英語史の大きな潮流の1つに，標題の現象がある．この潮流は，ゲルマン祖語の昔から現代英語にかけて非常に長く続いている傾向であり，drift （偏流）として称されることもあるが，とりわけ著しく進行したのは後期古英語から初期中英語にかけての時期である．Baugh and Cable (154--55) がこの過程を端的に要約している箇所を引こう．
Decay of Inflectional Endings. The changes in English grammar may be described as a general reduction of inflections. Endings of the noun and adjective marking distinctions of number and case and often of gender were so altered in pronunciation as to lose their distinctive form and hence their usefulness. To some extent, the same thing is true of the verb. This leveling of inflectional endings was due partly to phonetic changes, partly to the operation of analogy. The phonetic changes were simple but far-reaching. The earliest seems to have been the change of final -m to -n wherever it occurred---that is, in the dative plural of nouns and adjectives and in the dative singular (masculine and neuter) of adjectives when inflected according to the strong declension. Thus mūðum (to the mouths) > mūðun, gōdum > gōdun. This -n, along with the -n of the other inflectional endings, was then dropped (*mūðu, *gōdu). At the same time, the vowels a, o, u, e in inflectional endings were obscured to a sound, the so-called "indeterminate vowel," which came to be written e (less often i, y, u, depending on place and date). As a result, a number of originally distinct endings such as -a, -u, -e, -an, -um were reduced generally to a uniform -e, and such grammatical distinctions as they formerly expressed were no longer conveyed. Traces of these changes have been found in Old English manuscripts as early as the tenth century. By the end of the twelfth century they seem to have been generally carried out. The leveling is somewhat obscured in the written language by the tendency of scribes to preserve the traditional spelling, and in some places the final n was retained even in the spoken language, especially as a sign of the plural.
英語の文法体系に多大な影響を与えた過程であり，その英語史上の波及効果を列挙するとキリがないほどだ．「語根主義」の発生（「#655. 屈折の衰退＝語根の焦点化」 ([2011-02-11-1])），総合から分析への潮流 (synthesis_to_analysis)，形容詞屈折の消失 (ilame)，語末の -e の問題 (final_e) など，英語史上の多くの問題の背景に「屈折語尾の衰退」が関わっている．この現象の原因，過程，結果を丁寧に整理すれば，それだけで英語内面史をある程度記述したことになるのではないか，という気がする．
・ Baugh, Albert C. and Thomas Cable. A History of the English Language. 6th ed. London: Routledge, 2013.
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