As far as can be discovered, all muscles behave in this way. So do glands where they can be tested. It is said that a brass band can be reduced to silence by sucking a lemon in front of it, owing to the effect upon the salivary glands of its members; I confess that I have never verified this statement. But you will find the exact scientific analogue for dogs in Watson, p. 26. You arrange a tube in a dog's mouth so that saliva drops out at a measurable rate. When you give the dog food it stimulates the flow of saliva. At the same moment you touch his left thigh. After a certain length of time the touch on the left thigh will produce just as much saliva without the food as with it. The same sort of thing applies to emotions, which depend upon the ductless glands. Children at birth are afraid of loud noises, but not of animals. Watson took a child eleven months old, who was fond of a certain white rat; twice at the moment when the child touched the rat, a sudden noise was made just behind the child's head. This was enough to cause fear of the rat on subsequent occasions, no doubt owing to the fact that the adrenal gland was now stimulated by the substitute stimulus, just like the salivary glands in the dog or the trumpet player. The above illustrations show that "ideas" are not the essential units in association. It seems that not merely is "mind" irrelevant, but even the brain is less important than was formerly supposed. At any rate, what is known experimentally is that the glands and muscles (both striped and unstriped) of the higher animals exhibit the law of transfer of response, i.e. when two stimuli have often been applied together, one will ultimately call out the response which formerly the other called out. This law is one of the chief bases of habit. It is also obviously essential to our understanding of language: the sight of a dog calls up the word "dog", and the word "dog" calls up some of the responses appropriate to a real dog.
There is, however, another element in learning, besides mere habit. This is the element dealt with by Thorndike's "Law of Effect". Animals tend to repeat acts which have pleasant consequences, and to avoid such as have unpleasant
