In early England, many traditional songs were learned from printed "ballets" or handed out sheets of paper. Over the course of years they were changed. Barbara Allen would not have lasted unless someone printed the song out to be learned first.

There is an interaction between songs in print and songs in aural transmission.

Many songs started as poems such as "Home on the Range" or "Spanish is a Loving Tongue". Many came from the popular stage shows. Much of the dance music of Southern Appalachia stem from the songs of the Minstrel Shows and Uncle Tom's Cabin.

The main difference is that folk music extends beyond copyright laws and can be changed by people at their pleasure.

Schubert Lieder became Germany's folk music. Stephen Foster's song also went into aural transmission. Dick is right about the songs from American Songbag. I've changed a few of them, myself, sometimes unwittingly.

I'm reminded of Sam Hinton's famous analogy, "Folk songs in print are like a photograph of a bird in flight." The real deal goes on jumping off the printed page.