Animals do learn, all the vertebrates, at least, and many of the invertebrates. They often learn more slowly than men, but this is an advantage for our present purpose, since it makes the learning process easier to follow. Mere anecdotes of intelligent behavior in animals are of little value, but experimental studies, in which the animal's progress is followed, step by step, from the time when he is confronted with a perfectly novel situation till he has mastered the trick, have now been made in great numbers, and a few typical experiments will serve as a good introduction to the whole subject of learning. The negative adaptation experiment. Apply a harmless and meaningless stimulus time after time; at first the animal makes some instinctive exploring or defensive reaction; but with continued repetition of the stimulus, he ceases after a while to respond. The instinctive reaction has been detached from one of its natural stimuli. Even in unicellular animals, negative adaptation can be observed, but in them is only temporary, like the "sensory adaptation" described in the chapter on sensation. Stop the stimulus and the original responsiveness returns after a short time. Nothing has been learned, for what is learned remains after an interval of rest. In higher animals, permanent adaptation is common, as illustrated by a famous experiment on a spider. While the spider was in its web, a tuning fork was sounded, and the 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.