May I point you towards the chapter in Scholes' 'The Oxford Companion to Music' appertaining to Folk Song. It's too long (nearly 3.5 pages of small type) too reproduce here and looks at the subject from a 'Classical' music point of view, but it is an item of historical scholarship. It covers many of the points that have arisen here and is probably as debateable as all of them. One point that is made, which made my Mother-in-law change her description of "Dirt!", spat out as a derogatory epithet, to "I meant the soil from which all things grow" was the following: 'It is self-evident that the germ of all music lies in folk music.'
To which I would add that a song without a tune is a poem, not original I fear, but pertinent.