To be fair, tallhappy asked what the theory of evolution states, not for a definition of the word "evolution" as it relates to biology.
That sounds like lawyering to me.
There is evolution as a forensic statement about the history of life, and there is evolution as a theory of how populations change over time. I don't think the "theory" includes the forensic conclusion, even if the word evolution refers to more than one concept.
But as to what Darwin wrote, the answer is, he wrote a lot. There are few issues in biology that he didn't consider, and he changed his expectations over time, sometimes toward ideas that have since been discarded.
He moved form a saltationist position to a uniformitarianist position regarding the rate of evolution. He appears to have been wrong. Rates of change appear to vary, particularly after mass extinctions.
Darwin argued against periodic catastrophes followed by rapid evolution. He appears to have been wrong.