We gather here to mark the global trade that took some 15 million beloved human beings across the Atlantic in the hulls of barbaric ships, the largest forced migration in the history of the world, one that would reshape the entire Atlantic world and transform the global economy. We must never forget the scale and the depth of the horrors that people of African descent suffered in the name of profit, profit that enriched the European colonial powers and built the nascent economy of the United States. We must never forget how the systems of slavery collapsed, only to be reborn in other models of violent and racist economic exploitation, such as what we benignly call Jim Crow in the United States, but what is more aptly called apartheid.

At this place, liberty and life were stolen and sold. Human beings were delivered and sorted and weighed and branded with the marks of commercial enterprises and loaded as cargo on a voyage without return. One of the largest migrations of history was also one of the greatest crimes of history. Below the decks, the middle passage was a hot, narrow, sunless nightmare; weeks and months of confinement and abuse and confusion on a strange and lonely sea. Some refused to eat, preferring death to any future their captors might prefer for them. Some who were sick were thrown over the side. Some rose up in violent rebellion, delivering the closest thing to justice on a slave ship. Many acts of defiance and bravery are recorded. Countless others we will never know. Those who lived to see land again were displayed, examined and sold at auctions across nations in the Western Hemisphere. They entered society indifferent to their anguish and made prosperous by their unpaid labor.

The Great Migration in particular was not a seasonal, contained, or singular event. It was a statistically measurable demographic phenomenon marked by unabated outflows of black émigrés that lasted roughly from 1915 to 1975. It peaked during the war years, swept a good portion of all the black people alive in the United States at the time into a river that carried them to all points north and west.

Over the course of six decades, some six million black southerners left the land of their forefathers and fanned out across the country for an uncertain existence in nearly every other corner of America. The Great Migration would become a turning point in history. It would transform urban America and recast the social and political order of every city it touched. It would force the South to search its soul and finally to lay aside a feudal caste system. It grew out of the unmet promises made after the Civil War and, through the sheer weight of it, helped push the country toward the civil rights revolutions of the 1960s.

That it was precisely productive forces that left is not at all unique. It happens in all migrations. If you look at the period between 1500 and 1900, when that Atlantic slave trade took place, many more people left Europe exactly in the same age range, without us in Europe complaining about losing productive forces.

They traveled deep into far-flung regions of their own country and in some cases clear across the continent. Thus the Great Migration had more in common with the vast movements of refugees from famine, war, and genocide in other parts of the world, where oppressed people, whether fleeing twenty-first-century Darfur or nineteenth-century Ireland, go great distances, journey across rivers, desserts, and oceans or as far as it takes to reach safety with the hope that life will be better wherever they land.

Vast migrations of people -some voluntary, most not- have shaped the human condition. More of us flee from war, oppression and famine today than at any other time in human history.

There was an outflow of people from India before the fifteenth century BC.

So we were launched by the Elizabethan seamen upon our long career of adventure across the seas: adventure leading to discovery, discovery to exploration, exploration to colonisation and settlement, and so to Empire. So that you might say the British Empire was a product — almost a by-product — of adventure. It certainly was not planned; it came into being naturally, gradually, as the result of the adventurous spirit, the stout heart and courage of our forefathers looking for a livelihood and an outlet for their energy and spirits across the ocean.

Over the decades, perhaps the wrong questions have been asked about the Great Migration. Perhaps it is not a question of whether the migrants brought good or ill to the cities they fled to or were pushed or pulled to their destinations, but a question of how they summoned the courage to leave in the first place or how they found the will to press beyond the forces against them and the faith in a country that had rejected them for so long. By their actions, they did not cream the American Dream, they willed it into being by a definition of their own choosing. They did not ask to be accepted but declared themselves the Americans that perhaps few others recognized but that they had always been deep within their hearts.

In 1615 the British Isles had been an economically unremarkable, politically fractious and strategically second-class entity. Two hundred years later Great Britain had acquired the largest empire the world had ever seen, encompassing forty-three colonies in five continents.

It is the end of the British Empire.

They did what human beings looking for freedom, throughout history, have often done. They left.

They did not ask to be accepted but declared themselves the Americans that perhaps few others recognized but that they had always been deep within their hearts. NOTES ON METHODOLOGY I began this work because of what I saw as incomplete perceptions, outside of scholarly circles, of what the Great Migration was and how and why it happened, particularly through the eyes of those who experienced it. Because it was so unwieldy and lasted for so long, the movement did not appear to rise to the level of public consciousness that, by any measure, it seemed to deserve. The first question, in my view, had to do with its time frame: what was it, and when precisely did it occur? The Great Migration is often described as a jobs-driven, World War I movement, despite decades of demographic evidence and real-world indicators that it not only continued well into the 1960s but gathered steam with each decade, not ending until the social, political, and economic reasons for the Migration began truly to be addressed in the South in the dragged-out, belated response to the Civil Rights Act of 1964. The second question had to do with where it occurred. The migration from Mississippi to Chicago has been the subject of the most research through the years and has dominated discussion of the phenomenon, in part because of the sheer size of the black influx there and because of the great scholarly interest taken in it by a cadre of social scientists working in Chicago at the start of the Migration. However, from my years as a national correspondent at The New York Times and my early