Jim wrote -
It cannot be repeated enough that our knowledge of folk song does not precede the beginning of the twentieth century.

Outrageous - even by Jim's standards! Suggest you read pages 221 - 406 of the book under discussion for meticulously researched evidence of the state of folk song in England from the 16th to the 19th century.
Jim has also stated recently on Mudcat that he does not make pronouncements. In that case, I wonder what the above quotation is.