Eschewing Credit: Heywood, Shakespeare, and Plagiarism before Copyright

How, in a textual system which lacks a concept of authorial/intellectual property, and which operates largely through techniques of imitation, can there be such a thing as "plagiarism"? Can there be such a thing? What is being stolen? By whom? How? And how might it be useful to those of us living at the hypostasization of authorial/intellectual property at the end of the twentieth century to consider its prehistory in the early seventeenth?