Unlike Modern English, Old English is a language rich in morphological diversity. It maintains several distinct cases: the nominative, accusative, genitive, dative and (vestigially) instrumental. The only remnants of this system in Modern English are in the forms of a few pronouns (such as I/me/mine, she/her, who/whom/whose) and in the possessive ending -'s, which derives from the old (masculine and neuter) genitive ending -es. In Old English, however, nouns and their modifying words take appropriate endings depending on their case.
The Modern English ending -'s is derived from what ending in Old English?
-es