But the two ends are not only consonant; they are supplementary one to the other. During much of the life of the higher animals there is no need, immediately present and pressing, for the output of action to meet biological ends. There are periods of life and intervals of time when the sharp incidence of the struggle for existence does not call for the serious business of behaviour. But at these periods and in these intervals the animal is not inactive; indeed, it is restless in its activity. Unless it be weary with unwonted exertion, or basking in the psychical sunshine of content, due to the unsought advent of pleasant stimulation or the after-effects of previous behaviour (for example, when hunger has been relieved), the healthy animal must be up and doing. This familiar fact no doubt affords the basis in observation of the surplus-energy theory of play. But is it necessarily surplus energy? Is it not rather normal energy which expends itself in this way when there is no immediate and serious biological business on hand? And, as Professor Groos has pointed out, play is seen when we have every reason to suppose there is no surplus energy, nay, even when the normal energy is at a low ebb. There is no more pathetic sight than a sick kitten, with energy obviously much below par, utilizing its little remaining strength in feeble attempts to play. It is unnecessary to do more than remind the reader of the theory elaborated with so much skill and care by Professor Groos, that the forms assumed by play--in which, it will be remembered, he includes a very wide range of behaviour--have a very important indirect biological end in practice and experimentation. Our present point is, that its direct psychological end is the satisfaction it affords. Without this the individual would not be impelled to the continuance of performances which occupy a wide space in the field of animal behaviour in which the biological end has reference, not to present requirements, but to future needs. No one has given better expression to the sway of this psychological end than Mr. W. H. Hudson. “We see,” he says,[200] “that the inferior animals, when the conditions of life are favourable, are subject to periodical fits of gladness, affecting them powerfully and standing out in vivid contrast to their ordinary temper. And we know what this feeling is--this periodic intense elation which even civilized man occasionally experiences when in perfect health, more especially when young. There are moments when he is mad with joy, when he cannot keep still, when his impulse is to sing and shout aloud and laugh at nothing, to run and leap and exert himself in some extravagant way. Among the heavier mammalians the feeling is manifested in loud noises, bellowings, and screamings, and in lumbering, uncouth motions--throwing up the heels, pretended panics, and ponderous mock battles.