One of the earliest historians of the Civil War saw it as a fundamental clash between the peoples of different latitudes. Climate had made the antebellum North and South distinct societies and natural enemies, John W. Draper argued, the one democratic and individualist, the other aristocratic and oligarchical. If such were the case, the future of the reunited states was hardly a bright one. But Draper saw no natural barriers to national unity that wise policy could not surmount. The restlessness and transience of American life that many deplored instead merited, in his view, every assistance possible. In particular, he wrote, Americans needed to be encouraged to move as freely across climatic zones as they already did within them. The tendency of North and South to congeal into hostile types of civilization could be frustrated, but only by an incessant mingling of people. Sectional discord was inevitable only if the natural law that "emigrants move on parallels of latitude" were left free to take its course. These patterns of emigration were left free, for the most part, but without the renewed strife that Draper feared. After the war as before it, few settlers relocating to new homes moved far to the north or south of their points of origin. As late as 1895, Henry Gannett, chief geographer to the U.S. Census, could still describe internal migration as "mainly conducted westward along parallels of latitude." More often as time went on, it was supposed that race and not merely habit underlay the pattern, that climatic preferences were innate, different stocks of people staying in the latitudes of their forbears by the compulsion of biology. Thus, it was supposed, Anglo-Saxons preferred cooler lands than Americans of Mediterranean ancestry, while those of African descent preferred warmer climates than either. Over time, though, latitude loosened its grip and exceptions to the rule multiplied. As the share of the population in farming declined, so did the strongest reason for migrants to stay within familiar climates. Even by the time Gannett wrote, the tendency that he described, though still apparent, was weaker than it had been at mid-century. It weakened because a preference for familiar climates was not a fixed human trait but one shaped by experience and wants, and capable of changing as these variables changed.
The US and Extended Deterrence