"Demography is destiny," Auguste Comte, the 19th-century French mathematician, once wrote.

European politicians may be finding this old quote relevant today. British Prime Minister Tony Blair, for one, is worried that the European Union is losing control of its external borders. This week he won agreement from his Spanish counterpart, José Maria Aznar, for putting urgent action against illegal immigration at the top of the agenda of the EU next summit, to be held in Spain in June.

"We're not advocating a 'fortress Europe,' but what we are saying is there's got to be some order and some rules brought into the system whereby people come into Europe," Mr. Blair said.

He and other mainstream politicians are eager to prevent the far right from exploiting the immigration issue. Aftershocks of strong showings by anti-immigration parties in the recent French presidential elections as well as the Dutch parliamentary elections are still being felt.

Political parties stressing stricter controls on immigration, usually on the right, have been gaining ground in most of Europe, from Scandinavia to Spain.

The trend comes as no surprise to Joseph Chamie, top demographer at the United Nations. "Replacement migration," he says, promises to be one of the "most important controversial issues of this century." His office has just released a new report on this hot topic – one that has profound implications as societies struggle with issues of declining and aging populations and related labor issues.

In most of Europe, with birthrates so low that populations are or soon will be declining, there has been much discussion about whether to admit more immigrants from Africa, the Middle East, or eastern Europe to provide the workers needed to produce goods and services for retiring western Europeans.

Without enough workers, people may have have to wait until, say, age 75 or later to retire. But some wonder whether an inflow of migrants from poor countries will cause "cultural genocide," undercutting the ideas, religions, even national identities in the countries where they arrive.

New German legislation that would open the door to more skilled foreign workers promises to be a major political issue between the Social Democratic government of Chancellor Gerhard Schröder and the opposition Christian Democratic Union at election time.

In the United States, immigration has become a livelier issue because of the Sept. 11 tragedy with its foreign suicide hijackers. America's population will continue to grow, though only because of a high birthrate among recent immigrants. Native-born women average fewer than two children each.

The UN report looks at the number of immigrants that would be needed to offset declines in total population and working-age population in eight nations as well as the European Union and Europe as a whole.

In the US, the report notes, it would take 38 million immigrants, or 760,000 per year, between 2000 and 2050 to keep the population constant. That is less than the immigration level of about 1 million in recent years. The situation is similar in Britain and France.

In Italy, South Korea, and Japan, it would take a much higher level of immigration to stabilize the general population.

To stabilize the working-age population would require an even greater number of immigrants in the industrial nations, levels probably not politically feasible.

The aging of populations raises many critical questions, the report notes. What's the appropriate age for retirement? At 75, there would be enough workers to provide the same support for retirees as today in most countries studied.

Should present retirement and healthcare benefits for the elderly be maintained? Should workers and employers contribute more to support the increasing numbers of the elderly?

Immigration to rich countries won't solve the population problem in poor countries, at least not for many decades.

Population growth in the world, most of it in developing nations, runs about 76 million a year. It will take place chiefly in cities in developing countries. Another UN population report finds that in 2007, more than half of the world's population will live in urban areas.

By 2030, the number of people living in cities will reach 5 billion, up from 2.9 billion in 2000.

The rural population of the world will remain nearly stable, at between 3.2 billion and 3.3 billion in the years leading up to 2030. Many children of farmers will head for the cities, boosting the urban population 1.8 percent a year to 2030.

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