Marge,
Evolution is a wonderful thing and it hasn't stopped or slowed down in my opinion. I'm sure they made complaint in the 15th century when printing started, but the poets and printers' hacks just went right on rewriting the more popular stuff, and this alongside oral tradition, which incidentally is still as strong as it ever was. Some songs stay pretty much the same for centuries, others blossom into a myriad of variants. I love 'em all, well, most of those that survive.

As part of my research I have to plough through hundreds of thousands of songs from the 18th century, and boy am I glad most of these haven't survived. Those that have are jewels though (IMHO).