tnote 160: These confusions are more theoretical than real, however.
A language has countless methods of avoiding practical ambiguities.]

More often the phonetic drift is of a more general character. It is not
so much a movement toward a particular set of sounds as toward
particular types of articulation. The vowels tend to become higher or
lower, the diphthongs tend to coalesce into monophthongs, the voiceless
consonants tend to become voiced, stops tend to become spirants. As a
matter of fact, practically all the phonetic laws enumerated in the two
tables are but specific instances of such far-reaching phonetic drifts.
The raising of English long _o_ to _u_ and of long _e_ to _i_, for
instance, was part of a general tendency to raise the position of the
long vowels, just as the change of _t_ to _ss_ in Old High German was
part of a general tendency to make voiceless spirants of the old
voiceless stopped consonants. A single sound change, even if there is no
phonetic leveling, generally threatens to upset the old phonetic pattern
because it brings about a disharmony in the grouping of sounds. To
reestablish the old pattern without going back on the drift the only
possible method is to have the other sounds of the series shift in
analogous fashion. If, for some reason or other, _p_ becomes shifted to
its voiced correspondent _b_, the old series _p_, _t_, _k_ appears in
the unsymmetrical form _b_, _t_, _k_. Such a series is, in phonetic
effect, not the equivalent of the old series, however it may answer to
it in etymology. The general phonetic pattern is impaired to that
extent. But if _t_ and _k_ are also shifted to their voiced
correspondents _d_ and _g_, the old series is reestablished in a new
form: _b_, _d_, _g_. The pattern as such is preserved, or restored.
_Provided that_ the new series _b_, _d_, _g_ does not become confused
with an old series _b_, _d_, _g_ of distinct historical antecedents. If
there is no such older series, the creation of a _b_, _d_, _g_ series
causes no difficulties. If there is, the old patterning of sounds can be
kept intact only by shifting the old _b_, _d_, _g_ sounds in some way.
They may become aspirated to _bh_, _dh_, _gh_ or spirantized or
nasalized or they may develop any other peculiarity that keeps them
intact as a series and serves to differentiate them from other series.
And this sort of shifting about without loss of pattern, or with a
minimum loss of it, is probably the most im