It is not the kind of thing we Americans talk about much, but one of our oldest traditions is to fear tolerance. The habit goes back at least to the Pilgrims, who first headed for these shores not to shake off persecution, as we like to think, but to get as far as possible from tolerant people.

What we conveniently tend to forget in the Pilgrim saga is that those determined dissenters made their getaway from England to Holland, where they ran into unaccustomed acceptance. There was the rub: Many of their children, who found Dutch society attractive and open, were tempted to join it. The Pilgrims were threatened by assimilation.

As William Bradford recorded in Of Plymouth Plantation, 1620-1647 , "of all sorrows most heavy to be borne, was that many of their children, by these occasions and the great licentiousness of youth in that country, and the manifold temptations of the place, were dran away by evil example into extravagant and angerous courses, getting the reins off their neck and departing from their parents."

Enough of that tolerant society stuff. Off to the New World, where the Pilgrims could protect their religion, culture and kids, where they and many others would inflict intolerance -- and much worse -- on the peoples already there and yet to come, where group after group has similarly feared that more acceptance and success in the general society would doom its own special ways.

This persistent tension between assimilation and pluralism is at the heart of the Harvard Encyclopedia of American Ethnic Groups , a remarkable collection of 106 ethnic group histories (with 87 maps), plus 29 essays on often controversial subjects which continue to vex our days at home and abroad. o

The folks from Harvard have not given us a flawless work; the book misstates twice, for example, that the Supreme Court has requred schools to provide bilingual education to children with other languages who have little command of English. The court has done no such thing. But such errors of fact, which presumably will be fixed in future editions to satisfy those of us in the Nitpicker-Americans group, pale beside the immense value of this sweeping volume, six years in the making and the first to attempt to capture the entire mosaic of the American ethnic experience.

There's plenty here for those simply interested in their own groups' pieces of the mosaic, from 22 pages on Afro-Americans to 64 pages for the 173 American Indian nations to slightly more than a page for the little-known Zoroastrians. Southerners and Appalachians may be particularly pleased to learn that they are treated as ethnic groups, joining such other homegrown varieties as the Mormons.

There are also reams on the next guy's experiences, including intriguing material on additional little-known groups. Try, for example, the Wends (centered in Texas, originally from a land tucked in what is now East Germany), the Tri-Racial Isolates (more than 200 small groups of mixed ancestry, like the Brass Ankles and the Bushwhackers, many clustered in eastern swamps and mountain valleys), about 900 strong and living mainly in New Jersey and Pennsylvania).

Thankfully, though, the 120 contributors -- or at least the editors -- strive to avoid the these-are-a-terrific-and-proud-people type of ethnic sentimentality, and they generally succeed. Nor is the book designed to assist the politicaly active crowd that periodically pronounces the "melting pot" dead. Indeed, it includes early on a reminder that perhaps a majority of Americans, and certainly a large minority of many millions, no longer identifies with any ethnic group. They are just the "plain Americans."

Moreover, as one of the thematic essays notes, religious and racial intermarriage has been rising significantly in recent years. One might take this as a sign of reduced prejudice, as has customarily been the case, but it is another kind of tolerance that many of those pluralists devoted to preserving the ethnic group understandably would like to do without. People with dual or multiple ancestries tend to swell the ranks of the "plain Americans," that largest of groups which, ironically, we don't know all that much about.

Another entry does take on the seemingly neverending job of tryng to define just what an "American identity" is supposed to be. Although not entirely satisfying, it provides some provocative food for thought. Few realize, for one example, that the "cultural pluralism" argument from its start in 1915 has contained what is politely termed a "racialist" element, attributing "the distinctive characterists of peoples to inborn racial qualities," as the book puts it.

There is also much to chew on in other essays about controversial issues -- divided national loyalties, prejudice and discrimination, policies and prospects for languages other than English, immigration and naturalization questions, to name some -- that reamin unsettled. One of the most provocative aspects of the thematic essays is the mischief that results when ideologically opposed authors are writing on related issues.

As we continue on our way as a nation forever in search of itself, few things about the American ethnic future can be said with confidence.One is that we do not face some stark and simplistic choice between the "melting pot" and "cultural pluralism," as so many have believed. We have long had, and will long continue to have, differing degrees of both integration and separation, with each remaining a powerful force. As another essay aptly concludes, "The variety of styles in pluralism and assimilation suggests that ethnicity is as complex as life itself." Try stuffing that into a catch phrase.