We have seen that Professor Groos regards play as the practice and preparation for the serious business of animal life. Founded on instinctive tendencies, it has its biological value in the acquisition of practical acquaintance with the environment, and of skill in dealing with it effectually. It is an education in behaviour of the utmost service in view of the struggle for existence. It is full of the pleasure derived from the satisfaction of innate impulse, the success of conative effort, and the diffused sense of well-being which accompanies a life of action, free and unrestrained. This freedom and gladness lead us to call it play; but we must not draw the inference that the playful animal knows that it is playing, or forms any conception of the antithesis between work and play, which is a product of late development. In laying stress on the biological value of certain modes of behaviour which we thus call play, a value which lies in the practice and preparation they afford for life’s more earnest work, Professor Groos deserves our hearty thanks. Nor need our thanks be less hearty if we find that he has in some degree been anticipated by Darwin; for he has elaborated with systematic care what Darwin suggested incidentally. “Nothing is more common,” said Darwin,[135] “than for animals to take pleasure in practising whatever instinct they follow at other times for some real good. How often do we see birds which fly easily, gliding and sailing through the air, obviously for pleasure! The cat plays with the captured mouse, and the cormorant with the captured fish. The weaver bird, when confined in a cage, amuses itself by neatly weaving blades of grass between the wires of its cage. Birds which habitually fight during the breeding season are generally ready to fight at all times; and the males of the capercailzie sometimes hold their _Balzen_, or _leks_, at the usual place of assemblage during the autumn. Hence it is not at all surprising that male birds should continue singing for their own amusement after the season of courtship is over.”