How did such strikingly individual alternations as fot:  fet, fuoss:  fueesse develop?  We have now reached what is probably the most central problem in linguistic history, gradual phonetic change.  “Phonetic laws” make up a large and fundamental share of the subject-matter of linguistics.  Their influence reaches far beyond the proper sphere of phonetics and invades that of morphology, as we shall see.  A drift that begins as a slight phonetic readjustment or unsettlement may in the course of millennia bring about the most profound structural changes.  The mere fact, for instance, that there is a growing tendency to throw the stress automatically on the first syllable of a word may eventually change the fundamental type of the language, reducing its final syllables to zero and driving it to the use of more and more analytical or symbolic[149] methods.  The English phonetic laws involved in the rise of the words foot, feet, mouse and mice from their early West-Germanic prototypes fot, foti, mus, musi[150] may be briefly summarized as follows: 

[Footnote 149:  See page 133.]

[Transcriber’s note:  Footnote 149 refers to the paragraph beginning on line 4081.]

[Footnote 150:  Primitive Germanic fot(s), fotiz, mus, musiz; Indo-European pods, podes, mus, muses.  The vowels of the first syllables are all long.]

1.  In foti “feet” the long o was colored by the following i to long oe, that is, o kept its lip-rounded quality and its middle height of tongue position but anticipated the front tongue position of the i; oe is the resulting compromise.  This assimilatory change was regular, i.e., every accented long o followed by an i in the following syllable automatically developed to long oe; hence tothi “teeth” became toethi, fodian “to feed” became foedian.  At first there is no doubt the alternation between o and oe was not felt as intrinsically significant.  It could only have been an unconscious mechanical adjustment such as may be observed in the speech of many to-day who modify the “oo” sound of words like you and few in the direction of German ue without, however, actually departing far enough from the “oo” vowel to prevent their acceptance of who and you as satisfactory rhyming words.  Later on the quality of the oe vowel must have departed widely enough from that of o to enable oe to rise in consciousness[151] as a neatly distinct vowel.  As soon as this happened, the expression of plurality in foeti, toethi, and analogous words became symbolic and fusional, not merely fusional.

[Footnote 151:  Or in that unconscious sound patterning which is ever on the point of becoming conscious.  See page 57.]