Languages do not spread in a vacuum. They are not self-contained linguistic events but contingent on the history or prehistory more broadly, as Heggarty (2015:600) describes it, the result of:
[p]rocesses in the real-world context – demographic growth or collapse, migrations, conquest, or more subtle socio-political and cultural changes – are the cause; they alone determine entirely the linguistic effects of divergence, diversity and convergence.'
(Introduction)
