prior to any experience of these serious activities. The repetition (with a difference!) is in such cases not the re-enactment of what has been previously performed in full earnest by the individual, but rather the reappearance in the young of ancestral modes of procedure--in other words, its specific character is such because it is a piece of instinctive behaviour or arises from instinctive proclivities. And this is the central point of the interpretation elaborated with great skill and candour by Professor Groos. Play is instinctive; and its biological value lies in the training it affords for the subsequent earnest of life. Before leaving the surplus energy theory of play one more point made by Professor Groos may be mentioned. He contends that, though superabundant energy is a favouring condition of animal play (as it is, indeed, of all animal behaviour), still it is not a necessary condition. Animals often play when they are tired out. “Notice a kitten when a piece of paper blows past. Will not any observer confirm the statement that, just as an old cat must be tired to death or else already filled to satiety if it does not try to seize a mouse running near it, so will the kitten, too, spring after the moving object, even if it has been exercising for hours and its superfluous energies are entirely disposed of? Or observe the play of young dogs when two of them have raced about the garden until they are forced to stop from sheer fatigue, and they lie on the ground panting, with tongues hanging out. Now one of them gets up, glances at his companion, and the irresistible power of his innate longing for the fray seizes him again. He approaches the other, sniffs lazily about him, and, though he is evidently only half inclined to obey the powerful impulse, attempts to seize his leg. The one provoked yawns, and in a slow, tired kind of way puts himself on the defensive; but gradually instinct conquers fatigue on him too, and in a few minutes both are tearing madly about in furious rivalry until want of breath puts an end to the game. And so it goes on with endless repetition, until we get the impression that the dog waits only long enough to collect the needed strength, not till superfluous energy urges him to activity.”[128]