Why should play be instinctive in its forms, showing certain complex and ingrained channels of expression, if it were merely the discharge of surplus force?
We are more lively in the morning, but that does not explain our liking and indulging in certain sorts of complex games at all hours.

Moreover, animals and children will continue to play when greatly fatigued.

A dog, for example, which seems absolutely "used up," can not resist the renewed solicitations of his friends to continue the chase.

Furthermore, why is it that plays are characteristic of species, different kinds of animals having plays quite peculiar to themselves?
It is difficult to see how this could have come about unless there had been some deeper-going reason in accordance with which each species has learned the particular forms of sport in which it indulges.
The advocates of this theory attempt to meet these objections by saying that the imitative instinct accounts for the particular directions in which the discharges of energy occur.

A kitten's plays are like those of the cat tribe because the kitten is accustomed to imitate cats; when it falls to playing it is with cats, and so it sheds its superfluous energies in the customary imitative channels.