THE populations of the world are on the move, propelled by oppression and poverty in some countries; attracted by job opportunities or by the relatively generous welfare benefits available in the world’s richer countries; and facilitated by the declining cost of transportation, now organized by highly efficient “people-smuggling” gangs that control train, truck, bus, and shipping assets. The magnet countries are eager to welcome some immigrants, equally eager to bar others, and ambivalent about still others. So we are seeing a worldwide debate on immigration policy.