The history of the English language teems with such levelings or extensions. Elder and eldest were at one time the only possible comparative and superlative forms of old (compare German alt, aelter, der aelteste; the vowel following the old-, alt- was originally an i, which modified the quality of the stem vowel).  The general analogy of the vast majority of English adjectives, however, has caused the replacement of the forms elder and eldest by the forms with unmodified vowel, older and oldest. Elder and eldest survive only as somewhat archaic terms for the older and oldest brother or sister.  This illustrates the tendency for words that are psychologically disconnected from their etymological or formal group to preserve traces of phonetic laws that have otherwise left no recognizable trace or to preserve a vestige of a morphological process that has long lost its vitality.  A careful study of these survivals or atrophied forms is not without value for the reconstruction of the earlier history of a language or for suggestive hints as to its remoter affiliations.

Analogy may not only refashion forms within the confines of a related cluster of forms (a “paradigm”) but may extend its influence far beyond.  Of a number of functionally equivalent elements, for instance, only one may survive, the rest yielding to its constantly widening influence.  This is what happened with the English _-s_ plural.  Originally confined to a particular class of masculines, though an important class, the _-s_ plural was gradually generalized for all nouns but a mere handful that still illustrate plural types now all but extinct (foot:  feet, goose:  geese, tooth:  teeth, mouse:  mice, louse:  lice; ox:  oxen; child:  children; sheep:  sheep, deer:  deer).  Thus analogy not only regularizes irregularities that have come in the wake of phonetic processes but introduces disturbances, generally in favor of greater simplicity or regularity, in a long established system of forms.  These analogical adjustments are practically always symptoms of the general morphological drift of the language.

A morphological feature that appears as the incidental consequence of a phonetic process, like the English plural with modified vowel, may spread by analogy no less readily than old features that owe their origin to other than phonetic causes.  Once the e-vowel of Middle English fet had become confined to the plural, there was no theoretical reason why alternations of the type fot:  fet and mus:  mis might not have become established as a productive type of number distinction in the noun.  As a matter of fact, it did not so become established.  The fot:  fet type of plural secured but a momentary foothold.  It was swept into being