"The old doctrine that the slavery of the black, is essential to the freedom of the white race, can maintain itself only in the presence of slavery, where interest and prejudice are the controlling powers, but it stands condemned equally by reason and experience. The statesmanship of to-day condemns and repudiates it as a shallow pretext for oppression. It belongs with the commercial fallacies long ago exposed by Adam Smith. It stands on a level with the contemptible notion, that every crumb of bread that goes into another man's mouth, is just so much bread taken from mine. Whereas, the rule is in this country of abundant land, the more mouths you have, the more money you can put into your pocket, the more I can put into mine. As with political economy, so with civil and political rights."This also echoes the views of both Thomas Malthus and Thomas Jefferson on population in the United States (notice the phrase "the more mouths you have"). Both (Malthus in later editions of his Essay on Population, and Jefferson in a letter to Jean Baptiste Say) were of the view that limitations on population in the form of subsistence wages were not in play in the United States in the same way that they were in Europe, and that this allowed for general opulence and growth. The key, of course, is abundant land. Data from the United States played an important role in proving Malthus's theory, demonstrating that the controlling factor in population was the capacity of a society to produce food.
I find the last sentence interesting from a rights theory perspective too.