If economics do not offer great reasons to halt immigration, what about culture? Consider Los Angeles. No city has felt the impact of immigration more. Almost four out of every ten new immigrants went to California during the 1980s, and the largest share of these ended up in Southern California. They have transformed the region. In 1970, 4.7 percent of the American population was foreign-born. Today at least 8 percent is. In L.A., the proportion is about one-third.

As every traveler knows who has landed at the airport and set out for Disneyland in a rental car, L.A. on a map is a mosaic of pink, yellow, tan, and blue blobs strung together by a web of freeways. Today, those colored shapes represent not merely civic boundaries, but ethnic enclaves. L.A. is no longer the creamy city of the Beach Boys and endless identical suburbs. The texture is grittier, lumpy even.

While working on this article I set out in my rental car one morning from the posh, Anglo city of La Canada, crossing into Latino and African-American northwest Pasadena. Driving east across Lake Avenue, I entered an Armenian zone, then turned south to reenter another Anglo area. I passed through Monterey Park, a majority Asian (mostly Chinese) city, then into solidly Latino neighborhoods in Montebello. Each community I drove through has its own grocery stores (these are not mere corner grocery stores, but ethnic supermarkets), its own newspapers, its own politics.

You have a choice of how to read this polyglot experience. You can cruise the roadways watching the signs change from Chinese to English to Spanish to Hindi and think you are on a toy-sized global holiday, a Disneyland-esque "Small World." Or, you can think of Beirut. Is this, as some charge, a new tribalism? ...