Donald Francis Tovey

“ The modern conception of melody is based on the interaction of every musical category. For us a melody is the surface of a series of harmonies, and an unaccompanied melody so far implies harmony that if it so behaves that simple harmonies expressing clear key-relationships would be difficult to find for it, we feel it to be, strange and vague. Again, we do not, feel music as melodious unless its rhythm is symmetrical; and this, taken together with the harmonic rationality of modern melody, brings about an equally intimate connexion between melody on a large scale and form on a small scale. ”

“ In short, melody is the surface of music.
We may here define a few technicalities which may be said to come more definitely under the head of melody than any other; but see also Harmony and Rhythm.
1. A theme is a melody, not necessarily or even usually complete, except when designed for a set of variations (q.v.) , but of sufficient independent coherence to be, so to speak, an intelligible musical sentence. Thus a fugue-subject is a theme, and the first and second subjects in sonata form are more or less complex groups of themes. ”

“ Lastly, the element that appears at first sight most strictly melodic, namely, the rise and fall of the, pitch, is intimately connected by origin with the nature of the human voice, and in later forms is enlarged fully as much by the characteristics of instruments as by parallel developments in rhythm, harmony and form. Thus modern melody is the musical surface of rhythm, harmony, form and instrumentation ”