Coming now to Professor Groos’s interpretation of play, we find in it, perhaps for the first time in the literature of the subject, adequate stress laid on its biological value. “The play of young animals,” he says,[129] “has its origin in the fact that certain very important instincts appear at a time when the animal does not seriously need them.... Its utility consists in the practice and exercise it affords for some of the more important duties of life, inasmuch as selection [in the higher animals] tends to weaken the blind force of instinct, and aids more and more the development of independent intelligence as a substitute for it. At the moment when intelligence is sufficiently evolved to be more useful in the struggle for existence than the most perfect instinct, then will selection favour those individuals in whom the instincts in question appear earlier and in less elaborated forms--in forms that are merely for practice and exercise,--that is to say, it will favour those animals which play.... Animals cannot be said to play because they are young and frolicsome, but rather they have a period of youth in order to play; for only by so doing can they supplement the insufficient hereditary endowment with individual experience, in view of the coming tasks of life.” Some stress is here laid on the fact that important instincts appear at a time when the animal does not seriously need them. It seems to imply the doctrine of what biologists term “acceleration”--which means the development of an organ or mode of behaviour at an earlier period in the descendants than that at which it appeared in the ancestors. Thus the adult fighting or hunting instinct of past generations appears in the young to-day as a fighting-play or a hunting-play. It is open to question, however, whether either the instinctive behaviour or the conscious situation in the one case and the other is so nearly identical that the playful fight or hunt can fairly be called the same instinctive procedure as the serious combat or chase. We may hold, with Professor Groos, that the one is an invaluable preparation for the other without identifying them as the same behaviour under different conditions. Indeed the conditions are so different that the identification seems strained. The question may be left open, however, without impairing the value of Professor Groos’s suggestion. And we may divide the preparatory behaviour in what is commonly called play under two heads: first, general preparation for varied modes of serious effort in after-life; and secondly, special preparation for particular forms of this after-effort. Under the first will fall what Professor Groos terms experimentation and movement play, including what Dr. Stout, who fully realizes its importance, calls “manipulation”;[130] under the second, such forms as hunting-play and fighting-play.