s is _mys_ (i.e.,
_mues_) to _mus_. Middle High German _ue_ (Modern German _u_) did not
develop from an "umlauted" prototype of Old High German _uo_ and
Anglo-Saxon _o_, but was based directly on the dialectic _uo_. The
unaffected prototype was long _o_. Had this been affected in the
earliest Germanic or West-Germanic period, we should have had a
pre-German alternation _fot_: _foeti_; this older _oe_ could not well have
resulted in _ue_. Fortunately we do not need inferential evidence in
this case, yet inferential comparative methods, if handled with care,
may be exceedingly useful. They are indeed indispensable to the
historian of language.]

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_, _fodia