This week, four presidents journeyed to Austin, Tex., to address the Civil Rights Summit and remark on President Lyndon B. Johnson’s legacy on the 50th anniversary of the passage of the Civil Rights Act.

That landmark act brought an end to legal racial segregation in public places.

But now we are facing another, worsening kind of segregation, one not codified but cultural: We are self-sorting, not only along racial lines but also along educational and income ones, particularly in our big cities.

Our cities are increasingly becoming vast outposts of homogeneity and advantage, arcing ever upward, interspersed by deserts of despair, all of which produces in them some of the highest levels of income inequality ever seen in this country.

Some call this progress; I call it a perversion, at least of the concept of diversity — of race, culture, identity and class — that dynamic engine that built urban identities and that is now being erased out of them.