Play, he finds, is not "an aimless activity carried on for its own sake"; neither is it the product of surplus physical energy, as Mr. Spencer defines it, for in youth there is playfulness without this condition. Instincts useful in preserving the species appear before they are seriously needed, and are utilized in play, which serves as preparation for the tasks of life. "Animals do not play because they are young, but have a period of youth in order to play." The special ends accomplished by play are control of the body, command of the means of locomotion, agility in pursuit of prey and in escaping danger, and prowess in fighting. The games pursued in attaining these ends are classified in nine groups, beginning with those of experimentation and ending with those referred to curiosity. They include plays of movement, hunting, fighting, love, construction, nursing, and imitation. For all of these Professor Groos finds but one instinct of play responsible, supplemented by the instinct of imitation. He enters into an elaborate discussion of instinct, giving an outline of Weismann's theory of heredity and the views of various writers. He adopts Herbert Spencer's definition of instinct as a complex reflex act, referring its origin to the operation of natural selection, acknowledging the process to be beyond our grasp. In seeking to explain bird song and the love play of animals, the theory of sexual selection is not accepted by him without qualification; a modification of the Darwinian principle is suggested in which the female exerts an unconscious choice. The psychic characteristics of play are the pleasure following satisfaction of instinct, energetic action and joy in the acquirement of power. The animal at first masters its own bodily movements, then seeks the conquest of other animals and inanimate objects. When a certain facility in play has been gained a higher intellectual stage is entered upon, that of make-believe, or playing a part. This state of conscious self-illusion is reached by many of the higher animals. Psychically, it indicates a divided consciousness, and occupies a place between the ordinary state and the abnormal ones of hypnosis and hysteria. To this condition Professor Groos ascribes the genesis of artistic production, an hypothesis that he has elaborated more fully in _Einleitung in die Aesthetik_. The experimental plays of animals, divided into those of courtship, imitation, and construction, correspond to the principles of self exhibition, imitation, and decoration, which are claimed to be the motives of human art. The acquirement of power through play develops a feeling of freedom, and this the artist likewise seeks to realize in the world of ideals.