The fate of the West in the 21^st century may depend on how well its nations integrate ambitious people from the rest of the world into its fold. No advanced Western country—not even America—produces enough children to keep itself from becoming a granny nation by 2050. So unless indigenous birth rates rise beyond pattern and probability, only immigration—and the industry and energy these newcomers and their children bring—can provide the spark to keep Western societies vital and growing.

We see the dynamism of immigrant culture already before our eyes. Many of the most bustling sections of Western cities today, from Belleville in Paris to the revived communities along the 7 train in Queens, are precisely those dominated by immigrant enterprise. Sergio Muñoz, a Mexican journalist and a long-time resident of Los Angeles, calls what is happening in these and so many other places "the multiculturalism of the streets." These are the true laboratories of successful ethnic integration—a form of multiculturalism that takes place through face-to-face contact, informal cultural exchange and, above all, capitalist commerce.

This "multiculturalism of the streets" differs enormously from the political variety of multiculturalism taught in ethnic studies programs or embraced by governments in racial quotas and "official" Islamic councils. It is also very different from the futile French cult of enforced secularism, which denies ethnic differences and bans individual expression such as the cross, kippah or headscarf. Whenever multiculturalism is formally enforced or officially banned, it distorts natural impulses to ethnic association and invariably causes problems. This is particularly true when the chance to operate a street-level economy is stifled by state intervention—through taxes, labor regulations, certifications—as it is in much of western Europe.

Here in America, as well, we have distorted the benign multiculturalism of the streets in other ways, through militant ethnic studies programs at many American universities, racial quotas and sectarian politics, all of which are associated with the Left and with parts of the Democratic Party. The cadences of America's culture wars being what they are, such manifestations of institutional multiculturalism have evoked dire warnings from the Right about the dangers to national unity posed by our increasingly diverse population. These concerns, raised in works such as Samuel Huntington's Who Are We? and Victor Davis Hanson's Mexifornia, focus primarily on ideological and linguistic perspectives. Huntington worries about the future of Anglo-Saxon democracy and fears that our newcomers—whom he calls ominously a "migrant tide"—will become part of "a continuous Mexican society from the Yucatan to Colorado." Hanson focuses largely on the Hispanic population in places like his rural homeland near Fresno, California. He plays back the pronunciamentos of some Latino...