Edward Sapir (18841939). Language: An Introduction to the Study of Speech. 1921.
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 by one of the surface drifts of the language, to be swept aside in the Middle English period by the more powerful drift toward the use of simple distinctive forms. It was too late in the day for our language to be seriously interested in such pretty symbolisms as foot: feet. What examples of the type arose legitimately, in other words via purely phonetic processes, were tolerated for a time, but the type as such never had a serious future.
It was different in German. The whole series of phonetic changes comprised under the term umlaut, of which u: ü and au: oi (written äu) are but specific examples, struck the German language at a time when the general drift to morphological simplification was not so strong but that the resulting formal types (e.g., Fuss: Füsse; fallen to fall: fällen to fell; Horn horn: