I want to know the answer to the following question:
With the passage of time what particular things phonemic in a language are known to change?
All the background I'm provided with is that "At one time, [f] and [v], two sounds that have the same place and manner of articulation and differ in voicing only, were allophones of the same phoneme in English, but later came to belong to separate phonemes.". Is that enough to answer the question?