Yet this theory is decidedly partial, for among primitive people, besides mamma, papa, adda, etc., other sounds depending on neither interjections nor imitation, but purely the result of experimentation, get a meaning from the simple relation of mother and child, and so attain at least a place in their vocabulary and surely form one of the grounds for the explanation of the growth of language. It is not maintained that the child first learns the art of imitating sound from his elders, for without doubt he is often the originator, as in the case of mamma and papa, which he has taught them. For us the interesting question here is that of recognition which we find again the object of playful activity. The “Bow-wow” theory sounds perhaps improbable, or even ridiculous when we think of its being used by adults,[560] but when confined to children all this is changed. It works somewhat in this way: The child learns through imitation to produce all sorts of sounds—the crash of falling objects, the rumble of rolling ones, cries of animals, the gurgling of water. His mother’s play with him adds to the value of such imitations, since in their play the imitative sound comes to stand for its object just as symbolism arises from the effort to express qualities. Imitation makes this intelligible, since every copy is a symbol of the thing copied. Even the interjection and the experimental sound can only be elements of speech by imitation or repetition. Thus Jodl rightly says (following Marty) of the imitation of sound, “As soon as their power of adjustment, their reason, is sufficiently developed, they derive from free play the means consciously employed for the acquisition of varied experience.”[561] Therefore I maintain that imitation is an indispensable condition in the explanation of the origin of language, its objects being threefold: (1) All the acoustic models afforded by the environment; (2) interjectional sounds; (3) experimental sounds. It is as assured a fact that children practise the first as that they playfully repeat their own experiments. Playful imitation of interjection is not to my knowledge indulged in by very young children, but using the sound to signify the thing from which it proceeds is natural enough. On the whole, then, it seems that while imitation plays an important part in the origin of language, as many investigators testify, to make it the only factor would be an act of presumption. As this impulse for acoustic repetition is weaker in adults than in children, I need only mention the playful use of it in poetry where it is agreeable to all. I have already had occasion to remark that poetry written for children is especially rich in such imitation. Animal cries and bird notes figure largely. Rückert’s poem Aus der Iugendheit makes use of a very common metre to imitate the whirring call of the swallow, thus: