But Steve is absolutely right: the fact that two songs share a particular widespread theme [here animal/human metamorphosis, what one might perhaps call 'the werewolf motif'] clearly doesn't lead inevitably to the conclusion that one has to 'derive' from the other. We don't go around saying that every song in which a young woman gets screwed and falls pregnant* must all derive from some ur-original, do we?

≈M≈

*Drift observation. Aren't folkloric females fantastically fertile? Only ever needs a single casual sex-act and away their wombs are bound to go!