Nothing is more characteristic of the young of intelligent animals than the variety and persistency of their behaviour, their sensitiveness to stimuli of many different kinds, their restlessness of swiftly changing attention and response, with occasional pauses of continued effort in some special direction. Constantly on the alert, they exhibit in all its shifting phases behaviour which we interpret as indicating curiosity, inquisitiveness, love of mischief, destructiveness, and so forth. The facts are so familiar to every observer of young animals that it is unnecessary to give any detailed illustration. Watch a kitten in this stage of its development and carefully note its behaviour during half an hour; the variety of effort, the _rôles_ played by trial, failure, and success, the gain of skill and control over behaviour, will at once be evident. Or devote an equal space of time to observing young jays, magpies, or jackdaws. Every projecting piece of wire or bit of wood in their cage is pulled at this way and that way, from above, from below, from the side. Now one, then another, loose object is picked up and dropped, turned over, carried about, pulled at, hammered at, stuffed into this corner and into that, and experimented with in all possible ways. Then the wise bird goes to sleep, and wakes up again only to resume with new zest its persistent and varied efforts, by which it becomes acquainted with all the details of its environment. Watch young birds on the wing gaining their mastery of the air in flight, young seals tumbling in the water, young foals scampering and kicking up their heels in the meadows. A little observation, as occasion serves, a little attention to the progress towards an adequate experience of the meaning of things, towards more complete control, and increased nicety of behaviour, whether in reference to their surroundings, or in powers of finished locomotion, will serve to bring home what Professor Groos includes under experimentation and movement plays. He regards it all as play, since it seems to have no serious end, and is just a preparation for the sterner realities of adult life. And for human beings, whose work is so largely enforced, the freedom and evident joy of it all suggests the play which has acquired for us the meaning of relaxation from irksome effort, and glad abandonment to less constrained modes of behaviour. But in young animals such play is, after all, the serious business of their time of life. Its import for their future welfare can scarcely be overestimated.