Human migration is as old as history. Even migration to distant places and remote cultures is nothing new. In the nineteenth century, millions of Europeans sought liberty and prosperity in the Americas, notably in the United States. What is new today is the scale of migration, often across huge cultural divides - and often without a definite aim.
The African boat people in the Mediterranean are often not even sure whether they want to be in Italy, Germany, or Britain. Even those who are certain, like North Africans in Spain and France, or Turks in Germany, had as their priority escaping the hopelessness of their home countries, not arriving at a particular destination.
This modern form of migration raises massive problems for countries on the receiving end. In Europe, it is probably the most serious social issue today, because no one has a clear idea about how to manage the resulting clash of cultures.
Once upon a time, North America, notably the US, seemed to provide the answer. It was that of the "melting pot": different peoples made their own contribution to American culture, but, above all, they made every effort to accept what they found and integrate. "No," the Russian woman who came to the US in the early twentieth century replied to the grandchild who asked whether her ancestors arrived with the Pilgrims on the Mayflower. "Our ship had a different name, but now we are all Americans."
More recently, this has changed, giving rise to a process described by Arthur Schlesinger, the historian and former aide to President John F. Kennedy, in his book The Disuniting of America . No longer are all US citizens Americans. They have become hyphenated Americans: Italian-Americans, African-Americans, Hispanic-Americans, and so on. The ingredients of the melting pot are separating.
Even in Israel, the last true immigration country - at least for Jews - assimilation is no longer so easy. Recent newcomers from Russia have their own political party, and old Europeans have become a distinct minority.
Israel and America continue to have mechanisms to integrate new migrants. Language is an important underlying factor, and in Israel, there is the army, while in America, the values embodied in the Constitution still represent a shared secular faith.
But these mechanisms are weakening everywhere, and are virtually non-existent in European countries. Modern societies are characterized by acute problems of belonging. They don't offer the implicit, unconscious ties of community that citizens felt in the past. As a result, people have begun to cling to other, more primordial group identities. They resist assimilation, fearing that it will rob them of their identity without offering a new one.
What then is the alternative to assimilation? The "salad bowl" of so-called multiculturalism is no real alternative, because it does not provide the necessary glue that binds communities together. All the ingredients remain separate from the outset.
The only viable alternative for which there are examples is probably that of London or New York. The main characteristic of this alternative is the coexistence of a common public sphere shared by all and a considerable degree of cultural separation in the "private" sphere, notably in residential areas. The public space is multicultural in terms of people's backgrounds, but is governed by agreed values, even a common language, whereas the people's private lives are - to use an ugly word - ghettoized.
In theory, this is a distinctly second-best solution to the cultural consequences of migration; in practice it is the best answer we have. But it cannot be had for nothing. Even the necessary minimum of a common language requires a deliberate effort, to say nothing of certain rules of behavior.
Living in London, I marvel at the way in which we Londoners have come to terms with Indian family shops and West Indian-run public transport, while not asking many questions about whole districts that are Bangladeshi or Chinese. No one has yet found a name for this new version of the "separate but equal" doctrine that some of us fought so hard against in the 1960's: separate private lives in a common public space that is equal for all.
This is clearly easier in London and New York than it is in smaller towns or even in the capitals of countries where the world language of English is not spoken. Berlin's Turkish community and the North African communities around Paris seem increasingly separate, with their own public sphere and often language. Where this happens, an explosive condition can arise, a kind of separatism within, not by historically separate groups but by newcomers against natives.
If we are forced to abandon the hope of assimilation, our efforts should concentrate on creating a public space to which all contribute and that all enjoy. Ideally, this should be an expanding public space, for in the end, the element of unity in a modern society is the guarantee of its citizens' liberty.