[...] in the development of Anglo-Saxon long 'o' to long 'e', via 'ö', under the influence of a following 'i'. [...] the old long 'o' 'split' into two sounds -long 'o', eventually 'u', and long 'e', eventually 'i'.
- Sapir (1921), Pag. 180

If neither of the two sounds into which an old one 'splits' is a new sound, it means that there has been a phonetic leveling [...]
- Sapir (1921), Pag. 181

[...] shifting about without loss of pattern [...] is probably the most important tendency in the history of speech sounds. Phonetic leveling and 'splitting' counteract it to some extent but, on the whole, it remains the central unconscious regulator of the course and speed of sound changes.
- Sapir (1921), Pag. 182