In these the animals co-operate, but do not take the same parts.

The young perform actions in the game which the mother does not.
All this goes to support another and most serious objection to this theory--in the mind of all those who believe in the doctrine of evolution.

The Surplus-Energy Theory considers the play-impulse, which is one of the most widespread characters of animal life, as merely an accidental thing or by-product--a mere using-up of surplus energies.
It is not in any way important to the animals.

This makes it impossible to say that play has come to be the very complex thing that it really is by the laws of evolution; for survival by natural selection always supposes that the attribute or character which survives is important enough to keep the animal alive in the struggle for existence; otherwise it would not be continued for successive generations, and gradually perfected on account of its utility.
On the whole, therefore, we find the Surplus-Energy Theory of play quite inadequate.
II.

Another theory therefore becomes necessary if we are to meet these difficulties.