Preceding the opening credits is this quote: "We must suppose that American animals...slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves." Attribution is to Charles Darwin, from his seminal work, "On the Origin of Species."
After the Charles Darwin quote, the first credit on the screen is "This Is Not Based On A True Story," which then changes to "This Is...A True Story" by means of the disappearance of the relevant words.