In view, then, of these several considerations, I am disposed to think that the progress of mental evolution from the brute to the man most probably took place by some such stages as the following. Starting from the highly intelligent and social species of anthropoid ape as pictured by Darwin, we can imagine that this animal was accustomed to use its voice freely for the expression of its emotions, uttering of danger-signals, and singing.[324] Possibly enough, also, it may have been sufficiently intelligent to use a few imitative sounds in the arbitrary way that Mr. Darwin suggests; and certainly sooner or later the receptual life of this social animal must have advanced far enough to have become comparable with that of an infant at about two years of age. That is to say, this animal, although not yet having begun to use articulate signs, must have advanced far enough in the conventional use of natural signs (or signs with a natural origin in tone and gesture, whether spontaneous only or intentionally imitative), to have admitted of a tolerably free exchange of receptual ideas, such as would be concerned in animal wants, and even, perhaps, in the simplest forms of co-operative action.[325] Next, I think it probable that the advance of receptual intelligence which would have been occasioned by this advance in sign-making, would in turn have led to a further development of the latter—the two thus acting and re-acting on one another, until the language of tone and gesture became gradually raised to the level of imperfect pantomime, as in children before they begin to use words. At this stage, however, or even before it, I think very probably vowel-sounds must have been employed in tone-language, if not also a few of the consonants. And I think this not only on account of the analogy furnished by an infant already alluded to, but also because in the case of a “singing” animal, intelligent enough to be constantly using its voice for semiotic purposes, and therefore employing a variety of more or less conventional tones, including clicks, it seems almost necessary that some of the vowel sounds—and possibly also some of the consonants—should have been brought into use. But, be this as it may, eventually the action and reaction of receptual intelligence and conventional sign-making must have ended in so far developing the former as to have admitted of the breaking up (or articulation) of vocal sounds, as the only direction in which any further improvement of vocal sign-making was possible. I think it not improbable that this important stage in the development of speech was greatly assisted by the already-existing habit of articulating musical notes, supposing our progenitors to have resembled the gibbons or the chimpanzees in this respect. But long after this first rude beginning of articulate speech, the language of tone and gesture would have continued as much the most important machinery of communication: