In order to solve this problem we shall have first to go beyond the individual and appeal to the history of the race. What the race has found serviceable, the individual repeats. But even then it is hard to see why the particular type of physical response such as shrinking, pallor, and trembling, which naturally follow stimuli threatening harm, should be the best. It is easy to see, however, that the feeling which prompts to flight or serves to deter from harm's way might be useful. It is plain that there is an advantage in the tense muscle, the set teeth, the held breath, and the quickened pulse which accompany the emotion of anger, and also in the feeling of anger itself, which prompts to the conflict. But even if we are not able in every case to determine at this day why all the instinctive responses and their correlate of feeling were the best for the life of the race, we may be sure that such was the case; for Nature is inexorable in her dictates that only that shall persist which has proved serviceable in the largest number of cases. An interesting question arises at this point as to why we feel emotion accompanying some of our motor responses, and not others. Perceptions are crowding in upon us hour after hour; memory, thought, and imagination are in constant play; and a continuous motor discharge results each moment in physical expressions great or small. Yet, in spite of these facts, feeling which is strong enough to rise to an emotion is only an occasional thing. If emotion accompanies any form of physical expression, why not all? Let us see whether we can discover any reason. One day I saw a boy leading a dog along the street. All at once the dog slipped the string over its head and ran away. The boy stood looking after the dog for a moment, and then burst into a fit of rage. What all had happened? The moment before the dog broke away everything was running smoothly in the experience of the boy. There was no obstruction to his thought or his plans. Then in an instant the situation changes. The smooth flow of experience is checked and baffled. The discharge of nerve currents which meant thought, plans, action, is blocked. A crisis has arisen which requires readjustment. The nerve currents must flow in new directions, giving new thought, new plans, new activities--the dog must be recaptured. It is in connection with this damming up of nerve currents from following their wonted channels that the emotion emerges. Or, putting it into mental terms, the emotion occurs when the ordinary current of our thought is violently disturbed--when we meet with some crisis which necessitates a readjustment of our thought relations and plans, either temporarily or permanently.