The Columbian Exchange

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The transfer of organisms — crops, livestock, weeds, pathogens and people — between hemispheres that had been biologically separated since the flooding of Beringia, and the organising concept of this Journey since Alfred Crosby named it in 1972. It ran in both directions and it did not run symmetrically. Westward went wheat, barley, sugar cane, coffee, bananas, citrus, and above all the large domesticated animals the Americas lacked: cattle, pigs, sheep, goats and horses, which transformed American landscapes and, in the case of the horse, remade the societies of the Great Plains and the Pampas within a few generations. Eastward went maize, potatoes, sweet potatoes, cassava, tomatoes, chillies, beans, squash, cacao, tobacco and quinine — a set of crops that, crucially, tolerate ground that Old World staples cannot use, and so added calories rather than merely substituting for them. And westward also went the epidemic diseases of Eurasia, which is the limb of the exchange that killed. The framing's value is that it makes the period legible as biology and not only as politics; its limitation is that "exchange" is a mild word for a transfer in which one side received food and the other received epidemics.