The life of the immigrant was that of a man diverted by unexpected pressures away from the established channels of his existence. Separated, he was never capable of acting with the assurance of habit; always in motion, he could never rely upon roots to hold him up. Instead he had ever to toil painfully from crisis to crisis, as an individual alone, make his way past the discontinuous obstacles of a strange world.
But America was the land of separated men. Its development in the eighteenth century and the Revolution had set it apart from Europe; expansion kept it in a state of unsettlement. A society already fluid, the immigrants made more fluid still; an economy already growing, they stimulated yet more rapid growth; into a culture never uniform they introduced a multitude of diversities. The newcomers were on the way toward being Americans almost before they stepped off the boat, because their own experience of displacement had already introduced them to what was essential in the situation of Americans.
—Oscar Handlin in The Uprooted: The Epic Story of the Great Migrations That Made the American People