IT IS ALL astonishing. In a country long devoted to dulling the sense of the historical past and denying the continuity of experience from Europe to America; in a country where the young can hardly remember the name of Franklin Delano Roosevelt and are by no means sure in which century World War I was fought or who fought in it - in this very country groups of people now seek to define themselves through a deliberate exculsion from the dominant native stock which, only yesterday, had been taking pains to exclude them. These ethnic groups now turn back - and, as they nervously insist, "with pride" - to look for fragments of a racial or national or religious identity that moves them to the extent that it is no longer available. Perhaps, also, because it is no longer available.

Some of this turning back strikes me as last hurrah of nostalgia. Each day, necessarily, it keeps getting weaker and siller. Some time ago, I attended a pageant in an Eastern city recreating the Lower East Side: pushcarts, onion rools, flexibly priced suits, etc. Someone asked me whether anything was missing and I answered, not very graciously, that a touch of reality might have been added by a tubercular garment worker spitting blood from his years of exhaustion in a sweat shop.

Sentimentalism is the besetting sin of the Jewish turn to enthicity, a sentimentalism that would erase memories of ugliness and pathology, disputation and radicalism. Among the blacks, things are different. Having been deprived of their history in more brutal ways than anyone else in our society, they have to engage in more extreme measures to retrieve it.

Still, one wonders whether some recent assertions of roots are a conquest of history or an improvisation of myth. Tens of thousands of black Americans are expected to be visiting Gambia this summer and, while that is likely to be a boon for the tourist industry of a country that needs every break it can get, one is less certain about what it will do for the tourists. Will it lead to a growing moral strength with which to confront American realities or will constitute a pleasant style of evasion?

Still, I would be the last to deny that there are serious meanings behind ethnic nostalgia. We are all aware that our ties with the European past grow increasingly feeble. Yet we feel uneasy before the prospect of becoming "just Americans." We feel uneasy before the prospect of becoming as indistinguishable from one another as our motel rooms are, or a flavorless and mass-produced as the bread many of us eat.

THE FAMOUS melting pot of American society could grow very hot - indeed, too hot for those being melted. Usually, it was the immigrants and their children who were the meltess, while the temperature was being regulated by the WASP melters. So, by now, many of us are righly suspicious about easy notions concernings cultural assimilation, what might be called the bleaching of America.

Some of us remember with discomfort our days in high school, when well-intentioned but willful teachers tried to smooth the Jewish creases out of our speech and psyches. We don't want to be smoothed out - at least entirely, at least not yet. We don't want to yield outselves completely to that "destruction of memories" which the great sociologist, W.I. Thomas, once said was the essence of the Americanizing process.

Neither should we succumb to the current uncritical glorification of ethnicity. The ethnic impulse necessarily carries with it dangers of parochialism: the smugness of snug streets as against the perilous visions of large cities, the indulgent celebration of hibitual ways simply because they are habitual. The ethnic community always runs the danger that it is not really perserving the riches of an old-world culture; it is merely clinging to some scraps and debris of that culture which were brought across the ocean. At a time when the fate of mankind is increasingly, for better or worse, an international fate, the ethnic community too often shuts its eyes or buries its heads while clinging anxiously to received customs - as if there were no more important things in the world than customs!

The great weakness of the turn to ethnicity is that it misreads or ignores the realities of power in America. The central problems of our society have to do not with ethnic groupings, but the economic policy, social rule, class relations. They have to do with vast inequities of wealth, with the shameful neglect of a growing class of subproleterians, with the readliness of policy-makers to tolerate high levels of unemployment. They have to do with "the crisis of the cities," a polite phrase masking a terrible reality - the willingness of this country to dump millions of black (and white) poor into the decaying shells of once thriving cities.

Toward problems of this kind and magnitude, what answers can ethnicity offer? Very weak ones, I fear. Common action by the poor, major movements for social change require alignments that move past ethnic divisions. They require a tougher perception of the nature of American society than the ethnic impulse usually enables. The dominant powers of American society would be perfectly delighted if, for example, American blacks were to divert themselves out the next 10 or 20 years in seeking their roots in distant Africa, especially if this for the jobs, the housing, the opportunites they need here. (Who can say with any assurance that the vivid, if coarse, evocation of the black ordeal in the recent TV version of "Roots" did very much, or anything at all, to persuade white Americans that this society owes a debt to its black minority?)

This is not just a problem in social strategy; it has also to do with human awarness and self-definition. We want to remain, for the little time that we can, whatever it was to remain, for the little time that we can, whatever it was that we before they started pressurizing us in those melting pots. So let's try, even if the historical odds are against us.

But there is also another moral possibility, one that we call in Yiddish being or becoming a mensch . You don't have to be Jewish (or non-Jewish), you don't have to be white (or black) in order to be a mensch. Keeping an eye upon the fading past and the other on the unclear future, enlarging ethnic into ethic, you can become a man or woman of the world, even as you remember - perhaps because you remember - the tongue your grandfather and grandmother spoke in, though in fact the words themselves are fadingfrom memory.