaction of
dropping to the ground. It climbed back to its web, the fork was
sounded again, the spider dropped again; but after several {303}
repetitions in quick succession, the spider ceased to respond. Next
day, to be sure, it responded as at first; but after the same
performance had been repeated on several days, it ceased permanently
to respond to this stimulus.

Negative adaptation is common in domestic animals, as well as in men.
The horse "gets used" to the harness, and the dog to the presence of a
cat in the house. Man grows accustomed to his surroundings, and to
numerous unimportant sights and sounds.

The conditioned reflex experiment.

Put into a dog's mouth a tasting substance that arouses the flow of
saliva, and at the same instant ring a bell; and repeat this
combination of stimuli many times. Then ring the bell alone, and the
saliva flows in response to the bell. The bell is a _substitute
stimulus_, which has become attached to the salivary response by dint
of having been often given along with the natural stimulus that
arouses this response. At first thought, this is very weird, but do we
not know of similar facts in every-day experience? The dinner bell
makes the mouth water; the sight of food does the same, even the name
of a savory dish will do the same.

Quite possibly, the learning process by which the substitute stimulus
becomes attached to the salivary reaction is more complex in man's
case. He may _observe_ that the dinner bell means dinner, whereas the
dog, we suppose, does not definitely observe the connection of the
bell and the tasting substance. What the experiment shows is that a
substitute stimulus can become attached to a reaction under very
simple conditions.

A conditioned reflex experiment on a child deserves mention. A young
child, confronted with a rabbit, showed no fear, but on the contrary
reached out his hand to take the rabbit. At this instant a loud
rasping noise was produced just behind the child, who quickly withdrew
his hand with {304} signs of fear. After this had been repeated a few
times, the child shrank from the rabbit and was evidently afraid of
it. Probably it is in this way that many fears, likes and dislikes of
children originate.

The signal experiment.

Place a white rat before two little doors, both just alike except that
one has on it a yellow circle. The rat begins to explore. If he enters
the door with the yellow sign,