From the Oxford English Dictionary, s.v. "in":
in ME. the distinction of in and on was gradually restored, though many traces of their former blending still remain. (See sense 2.)
The formal coincidence of in with the L. prep. in (with which it is originally cognate) led to its being employed, in translating from L., in senses or uses which were idiomatic in L., but not originally English. These also have affected the current contextual use of the preposition.
2. a. = ON (of position). Obs.
Partly a reaction from the blending of in with on in OE.; but partly also transl. L. in, and partly due to a different notion in reference to the n.
22. Formerly (and still sometimes) used, where at, on, during, for are now in use, or where the preposition is omitted.
32. The sense of motion or direction formerly gave rise to various modifications. [Cf. L. in with acc.] Obs. a. = upon, on.
in ME. the distinction of in and on was gradually restored, though many traces of their former blending still remain. (See sense 2.)
The formal coincidence of in with the L. prep. in (with which it is originally cognate) led to its being employed, in translating from L., in senses or uses which were idiomatic in L., but not originally English. These also have affected the current contextual use of the preposition.
2. a. = ON (of position). Obs.
Partly a reaction from the blending of in with on in OE.; but partly also transl. L. in, and partly due to a different notion in reference to the n.
22. Formerly (and still sometimes) used, where at, on, during, for are now in use, or where the preposition is omitted.
32. The sense of motion or direction formerly gave rise to various modifications. [Cf. L. in with acc.] Obs. a. = upon, on.