The other explanation makes consonance due to the identity of partial tones. When two tones have one or more partial tones in common they are said to be related; the amount of identity gives the degree of relationship. Physiologically, one or more basilar membrane fibres are excited by both, and this fact gives the positive feeling of relationship or consonance. Of course the obvious objection to this view is that the two tones should be felt as differently consonant when struck on instruments which give different partial tones, such as organ and piano, while in fact they are not so felt. But it is not after all essential to the aesthetics of music that the physiological basis of harmony should be fully understood. The point is that certain tones do indeed seem to be "preordained to congruity," preordained either in their physical constitution or their physiological relations, and not to have achieved congruity by use or custom. Consonance is an immediate and fundamental impression,--psychologically an ultimate fact. That it is ultimate is emphasized by Stumpf<1> in his theory of Fusion. Consonance is fusion, that is, unitary impression. Fusion is not identical with inability to distinguish two tones from each other in a chord, although this may be used as a measure of fusion. Consonance is the feeling of unity, and fusion is the mutual relation of tones which gives that feeling. <1> _Beitrage zur Akustik u. Musikwissenschaft_, Heft I, Konsonanz u. Dissonanz, 1898. The striking fact of modern music is the principle of tonality. Tonality is said to be present in a piece of music when every element in it is referred to, gets its significance from its relation to, a fundamental tone, the tonic. The tonic is the beginning and lowest note in the scale in question, and all notes and chords are understood according to their place in that scale. But the conception of the scale of course does not cover the ground, it merely furnishes the point of departure,-- the essential is in the reference of every element to the fundamental tone. The tonic is the centre of gravity of a melody.