It is probably [...] variability of value in the significant compounds of a general linguistic drift that is responsible for the rise of dialectic variations. Each dialect continues the general drift of the common parent, but has not been able to hold fast to constant values for each component of the drift.
- Sapir (1921), a pag.161

[...] knowledge of the general drift of a language is insufficient to enable us to see [...] what the drift is heading for. We need to know something of the relative potencies and speeds of the components of the drift.
- Sapir (1921), a pag.162