Darwin's evolutionary theories altered the way scientists and the natural world were studied.
Furthermore, Ernst Mayr discovered that "the basic theory of evolution" has been confirmed in such a way that modern biologists regard evolution as a fact.
In this context, evolution is both a fact and a theory.
It is an undeniable fact that organisms have changed, or evolved, over the course of Earth's history and biologists have discovered and studied mechanisms that can explain major patterns of change.
The process of general evolution could theoretically be replicated by experimentation, but it has never been done.