The relation of tonic to dominant is the foundation of Western triadic tonality. The attempt of the early nineteenth century to substitute third or mediant relationships for the classical dominant amounted to a frontal attack on the principles of tonality, and it eventually contributed to the ruin of triadic tonality. This ruin was accomplished from within the system, however, as mediant relationships were essential to tonality as conceived in the eighteenth century.

Ch. 4 : Formal Interlude - The Romantic Generation (1995)