The quote mentioned in the episode about immigration in the early 19th century:

“Entering the United States, these immigrants were as free as air, free as the Indians in their forest heyday, for no papers were required of them, there were no regulations to bind them, there were no privileges of birth, no tithes, no guilds. No one asked them even to be naturalized and there was work and abundance for all, for the labour-shortage was acute, with turnpikes and canals a-building, and presently railroads. The nation was growing by leaps and bounds, and cities sprang up overnight, while the older towns could scarcely meet the demand for new streets, houses, docks and stores. For these were the days of Andrew Jackson, and a fury of energy drove the people, who felt that the nation belonged to them at last. Travellers observed that Americans lived twice as much as other folk and accomplished twice as much in the span of their lives, for they plunged into the stream of enterprise in their early teens; and David Crockett’s “Go ahead” had become a national slogan that often omitted [his] words “Be sure you’re right.””

From The World of Washington Irving, by Van Wyck Brooks.