poppagator, excellent point,Music other than classical music[where the composer has stipulated certain interpretations]evolves and changes.,and as you say just because it was sung in a certain way in 191O,it doesnt mean it was performed the same way thirty years previously.
an intersting remark that Alistair Anderson made,is that the styles traditional musicians play in, is affected by their geographical environment,someone playing and living in the fens of east anglia will play very differently from someone living in northumberland.