ONE of the little analyzed recent changes on the American scene is the rapid spread of legal gambling. As of this writing, 35 states have some form of legal gambling: 31 allow pari-mutuel betting at race tracks, eight have state lotteries, three permit off-track betting on horse races, three permit only charity bingo games, and one has casinos. Changes are coming rapidly.  New Hampshire established its lottery in 1963, followed by New York in 1967, New Jersey in 1970, and Pennsylvania, Massachusetts, Connecticut, Michigan, and Maryland in 1971-73. New York passed a law in 1970 permitting off-track betting on horse races on local option, and in 1971 the New York City Off-Track Betting Corporation began operations. New Jersey began a daily lottery and Connecticut legalized off-track betting in 1972. In 1973, voters in Maine approved a lottery. In New York, the first stages of legislation to permit casinos have passed, and a referendum on casinos will be held this November in New Jersey. Moves to legalize gambling are under study in several other states, including Kentucky, Rhode Island, and Georgia.