Does a runner have to advance?

Question
Situation: Runner on first and the batter hits a ground ball. The fielder throws to first base getting a force on the batter/runner. The ball is then thrown to second base. The runner who was initially on first sees that the throw is going to beat him and retreats to first base. Is this a legal play?

Thank you,
Tony

Answer
This is legal.  A runner is only forced to run when each base behind him is occupied (with the batter always having the right to take first base).  When the batter was forced out at first base, that eliminated the force on the runners in front of him and hence they have the right to go back.  A prime example of the force going away is when with a man on first on less than 2 outs, the first baseman fields a grounder, tags first, and throws to second for the double play.  Here, since the force is off, the player covering second would have to tag the runner advancing.