Under today’s copyright regime, it’s extremely difficult to to build upon prior artistic works. It was different in Shakespeare’s day.
Recently my wife was browsing an antiquarian bookstore and came across a ten volume set, “The Best of the World’s Classics,” Henry Cabot Lodge, Editor-in-Chief, published by Funk & Wagnalls in 1909. These are beautiful little hardcover books (sextodecimo or 16mo format), full of wonderful, well, classics! — texts that I would know better if I hadn’t coasted through much of my early education.
From volume 1, page 190, the editor’s footnote 2,
From “The Lives of the Noble Grecians and Romans Compared together by that Grave, Learned Philosopher and Historiographer Plutarch of Chaeronea,” translated by Sir Thomas North. North was born about 1535, his translation being first published in 1579. Written throughout in the best prose of the Elizabethan period, North’s version will always have another and very special interest as the storehouse from which Shakespeare obtained his knowledge of antiquity. It has been asserted that to this book we really owe the existence of “Julius Caesar,” “Coriolanus,” and “Anthony and Cleopatra.” In “Coriolanus” whole speeches have be taken bodily from North, while in “Anthony and Cleopatra” North’s diction has been closely followed.
These plays were likely written in 1599-1606, more than two decades after North (1535? - 1601?) published his translation. In the US, Congress has continually extended copyright terms (eleven times in the past forty years) so, if current US law applied, North’s copyright would have lasted until 1671, long after Shakespeare’s death. Luckily for Shakespeare, copyrights didn’t emerge until long after his death.