Blowzabella uses a nice image of a river. But his image specifies only one source. One could argue this as a very eurocentric notion of a river if one compares it with a different notion widespread among Australian Aborigines. Because rivers of any great size (OK, we've only got a couple) run through several different language groups (about as different from each other as European languages) the different parts of a river each had specific and different names. When Europeans came and imposed 'the one true source' notion and eradicated many of the separate names, the sense that each part of the river's extent was distinctive became diminished, if not lost.

It seems to me that the Aboriginal view stands as a fairly good metaphor for many variants/performances of a song. They're all connectable and may take on widely variant appearances. Very few "rivers" have a single "source" that has the same dimension as the later development of the watercourse. Even if you can detect the location of the spring, that isn't necessarily the source; it's only where the water comes into easy view from underground. Songs, both traditional and recent, may have an extensive "underground" component before they come into view. And then they change.

And in 500 years' time, will our current pedantries matter?

Cheers, Rowan