One wonders what Martin Luther King Jr. would have made of last week's event at the Lincoln Memorial. The March on Washington for Jobs and Freedom of Aug. 28, 1963, is remembered partly for his "I Have a Dream" speech. Could he have dreamed that within two generations a black American president would stand at the lectern to praise his vision? And how far would that remarkable fact atone, in his view, for the country's lingering failure to give black Americans their full measure of economic and social equality?

U.S. President Barack Obama has often emphasized the paradox. The country that elected him to its highest office is still divided by race — culturally, socially and economically.

There has been progress, and not just in attacking legally sanctioned discrimination. Physical segregation has gently declined, decade by decade. Blacks go to college, advance to the middle class and hold political office in vastly larger numbers than in the 1960s and 1970s. Yet the black-white gap on measures such as income, employment and intergenerational social mobility persists, and in some respects may have worsened in recent years.