The focus of this chapter is on how languages move and change over time and space. The perceptions of historical linguists have been shaped by what they were observing. During the flowering of comparative linguistics, from the late 19th into the 20th century, the dominant view was that in earlier times when people moved, their languages moved with them, often over long distances, sometimes fast, and that language change was largely internal. That changed in the second half of the 20th century. We now recognize that in recent centuries and millennia, most movements of communities and individuals have been local and shorter. Constant contact between communities resulted in features flowing across language boundaries, especially in crowded and long-settled locations such as most of Central and West Africa. Although communities did mix and people did cross borders, it became clear that language and linguistic features could also move without communities moving.
Language Boundaries Driven by Surface Tension
