AS Leopold plies Paris' streets in his dark gray Renault, the Senegalese cab driver who came here 16 years ago says he can't complain about life in France.And yet, the ebony-skinned father of two feels, "It's no exaggeration to say things were better here for black people 10 years ago. It's just a fact." "We get lumped in with the Arabs, and the Arabs get lumped in with everybody else from the underdeveloped world," Leopold says. "People don't like us because we're a reminder that there are a whole lot more like us - hungry and desperate for work - where we came from." As the issue of immigration has climbed to the top of Europe's political agenda, the question of immigration from the South, especially Africa, has emerged as both the hottest aspect of the debate and the least amenable to solutions. Simple numbers help to explain why. While the population in Eastern Europe is either stable or falling - Hungary's population, for example, shrinks by about 20,000 a year - very much the opposite is true in the South. Africa's population is growing at the rate 3 percent a year, generally outstripping economic growth. The average African woman still has six children - double the average for Latin America, and even higher by a third than the Asian average. The North African countries alone, which counted 62 million people in 1950, should grow to 285 million by 2025. And with poor local economies and 2 million people of North African birth already living in Europe (not counting Europeans of North African descent), the pattern for continued northward migration is set. Although the opening of Eastern Europe since 1989 has raised new concerns about European immigration, it is immigration from the South that most starkly poses the question of the West's responsibilities toward the world's poorer regions - an especially wrenching issue for Western Europe because so many of the third-world emigrant countries are former European colonies. Perhaps most troubling of all, immigration from the South has forced to the surface difficult questions about the ultimate compatibility of widely differing cultures. As challenging as accommodating tens of thousands of immigrants from Eastern Europe - or even hundreds of thousands from the Soviet Union - may seem to some Europeans, many experts say similar cultural and religious backgrounds spanning Europe's two halves will facilitate their assimilation. The same is not true for immigrant populations from the South. "Immigration from third-world countries will continue to increase for the simple reason that people have no other choice," says Karin Konig, director of the Committee to Support Politically Persecuted Foreigners, a private immigrant support organization in Vienna. "But that is going to mean growing problems of the kind we see across Europe - the street battles, and crime, and ghettos, the xenophobia and far-right political movements - because the cultures involved are so different." S. Konig notes that her organization, set up in 1986 primarily to assist Iranians, has seen its clientele shift to a majority of southern immigrants, largely from Africa. She expects an accentuation of that trend, she says, "because I don't see any prospect that the economies in those countries will improve drastically, and that's what it would take." In fact, as Western Europe has shifted its attention to Eastern Europe, a growing chorus of third-world leaders has stepped up warnings of mass South-to-North migrations if the South's economic difficulties are not addressed. "You risk being invaded tomorrow by multitudes of Africans who, pushed by misery, will land in waves in the countries of the North," Senegalese President Abdou Diouf recently told Le Figaro, a French daily. "You can pass all the immigration legislation you want, you won't stop the tide." The Mediterranean Sea, which at its narrowest point separates North Africa from Europe by only 12 miles, "is right now becoming our Rio Grande," says Nadji Safir, a fellow at National Institute for Strategic Global Studies in Algiers, a presidential think tank. Either Europe "transfers a part of its wealth to the South, or inevitably people will go North to get it." That argument is echoed by President Diouf, who says it is in Western Europe's interest to aid Africa's development. Europe's governments and industries have no choice but to create the conditions allowing Africa's masses to stay at home "so they don't come to assault you on your continent," he says. Noting that Algeria's troubled economy would have to create millions of jobs by the year 2000 to satisfy its youthful population, Mr. Safir says, "It is not with symbolic action that the problems of this magnitude will be addressed." Still, no one believes that even massive economic aid to the South will stop migration northward. "The traditional argument has always been that if you help them realize strong economic growth, foreigners will stay home," says Gudrun Biffl, a labor economist at the Austrian Institute of Economic Research, a government organization in Vienna. "But if you take the case of Asia, you see there has been 8 to 10 percent economic growth, [in] 'miracle' countries like Korea and Thailand, but still they have emigration," he says. What is needed from Europe, according to Dr. Biffl, is a strategy that links investment in the South with population control measures; and in Europe, integration programs to promote social harmony should be coupled with employer sanctions and tough border controls that "do work as a brake." Yet even with that, she says, "you can't expect dramatically quick results, but have to realize it will take quite a long time" to bring together such factors in the South as reduced fertility, economic growth, and skills development, to reduce the prob-lem. Other experts say that if Europe is serious about stemming Southern migration, it will take a change in thinking about trade that accepts a further shift of certain industries to the South, and a reevaluation of prices for the raw materials and agricultural products that are so much of the third-world's wealth. But this will take time, and meanwhile, immigration from the South will continue. The challenge for governments and public officials will be to make the bumpy road to a multicultural European society as smooth and accident-free as possible.