This Saturday at Santa Anita could prove something of a watershed moment in North American horse racing--the carding of two handicaps built upon a new national ratings system. One race, a $35,000 handicap over six furlongs for dirt horses rated between 70 and 79, has drawn seven entries. The next race on the card, a $65,000 handicap over 6 1/2 furlongs on the hillside turf course for fillies and mares rated between 80 and 95, has drawn six entries. The impetus for launching a ratings system in the U.S. has...