Rock ’n roll is really swing with a modern name. It began on the levees and plantations, took in folk songs, and features blues and rhythm. It's the rhythm that gets to the kids – they're starved of music they can dance to, after all those years of crooners.
The technical history of modern harmony is a history of growth of toleration by the human ear of chords that at first sounded discordant and senseless to the main body of contemporary professional musicians.
The effect of music is so very much more powerful and penetrating than is that of the other arts, for these others speak only of the shadow, but music of the essence.