The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
219. Prepositions were not originally distinguished
from Adverbs in form or meaning, but have become specialized in use. They
developed comparatively late in the history of language. In the early
stages of language development the cases alone were sufficient to indicate
the sense, but, as the force of the case-endings weakened, adverbs were
used for greater precision (cf. § 338). These adverbs, from their
habitual association with particular cases, became Prepositions; but many
retained also their independent function as adverbs.
Most prepositions are true case-forms: as, the comparative ablatives
suprá (for exterá, ínferá, superá), and the accusatives
circum, córam, cum (cf. § 215). Circiter is an adverbial
formation from circum (cf. § 214. b. N.); praeter
is the comparative of prae, propter
prope.[The case-form of these
prepositions in -ter is doubtful.] Of
the remainder, versus is a petrified
nominative (participle of vertó); adversus is a compound of
versus; tráns is probably an
old present participle (cf. in-trá-re); while the origin of the
brief forms ab, ad, dé, ex, ob,
is obscure and doubtful.