It makes the young animal flexible, plastic, and adaptable; it supplements all his other instincts and imperfect functions; it gives him a new chance to live, and so determines the course of evolution in the direction which the playful animal represents.

The quasi-social and gregarious habits of animals probably owe much of their strength to the play-impulse, both through the training of individual animals and through the fixing of these tendencies as instincts in various animal species in the way just mentioned.
In another place below I analyze a child's game and draw some inferences from it.

Here it may suffice to say that in their games the young animals acquire the flexibility of mind and muscle upon which much of the social co-operation, as well as the individual effectiveness, of their later life depends.

With children, it is not the only agency, of course, though its importance is not less.

We have to carry the children further by other means; but the other means should never interfere with this natural schooling.