William Tenn: Of All Possible Worlds: Changing the world is simple; the trick is to do it before you have a chance to undo it!

"If his great-grandfather had not volunteered for the earliest time-travel experiments way back in the nineteen-seventies, back even before the Blight, it would never have been discovered that he and his seed possessed a great deal of immunity to extra-temporal blackout.

And if that had not been discovered, the ruling powers of Earth, more than a century later, would never have plucked Max Alben out of an obscure civil-service job as a relief guard at the North American Chicken Reservation t...