One difference between Rugby Union and Walter Camp's version, is that in rugby play doesn't stop at the tackle, so the pile of bodies is actually a 'loose scrum' with both sides trying to win the ball. Possession is retained until they make a mistake or give it away. You're not likely to see many replays of scrums - the IRB keep fiddling with the laws and interpretations in an effort to make them comprehensible to referees.