THREE OF THE nation's mega-states -- Texas, California and Florida -- are in various stages of experimenting with a disarmingly simple alternative to traditional affirmative action in deciding whom to admit to their publicly supported systems of higher education. Instead of explicitly taking into account such factors as race and ethnicity, the use of which is under increasing attack in the courts, they use the proxy of geography. They offer automatic admission to the top graduates of every high school in the state. The result might be called the revenge of the neighborhood school. The residential segregation that persists in so much of the nation becomes an instrument for desegregating higher education, at least at the undergraduate level.

In Gov. (and presidential candidate) George W. Bush's Texas, where the experiment is farthest along, it seems to be working. When explicit affirmative action was abandoned in response to a court order several years ago, the already small percentages of minority groups in the entering classes at the University of Texas at Austin declined. Though it's still too early to be sure of causes and effects, with the institution of the new system in 1998, the figures turned back up.

Like all admissions systems, this one is imperfect, or arbitrary, in that some worthy applicants who would prosper under different rules are given a steeper hill to climb. All high schools are not equal. A top graduate from a poor one may well have weaker academic credentials and may not even have had to work as hard as a student who may likewise be a member of a minority group and who graduates with a lower class rank from a more competitive institution.

To keep the graduates of weaker schools from flunking out also may require more remedial courses; it has at the University of Texas. In that sense, the geographic system is a modified version of open admissions. The resort to geography also represents a retreat in principle if you believe, as some assert, that colleges have not merely the unique right but the duty to create diverse student bodies, because the diversity itself, as defined by the college, is a valuable part of the educational experience.

But if in fact the proxy turns out to get the job done without explicit resort to race or ethnicity, whose use the courts are narrowing anyway, why not? This is a case in which society has contradictory interests -- to maximize opportunity for members of minority groups even as it seeks to obliterate the use of minority-group membership as a criterion in admissions, hiring and other such decisions that help determine opportunity. A system that serves both interests at once can't be all bad.

A geographic system needs to be monitored to reduce its rough edges. The percentage of high school graduates automatically eligible needs to be attuned to the capacity of the higher education system so as not to take up all the available slots; sufficient room needs to be left for an equitable share of the capable applicants who finish lower down in stronger schools. And if, as the New York Times reported the other day from Texas, a side effect of a geographic system is to create a new incentive for the state to improve the quality of its weaker high schools, well, that can't be bad, either.