Further, the tendency toward the weakening of final syllables was very strong even then and had been manifesting itself in one way and another for centuries.  I believe that these further facts help us to understand the actual sequence of phonetic changes.  We may go so far as to say that the o (and u) could afford to stay the change to oe (and ue) until the destructive drift had advanced to the point where failure to modify the vowel would soon result in morphological embarrassment.  At a certain moment the _-i_ ending of the plural (and analogous endings with i in other formations) was felt to be too weak to quite bear its functional burden.  The unconscious Anglo-Saxon mind, if I may be allowed a somewhat summary way of putting the complex facts, was glad of the opportunity afforded by certain individual variations, until then automatically canceled out, to have some share of the burden thrown on them.  These particular variations won through because they so beautifully allowed the general phonetic drift to take its course without unsettling the morphological contours of the language.  And the presence of symbolic variation (sing, sang, sung) acted as an attracting force on the rise of a new variation of similar character.  All these factors were equally true of the German vocalic shift.  Owing to the fact that the destructive phonetic drift was proceeding at a slower rate in German than in English, the preservative change of uo to uee (u to ue) did not need to set in until 300 years or more after the analogous English change.  Nor did it.  And this is to my mind a highly significant fact.  Phonetic changes may sometimes be unconsciously encouraged in order to keep intact the psychological spaces between words and word forms.  The general drift seizes upon those individual sound variations that help to preserve the morphological balance or to lead to the new balance that the language is striving for.

I would suggest, then, that phonetic change is compacted of at least three basic strands:  (1) A general drift in one direction, concerning the nature of which we know almost nothing but which may be suspected to be of prevailingly dynamic character (tendencies, e.g., to greater or less stress, greater or less voicing of elements); (2) A readjusting tendency which aims to preserve or restore the fundamental phonetic pattern of the language; (3) A preservative tendency which sets in when a too serious morphological unsettlement is threatened by the main drift.  I do not imagine for a moment that it is always possible to separate these strands or that this purely schematic statement does justice to the complex forces that guide the phonetic drift.  The phonetic pattern of a language is not invariable, but it changes far less readily than the sounds that compose it.  Every phonetic element that it possesses may change radically and yet the pattern remain unaffected.  It would be absurd to claim that our present English pattern is identical with the old Indo-European one, yet it is impressive to note that even at this late day the English series of initial consonants: