The widespread interchange of crops, livestock and techniques accelerated
with the discovery of the Americas and the later migration of Europeans to all
parts of the world. Crops indigenous to the Americas have profoundly altered
the cropping patterns of the Old World. In Europe the potato was known but
little grown before the mid-eighteenth century, but became a major food crop
by the mid-nineteenth century (Langer, 1975). Maize spread through the
warmer southern parts of Europe in the eighteenth century; in the last 30
years the breeding of varieties capable of yielding in cooler, shorter summers
has seen its diffusion into northern France and even into southern Britain.
African consumption habits were profoundly changed in the slow spread of
maize and manioc in the nineteenth and early twentieth centuries, whilst in
parts of Asia sweet potatoes and peanuts, as well as maize, have become