But they're not newer than slavery, which is why the slavery map has density in the north. That matters to the ultimate point that fertile soil = large farms = slaves = high black population = democratic majority. Because in this case it's fertile soil = not as large farms = slaves = low black population = republican majority.

How can the size of the farms be relevant if both large and not large farms result in different black populations and therefore party majorities? In this case the most relevant statistic to the party is race. The proximity to agriculture means nothing outside of 'people live near agriculture and some of them may be democrats'