CENTRAL EUROPE is now anxiously facing the prospect of a great surge of population westward out of the Soviet Union. The Poles, accustomed to comparing their condition with Western Europe's and thinking of themselves as poor, have begun to realize that they are living well compared with the Soviets. Germany, rich by anybody's standard, is using its wealth to rebuild its eastern provinces and would not welcome the distraction of a wave of new arrivals. Suddenly Europe finds itself facing the same kind of population pressure on its eastern borders that, for example, the United States feels on its Mexican border.

A Soviet official, Vladimir Scherbakov, told a conference on migration last week that some 5 million Soviets had indicated a desire to leave the country, and by his estimate perhaps a third of them will actually go through with it. The possibility of a million and a half people fleeing westward from a collapsing economy and ethnic violence is enough to make any European minister of housing wince. As of the beginning of this year, Soviet law guarantees its citizens the right to depart at will, and a further law to accelerate their actual access to passports is to come into effect before the year's end. If Europe does not prepare to receive these people in an orderly way, Mr. Scherbakov coolly observed, it will have to deal with an increase in illegal immigration -- raising, he could have added, all the resulting moral perplexities with which the United States has become very familiar.

In the past couple of years, there have been two principal streams of emigration out of the Soviet Union. There have been Soviet Jews and, less widely noted, the ethnic Germans taking advantage of Germany's offer of citizenship to anyone of German descent. The next wave promises to be much more diverse and, in social terms, harder to absorb.

After decades of reproaching the Soviet Union for its closed borders and its harshly repressive restrictions on its citizens' freedom to travel, the West is now seeing those borders suddenly open and an uncomfortably large number of people preparing to pour across them. The Soviet people's wider freedom is greatly to be welcomed, but most of Central Europe hasn't the resources to deal with a flood of refugees. Massive migration would not necessarily be the most dangerous or dramatic consequence of deepening economic failure in the Soviet Union, but it would mean serious trouble for countries such as Poland that are themselves struggling to find their footing in a new world.