Why didn't Charles Darwin use the word "evolution" in his original publication of On the Origin of Species?

Evolution did not appear anywhere on the pages of Darwin’s seminal work on how the variation in life came to be. Instead he used the term, descent with modification. Even the word “evolve’ occurs only once, as the closing word of the closing sentence, “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

The word, evolution, was certainly know to Darwin; so why do you think he was careful to avoid its use.