The author discusses the inadequacies of all the theories and illustrates his remarks from the theories of Spencer and Bergson. Spencer stated that laughter affords an outlet for surplus nervous energy, escaping by the motor nerves in most frequent use, those supplying the muscles of speech and respiration. This theory is inadequate, because laughter occurs independently of this escape of energy. Bergson states that the essential function of laughter is disciplinary. Mc Dougall points out that this theory no more covers the essential facts of laughter than Herbert Spencer's. They both fail to answer the question: For what end did the human species acquire this capacity for laughter? Laughter is a highly complex co-ordinated series of movements, maintained by an impulse so strong and definite that it often defies the control of the will. The author proceeds to consider the conditions which excite laughter and the condition of laughter itself. Laughter interrupts the train of mental activity; it diverts or