Quote from The Long Nineteenth Century by Blackbourn

...Emigration was not unknown in earlier years, Germans having settled for centuries in eastern and southeastern Europe, and on the eastern seaboard of America. It was the volume that was unprecedented. The numbers began to rise in the 1840s, and the first wave of emigration saw 1.3 million people leave the German states between 1845 and 1858. In the peak year of 1854 the total reached nearly a quarter of a million...But who were the emigrants, why did they leave, and where did they go?...Most went for material reasons, the emigration statistics providing a yardstick of economic distress in the German regions. The first wave was made up disproportionately of small peasants and craftsmen from the overpopulated rural areas of partible inheritance in the southwest...the greatest volume of emigration occurred during relative upturns, after the opportunity had been taken to pay off debt and land could be sold on a more buoyant market. Hance the peaks in the 'good' years of 1854 and 1867, and the fall-off in numbers after the onset of crises in 1873...Of the roughly 4.5 million Germans who left their homeland between 1847 and the First World War, almost 4 million went to the United States...In the earlier stages, when the America-bound came from the southwest or west, Antwerp and Rotterdam handled much of the traffic. Later, as emigrants came increasingly from the north, Hamburg and Bremen became the great entrepôts of this human cargo...The communications revolution...made possible the letters home and the financial remissions of those who had already gone. The letters encouraged others to follow by painting an idealized picture of a land of opportunity; the prepaid ticket brought the opportunity within reach. By the 1880s the 'uncle in America' was a familiar figure...

Blackbourn is very readable and has a great deal more to say than what I have excerpted here. I can't recomend this book enough. The quotes above were taken from the chapter Economy and Society Transformed the section entitled Emigration, Migration, Urbanization, pages 191-207. He goes on to discuss the movement of people from the rural landscape to the cities in a most informative manner. Blackbourn paints a vivid picture of the conditions these emigrants were fleeing. Understanding the communities they established in the US and elsewhere and the patterns they followed yeilds great insight into the cultural heritage of anyone whose ancestors came to the US during those years between 1840 and 1880.