Language, in its broadest sense, includes any form of expression by which thoughts and feelings are communicated from one individual to another. Words may be spoken, gestures made, cries uttered, pictures or characters drawn, or letters made as means of expression. The deaf-mute converses with his fingers and his lips; the savage communicates by means of gesticulation. It is easy to conceive of a community in which all communication is carried on in sign language. It is said that the Grebos of Africa carry this mode of expression {122} to such an extent that the persons and tenses of the mood are indicated with the hands alone. It has been advocated by some that man first learned to talk by imitating the sounds of nature. It is sometimes called the "bow-wow" theory of the origin of language. Words are used to express the meaning of nature. Thus the purling of the brook, the lowing of the cow, the barking of the dog, the moaning of the wind, the rushing of water, the cry of animals, and other expressions of nature were imitated, and thus formed the root words of language. This theory was very commonly upheld by the philosophers of the eighteenth century, but is now regarded as an entirely inadequate explanation of the process of the development of language. It is true that every language has words formed by the imitation of sound, but these are comparatively few, and as languages are traced toward their origin, such words seem to have continually less importance. Nothing conclusive has been proved concerning the origin of any language by adopting this theory. Another theory is that the exclamations and interjections suddenly made have been the formation of root words, which in turn give rise to the complex forms of language. This can scarcely be considered of much force, for the difference between sudden explosive utterance and words expressing full ideas is so great as to be of little value in determining the real formation of language. These sudden interjections are more of the nature of gesture than of real speech.