At the bottom of the ninth in a 1-1 tie, with runners on first and third, it seemed the Giants would defeat the Cubs when Al Bridwell hit an apparent single to center. However, when Merkle (on first) saw Moose McCormick touch home plate with the "winning" run, he left the basepath before touching second base and headed for the clubhouse in center field at the Polo Grounds.
Chicago second baseman Johnny Evers called for the center fielder to throw him the ball so he could get a forceout at second on Merkle. The ball was thrown in, and in the tussle, pitcher "Iron Man" McGinnity, who had been coaching at third base, wound up with it and threw it into the stands. Somehow, though, a ball appeared in Evers' hand and he touched second base. Umpire Hank O'Day called Merkle out and, with the Giants already having left the field and the fans swarming it, called the game a 1-1 tie.
Later, National League president Harry Pulliam upheld O'Day's decision. The game was replayed after the regular schedule was finished, with the teams tied for first place. The Cubs won the replay to capture the pennant and went on to win the World Series. Ninety years later, they haven't won another Series.
"More than Merkle," a book discussing the 1908 baseball season.
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New York Times
A Google search for Fred Merkle