h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The reason for having an apostrophe as part of the Genitive case was once explained to me as follows:

The Genitive was originally expressed as in this example: "The man his dog". This contracts to "The man's dog".

(Sorry, ladies ... gender correctness was not available when the Genitive was invented)

The following is a paradigm for the Old English word for "fish" and the declensions for number and case.

Old English Masculine a-stem noun

....................Singular.....Plural
Nominative....'fisc'.......'fiscas'
Accusative....'fisc'.......'fiscas'
Genitive.........'fisces'...'fisca/fiscana'
Dative............'fisce'......'fiscum'
Instrumental....'fisce'....'fiscum'

The nominative/accusative plural form 'fiscas' is the source of the plural '-s' in Modern English, while the genetive singular '-es' form is the source of the Modern English '-'s' possesive.

SOURCE: "A history of English - A sociolinguistic approach", Barbara A. Fennell, 2001, Blackwell Publishers Inc. USA, p.65

It lines up in preview, at least...

Or perhaps back to the latin ...

Third declension (m&f) tend to have the plural in '-es' and the genitive singular in '-is'

exempla gratia

....................Singular.....Plural
Nominative......dux..........duces
Accusative......ducem......duces
Genitive..........ducis........ducum
Dative.............duci..........ducibus
Ablative..........duce.........ducibus

But I don't believe this explains genitive contraction. I vote for Il Duce's fishes!

Good point, but English isn't a daughter language of Latin... it just seems that way because we've stolen (sorry, 'borrowed') so many words

The explanation that Kes gives above for the genitive apostrophe is common but completely wrong.

Old English used endings on nouns to show the functions of nouns. To show possession, the ending was "-es". The man's dog was "mannes docg" and the e was pronounced, so it was like "man-ness dog". Gradually they stopped pronouncing the e, so the writers started to write an apostrophe to show that the e had been left out.