In 1810, more than four in five Americans lived in one of the original thirteen seaboard states. Half a century later, though those states had grown considerably, they held less than half of the nation's population. The reason lay in the post-1815 rush of settlers beyond the Appalachians into the continental interior, "one of the great immigrations in the history of the western world." Chaotic though this movement was in many ways, it showed at least one orderly pattern. Individually these settlers followed many paths, but the typical ones moved due west, erring to the north or south only when their path was blocked by mountains or water or political boundaries or when they were pulled aside by the easier travel routes along navigable rivers. Most of the inhabitants of every inland state in i860 came from the states to the east within its own latitudes. It was mostly New Englanders and upstate New Yorkers—themselves mostly of New England origin—who occupied the territories and states bordering on British North America. They left the central and southern parts of Ohio and Indiana and Illinois mainly to settlers from the middle states and the Chesapeake. The frontier of the Deep South was colonized from the far southern coastal states much more than from Virginia or North Carolina, states that furnished Kentucky and Tennessee and Missouri with the bulk of their inhabitants. "Ohio Fever" swept the rural Northeast after 1815, followed by "Michigan Fever" in the 1830s, but it was "Alabama Fever" and "Texas Fever" that gripped the southern states. Modern research has documented what many Americans at the time spotted for themselves, what some who could agree on little else agreed was a constant truth of human behavior growing out of a basic law of climate-society relations. "The great law that governs emigration," announced a Massachusetts congressman during an argument against the spread of slavery, "is this: that emigration follows the parallels of latitude." It was "a great law of emigration," "fixed and certain," echoed a Louisiana editor in a defense of the South and its institutions, "that people follow the parallels of latitude." People were presumed to do so in order to avoid the change of climate that traveling north or south would have entailed.
On the postglacial history of the Hudson River valley