Demography is destiny. The framers of the Constitution recognized this when they mandated for the first time in history that a census be conducted at regular intervals, and that representation in the lower house of Congress be based on its results. The growth that the Census Bureau has reported over the ensuing two centuries has been unique among nations: from 3.9 million in 1790, mostly clustered along the Atlantic coastline, to 50 million in 1880, 100 million in 1915, 200 million in 1967 and now 300 million in October 2006.

As George Washington expected, the United States has expanded across the continent, and even beyond. That expansion is a story with several different chapters, each one largely unexpected, and one still unfolding today. The first chapter, from 1790 to around 1840, is a story of fertility, of unparalleled natural increase. During this period, the average American woman gave birth to seven children. Not all survived, of course, but very many did.

Americans probably enjoyed the best nutrition in the world: While European peasants subsisted mainly on bread, American farmers had a plentiful supply of meat. Although there was little immigration -- never more than 79,000 a year, and as little as 6,000 -- our population increased by 28 percent to 31 percent per decade, probably the highest rate in history.

Then, in the 1840s, another chapter begins, a story of mass immigration. The potato famine in Ireland and the failed revolutions of 1848 in Germany resulted in a vast flow of immigrants across the Atlantic to the United States. In the years from 1847 to 1857, as Americans moved west and the nation grappled with the issue of slavery, 3.3 million immigrants arrived on our shores -- 6 percent of the pre-existing population.

The Civil War reduced the flow of immigration and cost the lives of 600,000 Americans in a nation of 31 million (a death rate that would translate to the loss of nearly 6 million today). The end of the war produced a third chapter, which lasted into the 1920s. Immigration continued and accelerated -- 2.7 million in the 1870s, 5.2 million in the 1880s, 3.7 million in the 1890s, 13.4 million from 1900 to 1914.

Starting around 1890, immigrants were increasingly people considered to be of a different race from most Americans -- Italians, Jews, Poles and other Eastern Europeans -- and they moved almost exclusively to big cities in the North. At the same time, even though wages were much higher in the North than the South, only a few Southerners -- black or white -- moved to the North.

Immigration was sharply reduced by restrictionist laws in the 1920s and plummeted further in the Depression decade of the 1930s, which saw the lowest population increase -- only 7 percent -- in American history. But this turned out to be a brief chapter. Contrary to almost universal expectations, post-World War II America was an era of boisterous economic growth and of a baby boom that began just after the war and lasted for about 20 years.

Americans were jostled out of their home states by the war and then continued moving, with huge flows to the West and, particularly after the civil rights movement got rid of racial segregation, to the South. California grew from 6.9 million to 10.6 million in the 1940s and 20 million in 1970; Texas from 6.4 million in 1940 to 11.2 million in 1970, and Florida from 1.9 million to 6.8 million in those years.

Demographers in the late 1960s expected the future to be like the recent past. They waited for the children of the baby boom to produce a new baby boom and discounted the likelihood of mass immigration. But a new chapter was beginning around 1970. Birth rates plummeted, and immigration -- mainly from Latin America and Asia -- surged.

The move to the South and West continued: In 2000, California had 34 million people, Texas 20.9 million, Florida 16 million. Hispanics now outnumber blacks.

Recently, we've seen signs of a new chapter. Divorce and abortion rates are down, and fertility rates are up, far higher than in other affluent nations. Democrats look for gains from Hispanics, Republicans in fast-growing exurbs (97 of the 100 fastest-growing counties since 2000 voted for George W. Bush in 2004). The lesson of the past is that America keeps changing and growing, often in ways we fail to anticipate.