Another aspect of the play-period is that it affords opportunity for testing new variations before the struggle for existence sieve becomes too fine in the mesh. Play affords elbow-room for new departures, and its value is particularly clear when the adult life is very varied, like an otter's, demanding plasticity and resource.; fulness. Here there is a marked contrast between games, which are restricted to mankind, and play, which children share with young animals. For the game has its rules and demands self-sub ordination, whereas play is spontaneous and allows of idiosyn crasies and experimentation. From the biological point of view it is clear that human games cannot fulfil all the functions of play.
According to Groos there is no general "instinct to play"; it is enough to suppose that each type of playing animal has its inborn or instinctive system or pattern of predispositions towards particular types of adult activity, and that the young are peculiarly sensitive to liberating stimuli. Play implies not only susceptibility, but precocity and plasticity. It secures a certain freedom for initiative before habituation sets in. And this, as has been said, is of especial value when the adult life demands con siderable versatility. In such cases, the animals that play best are also likely to work best.
Types of Play.—If play is anticipatory of future work, the different kinds of play will correspond to the chief activities of adult life. (a) Many forms of play are of the nature of experi ments in locomotion, as in aimless racing, rival jumping, riotous gambolling and feats of climbing or of flying. Here one pictures the behaviour of lambs, kids, calves, young antelopes, young chamois, foals, young squirrels and young monkeys. (b) On another line is sham-hunting, in which the young animal chases some moving object irrespective of all utility. A leaf blown by the wind or a ball of grass will pull the trigger as effectively as a small animal. The mother sometimes aids and abets, and here play may coalesce with education (see ANIMALS, EXPERIMENTS ON ). The kitten's play with the mouse, often absurdly misinterpreted as "delight in torture," is paralleled in many other young carnivores. It is justified in the present by the repetition of pleasurable excite ment, and in the future by the increased dexterity it develops. When the mouse-play is exhibited by cats of mature years, and apart from their education of their kittens, it is probably a relapse into youthful play, illustrated less poignantly in some other adults.
(c) A third form of play is the sham-fight, familiar in puppies. It has been described among lions, tigers, hyenas, wolves, foxes, bears and other carnivores; among lambs, kids, calves, foals, antelopes and other ungulates. It is also common among birds. Care must be taken to keep the sham-fight distinct from the corn bats of rival males, the first hints of which may begin early, as in bull-calves. And even apart from sex, it is not always easy to distinguish the sham-fight from serious combativeness. In his description of the behaviour of two young gluttons, Brehm says that nothing could be more playful, they were hardly at a rest for a minute, but every now and then the note of earnest was struck. Very curious, considering the level at which they occur, are the so-called sham-fights which several good observers have described among ants. There is energetic wrestling and the like, but no discharge of poison or actual wounding. (d) Perhaps one may recognize another type which may be called playful experi ment, when animals test things, often pulling them to pieces; or test themselves, often performing interesting but useless feats; or test their neighbours, discovering how they will respond to sundry provocations. The difficulty is to distinguish these playful experiments from the ways in which many well-endowed young animals feel their way about in their environment. But Hamerton describes how his young goats would spend hours in jumping in nd out of a basket, or would try to upset the artist by getting under his seat, or would tease the big dog to the limit of his endurance.
Along this line the subtlest forms of play are found in apes, where experimenting may go far, and sometimes become sheer mischief. Chimpanzees often show what looks like delight in being a cause, and an entirely useless activity may be repeated over and over again. Thus a chimpanzee will entice a hen with bread, and pull the reward away at the last minute, repeating the trick many times with evident gusto. Or it will attract a hen close to the cage and then give her a sudden poke with a stick when she is preoccupied with her food. This seems almost like a joke.