Parallel grammaticalizations in Tibeto-Burman : evidence of Sapir's 'Drift'

In chapters seven and eight of his book Language, Sapir talked about what he called ‘drift’, the changes that a language undergoes through time [...]. Dialects of a language are formed when that language is broken into different segments that no longer move along the same exact drift. Even so, the general drift of a language has its deep and its shallow currents; those features that distinguish closely related dialects will be of the rapid, shallow currents, while the deeper, slower currents may remain consistent between the dialects for millennia. It is this latter type that Sapir felt is ‘fundamental to the genius of the language’ (p. 172), and he said that ‘The momentum of the more fundamental, the pre-dialectal, drift is often such that languages long disconnected will pass through the same or strikingly similar phases’ (p. 172)