The kitten plays with the ball as the old cat handles the mouse; the little dogs wrestle together, and so learn to fight with teeth and claws; the deer run from one another, and so test their speed and learn to escape their enemies.

If we watch young animals at play we see that not a muscle or nerve escapes this preliminary training and exercise; and the instinctive tendencies which control the play direct the activities into just the performances which the animal's later life-habits are going on to require.
On this view play becomes of the utmost utility.

It is not a by-product, but an essential part of the animal's equipment.

Just as the infancy period has been lengthened in the higher animals in order to give the young time to learn all that they require to meet the harsh conditions of life, so during this infancy period they have in the play-instinct a means of the first importance for making good use of their time.

It is beautiful to see the adults playing with their young, adapting their strength to the little ones, repeating the same exercises without ceasing, drilling them with infinite pains to greater hardihood, endurance, and skill.
On this theory it is also easy to see why it is that the plays are different for the different species.