Referring again to the schema (page 88), it is needless to consider cases 1 and 2, for evidently these are on a psychological level in man and animals. Case 3, also, especially in the direction of its branch 4, is to a large extent psychologically equivalent in men and animals: so far as there is any difference it depends on the higher psychical nature of man being much more rich in ideas which find their natural expression in gestures or tones, and which, therefore, are impossible in brutes. But it will be conceded that here there is nothing to explain. The fact that man has a mind more richly endowed with ideas carries with it, as a matter of course, the fact that their natural expression is more multiplex. The case, however, is different when we arrive at conventional signs; for these attain so enormous a development in man as compared with animals, that the question whether they do not really depend on some additional mental faculty, distinct in kind, becomes fully admissible. The first thing, then, we have to notice with regard to conventional signs as used by man is, that no line of strict demarcation can be drawn between them and natural signs; the latter shade off into the former by gradations, which it becomes impossible to detect over large numbers of individual cases. With respect to tones, for example, it cannot be said, in many instances, whether this and that modulation, which is now recognized as expressive of a certain state of feeling, has always been thus expressive, or has only become so by conventional habit; although, if we consider the different tones by which different races of mankind express some of their similar feelings, we may be sure that in these cases one or other of the differences must be due to conventional habit—just as in the converse cases, in which all mankind use the same tones to express the same feelings, we may be sure that this mode of expression is natural. And so with gestures. Many which at first sight we should, judging from our own feelings alone, suppose to be natural—such, for instance, as kissing—are shown by observation of primitive races to be conventional; while others which we should probably regard as conventional—such, for instance, as shrugging the shoulders—are shown by the same means to be natural.[66]