We shall be helped to define a form in general by looking back to the metrical forms we have just been studying. These, we have seen, are groups or clusters of impressions, held together by some similarity, yet also differentiated by some contrast. The two or three beats of the measure-group are similar in duration, yet different in accent. And without both the similarity and the difference, the unity and the variety, they would not be a group. Without similarity they would be a haphazard collection, a chaos; without difference they would all fuse together in one indistinguishable mass. In other words, they exemplify a general fact about forms--namely, that the elements must be alike enough to be associated, and yet different enough to be discriminated. If we cannot associate them we cannot feel them as a group; they will not cohere. If, on the other hand, we cannot discriminate them, then do they equally fail to make up a form; they simply mingle together into a homogeneous lump. The organs of an organism must be, then, related, yet different; the elements of a form must be both similar and dissimilar. Unless they are both we cannot perceive them as linked, yet distinct. Bearing this general fact about forms in mind, we may investigate the kinds of form that underlie Song. Probably every one who has listened to the whistling of factories in a large city at noon has had the curious experience of suddenly hearing amid the meaningless din a pair of tones that mysteriously mate and merge. The other tones seem entirely accidental; they have no relation to each other, and give one merely a sense of vague annoyance. But these two form an intelligible group; we are able to grasp them together, and we take an indescribable pleasure in thus feeling them as parts of one whole. Here is an instance of another sort of musical form than the metrical, a sort that we may call harmonic. Here the grouping takes place on a platform not of time, but of pitch; the two elements of the group have no metrical relations, but in pitch they are somehow related. Now this sort of pitch relationship has played a vital part in music, a part hardly secondary to that of time relationship; so that an understanding of it is important enough to delay us here a moment with some rather dry technical facts on which it depends.