Thanks, Leeneia. I am reading a 'Prolegomena' (introduction/preparatory discussion) he wrote on the topic of the study of the principal 'melodic families' in British-American folksong.

After a long period of study, Bayard decided that the number of 'distinctly different melodies' circulating among 'modern folk singers' was far lower than many people believed. He felt that some melodies should be seen as 'variants' or 'versions' or 'closely related airs' and that this was a much larger proportion that many people believed. As I said, I'm still reading through it.

The Prolegomena don't set out a detailed study of such a family, more of an introduction to the ideas and general ideas eg the difficulties of tracing the 'history' of any particular melodic family and variants, the different ways, such as length, structure, etc in which a tune may get modified through what he calls 'oral recreation'.