There are, however, certain exceptions to the analyzing power of the ear. If two tones differ but little in pitch, they are not perceived as two, but a mean tone is heard beating as frequently in a second as the difference of the vibration rates indicates. The ear thus creates something new, but of course something definitely depending on the external processes. If two tones not quite so close in pitch are sounded, one or even several new tones are created, combination tones or difference tones, the pitch of the new tone being determined by the difference of the rates of vibration. These difference tones do not seem to serve any purpose in animal life. They are merely secondary phenomena, of little practical consequence, but of much interest to the student of the function of the organ of hearing. We have seen that the number of classes of sensations is fairly large; but to state this number exactly is impossible. According as we count the muscles, the joints, the lungs, the digestive organs as several sense organs or as a single group, the number of classes of sensations is larger or smaller. However, it matters little whether we count them or not. We know that provision is made for everything needed. Information about the most distant things is obtained through the eye; information about the things in contact with the body or the body itself comes through the cutaneous and organic sense organs. Most varied is the information about things at a moderate distance, obtained through eyes, ears, and nose combined. Many of the higher animals surpass man in one or the other respect through their sensory equipment. Many of the birds (for example, the carrier pigeons) have a sharper eye; dogs and other animals, a keener sense of smell. The sense of hearing in man seems to be equal to that of the higher animals, and the cutaneous sense perhaps superior. In one respect man is better equipped than his mode of living justifies, that is, in possessing the semicircular canals and the otolith organs, for which he has scarcely any use. In another respect he, as well as the animals, is very poorly equipped, that is, for the direct perception of the electromagnetic-optic phenomena of physics, only a small range of which can be perceived as a particular kind of sensations, namely, as colors. 3. _Temporal and Spatial Attributes_