Thus all the evidence at our disposal goes to show that, while the language of tone and gesture is distinctive, in its least-developed form, of a comparatively low grade of mental evolution, in all but its least-developed form it is not thus distinctive; for as soon as the language of gesture becomes in the smallest degree conventional, so soon is the psychological level sufficiently high to admit of the use of articulate sounds, vocal gestures, or words expressive of concrete ideas—always supposing that these are already supplied by the psychological environment. Whether or not articulate sounds are then actually made depends, of course, on conditions of a purely anatomical kind. And here it may be as well to remember the point previously mentioned, namely, that although no existing quadrumanous animal has shown itself able to articulate, we may be quite sure that this fact depends on anatomical as distinguished from psychological conditions; for not only are the higher monkeys much more intelligent than talking birds, but they are likewise much more imitative of human gestures; and for both these reasons they are the animals which, more than any others, would be psychologically apt to learn the use of words from man, were it not for some accident of anatomy which stands in the way of their uttering them. And in this connection it is worth while to bear in mind the remark of Professor Huxley, that an imperceptibly small difference of innervation, or other anatomical character of the parts concerned, might determine or prevent the faculty of making articulate sounds. * * * * * Looking to the direction in which my argument is tending, this appears to be the most convenient place to dispose of a criticism that is not unlikely to arise. It may be suggested, by way of objection to my views, that if all the foregoing discussion is accepted as paving the way to the conclusion that human intelligence has been developed from animal intelligence, the discussion itself is proving too much. For, if animals possess in so conspicuous a degree the germ of the sign-making faculty, why, it may be asked, has this germ been developed only in the case of our own ancestors?