The Continent's Muslim population, however, represents a stark exception to this trend. Europe's Muslims are considerably younger than its non-Muslims, and their overall birth rate is roughly three times as high. Small wonder that by 2015, when Europe's population will have fallen by roughly 3.5 percent from the present level, the Muslim population is expected to have doubled—and that's assuming no significant increase in the conversion rate on a continent where Christianity's decline in recent decades has left an unprecedented religious vacuum.

In addition, about 900,000 immigrants reach European shores each year—and Muslims from the Middle East and North Africa make up an increasing proportion of these new arrivals. So far this influx has been driven largely by conditions in the Muslim world, but it is likely to be amplified by a demand for immigrant workers in European nations in the decades ahead, as the graying of Europe's population puts an increasing strain on its welfare-state economy. According to a 2000 UN projection, the EU states would need 949,000 immigrants a year to maintain their 1995 population, 1,588,000 a year to maintain their 1995 working-age population, and a stunning 13,480,000 a year to maintain the 1995 ratio of working-age to retired residents.

Relying too heavily on demographic projections can be a fool's game, of course; recall that the feared "population bomb" of the 1970s never detonated. But integrating even existing Muslim minorities already poses a serious cultural and political challenge for European society. The relatively small size of the Islamic population is counterbalanced by its concentration in a few Western European nations—particularly France, Germany, and the Low Countries—and by the tendency of Muslim immigrants to cluster around large cities (two fifths of Britain's Muslims live in greater London, and a third of France's Muslims in the Paris area), often in neighborhoods and vast housing projects that are culturally and economically isolated from the wider society.

The inhabitants of these Muslim ghettos are vulnerable to the woes that usually afflict marginalized populations: high crime rates, poor education, rampant unemployment. And this, in turn, may make them receptive to the lure of radical Islam. Only a small minority of European Muslims are hard-core Islamists. But the radical strain in what Olivier Roy, of the French National Center for Scientific Research, calls "EuroIslam" is potent enough to make Europe both an ideal training and staging ground for terrorism (since 9/11 European authorities have reportedly arrested roughly twenty times as many terror suspects as the United States has) and a target of opportunity for would-be jihadis (as in last spring's Madrid bombings). Unsurprisingly, these factors also tend to spur anti-Muslim, anti-immigrant backlash—exemplified by the high vote total earned by Jean-Marie Le Pen's quasi-fascist National Front in France's most recent presidential election.