From the time of Columbus to the late 19th century some 12 million black Africans were shipped across the Atlantic. Those who survived were unloaded by the Portuguese, Dutch, British, French and Spanish in various ports in the Americas, and the "pieces" (as the Spanish called the slaves) were put to work. Up to 20 hours a day.

Diaspora: a people dispersed. What would the future bring for a slave whose chain was unlocked by an Englishman in Barbados as opposed to one who first saw the sun in Portuguese Brazil? Would their descendants feel any rapport with each other? In what conditions are these descendants living in their different parts of the world today? How has the black diaspora changed these societies and their cultures - for better and for worse?

Ronald Segal, who attributes his interest in race to being a white South African and a Jew, first thought up this huge project in the early 1960s. At that time he suggested it to someone else, who told him, "Such a book needs 100 scholars working for 20 years." In 1987 Segal decided to write it himself. We have been fortunate. I know of no other book on slavery that spans such a geographical spread. (Lerone Bennett's classic Before the Mayflower (1962) focuses on North America.) It is dense with detail but eminently readable. The account of the appalling black oppression and suffering is delivered without sentimentality. And it brings us right up to the present: the chapter on Haiti ends with the return of Aristide in October 1994.

Because it is comparing different societies, the book is organised in short chapters. Although no colonising power comes out looking much better than another, conditions varied hugely in different locations, with social implications that would shape national histories forever. In the Cuban sugar industry, it was accepted that slaves needed only four hours of sleep a night. "Brutality," writes Segal, "would have an influence on Cuban social and political development long after the abolition of slavery in 1886." Until the 18th century Portuguese emigrants to Brazil were almost all men; consequently, African slave women were made into concubines and wives with the blessing of the law. Because miscegenation often meant freedom for the mothers and children, by the beginning of the 19th century a considerable portion of the black and mulatto population was free.

ÁñÁ«ÊÓƵ

But those slaves, in Brazil or elsewhere, who challenged the iron-clad system of white supremacy risked the most gruesome tortures: lashing to death, breaking on the wheel, castration. "A Negro is not a Man", John Locke argued in 1690, in his "Essay Concerning Human Understanding." The notion of white supremacy was so all-inclusive that, inevitably, many blacks themselves believed it. And though laws were gradually changed, the racial hierarchy never did. Emancipation, Segal shows, merely brought with it a different set of chains.

In a marvellous section called "Travels in the Historical Present", Segal discusses the conditions and achievements of blacks today - in the Americas, in Britain and Canada. If this is the liveliest part of the book, it is because Segal steps outside libraries to observe for himself, and brings himself modestly into the narrative. We see him, the roving journalist-scholar, talking to taxi drivers, watching television, interviewing local intellectuals and business people, reflecting on the ways the past has influenced the present.

ÁñÁ«ÊÓƵ

Christopher Columbus waxed lyrical about the island he named Hispaniola, writes Segal. "The rivers could scarcely be counted, the trees reached to the stars, and the nightingales were always singing." Today, Segal observes, it is the island's open sewers that can scarcely be counted; there are few trees left; and Haiti, once the richest of colonies, is the poorest country in the hemisphere with disparities of income among the widest in the world.

In the 1990s, Fort-de-France, the capital of Martinique, is choked with cars, a surprising number of which are Mercedes-Benz, owned by a French-speaking elite whose lavish consumption is supported by French subsidies. Martinique lacks any sense of its own identity. Creole, the living language of the people, is disparaged (here and elsewhere) as the speech of the uneducated poor.

The black inhabitants of the favelas (slums) of Brazil consume television soaps as avidly as people do elsewhere in the world, and yet, Segal observes, the actors whose dramas they follow "could scarcely be whiter". He was forcibly struck, on his travels to Brazil, by the invisibility of blacks in most domains of Brazilian life. Successful blacks or mulattos have been "effectively bleached".

What about North American cities since the civil rights movement clamoured for integration? Today, Detroit's empty skyscrapers look like "monolithic gravestones to the death of a city". After the severe race riots in that city in the late 1960s, the middle class stampeded to the suburbs. Detroit is now without a downtown shopping area, without a subway system (the plan was vetoed as too dangerous), and without so much as proper bus transport. The million or so left in the inner city, half the number once living there, are 80 per cent black. The predictions Segal made about the evolution of American cities in his book The Race War, written 30 years ago, have proved depressingly accurate.

ÁñÁ«ÊÓƵ

Although this unending narrative of brutal and insidious racism from the African slave trade to the present time is intensely depressing, the spirit that shines through The Black Diaspora is uplifting. Ultimately, we, along with Segal, are awed "less at the depths which humanity has plumbed than at the heights its spirit can scale". The moral vision evident in the book, the obvious personal and political commitment underpinning Segal's writing, is admirable.

As a Jew growing up in South Africa at a time when the Afrikaner Nationalist Party was declaring its affinity with Nazism, Ronald Segal experienced his racial-cultural identity as both privileged and precarious. It made him acutely aware of the determining factor of race and led him, from his student days at the University of Cape Town onwards, to become an activist. He was sufficiently outspoken to be stripped of his passport and subjected to frequent death threats. After the 1960 Sharpeville massacre, he drove across the border with the African National Congress leader Oliver Tambo and continued the struggle from England. "A people with a past infused by oppression and suffering is charged with a special responsibility," he believes, "to remember and remind."

Segal agrees with Cornel West, a prominent black intellectual at Harvard: "If black folk cannot love themselves, there will be no black freedom." But there is not much of Martin Luther King's dream left in today's black ghettoes. And Segal sees little future in the current trend - he calls it "Afrocentrism" - which "ascribes an African genesis to virtually anything of value in white western culture." Surely, encouraging black people "to love an idealised past instead of themselves" is merely a way to a new captivity, writes Segal.

Idealising the African past means closing one's eyes to what came after: the extraordinary achievements of the black diaspora and the "epic quality" of its history. The descendants of black slaves who are scattered throughout the world today have much more to be proud of than their African heritage. The black diaspora is about struggle, courage and achievement. It is about a "commitment to endure and resist, surmount and create, and above all else to be free".

ÁñÁ«ÊÓƵ

Hazel Rowley is a senior lecturer, Deakin University, Melbourne, Australia.