I'm writing a fantasy novel, in which the story progresses over a number of years. Throughout the story, I want to display an evolution of the magic, much like technology evolves in our world.
The speed of the magic evolution would begin slowly but would eventually progress very quickly, due to reasons to do with the plot and the main characters of the story, therefore there will be a great many more things that the magic allows close to the end of the story than the beginning.
One of my worries is that it may seem as though I'm making up additional rules for my magic as I go along, due to the fact that it will seemingly appear from nowhere (the protagonists contribute some of the magic evolution, but the majority of it is done by the antagonist behind the scenes).
How can I foreshadow, or introduce early into the story, that the magic in my world will advance quickly in the future, and entirely new realms of my magic system will eventually become available, without it appearing as though I'm making up new rules for the magic system as I go?
For reference, imagine that the technology equivalent would begin in a medieval - flintlock era, and progress to basic robotics in a span of years (not necessarily in the same order as in our world, there will be some steps missed out, as the robots will be built using magic and not technology, therefore computers will still not exist).