Darwin himself wrote in his autobiography about his own intelligence and abilities and looked back on his long career about five years before his death.(He did not write this for publication; it was intended for his children and descendants.) I’m not going to quote the whole thing – the text is here, if you want it: page on Darwin Online But here’s a passage example (pages 141-145 in the first issue at the end of the book):
I don’t have much speed of reflection or wit, which is so remarkable for some smart men, such as Huxley.I am therefore a bad critic: a newspaper or a book generally arouses my admiration when I first read it, and it is only after a long period of reflection that I see the weaknesses. My ability to follow a long and purely abstract line of thought is very limited. I would never have succeeded with metaphysics or mathematics. My memory is extensive yet thirsty: it is enough to be careful by vaguely telling myself that I have seen or read something that contradicts the conclusion I draw, or on the other hand in favour of it. and after a while, I can generally remember where to look for my authority. In a sense, my memory is so poor that I could never remember a single date or a series of poems for more than a few days.
Some of my critics have said, “Oh, he’s a good observer, but he has no reasoning.” I do not believe that this can be true, because the origin of the species is a long argument from start to finish, and it has convinced quite a few capable men.No one could have written it without having an argumentative capacity. I have a fair share of inventions and common sense or judgment, as any fairly successful lawyer or doctor needs, but I do not believe to a greater extent.
On the favorable side of balance, I think I am superior to the usual race of people when I perceive things that elude attention and by observing them attentively.My industry was almost as big as it could have been when it was observing and gathering facts. More importantly, my love of science has been consistent and passionate. However, this pure love was greatly supported by the desire to be appreciated by my friends of nature. Even in my early youth, I had the greatest desire to understand or explain everything I observed, namely to classify all facts under some general laws. These causes have given me the patience to think or think about any unresolved problem for any year. As far as I can judge, I am not inclined to blindly follow the leadership of other men. I have constantly tried to keep my mind free to give up any hypothesis that I love (and I can’t resist making one on every subject) once it’s shown that the facts are against it. . . .
My success as a scientific man, whatever that may have been, was determined, as far as I can judge, by complex and varied mental characteristics and conditions.Of these, the most important – the love of science – were the unrestricted patience to think long and long about every subject – the industry to observe and collect facts – and a good share of inventions and common sense. With such moderate skills that I have, it is really surprising that I would have had a significant impact on the beliefs of scientific men on some important points.
So your answer depends on what you think is “intelligent.”He was not a lightning-fast, intuitive thinker, nor was he good for abstract thinking. He was a turtle, not a hare. But he was very good at observing – he even noticed seemingly banal things that no one else was concerned with; tracking of clues and tracking them; careful reflection; patience and perseverance. His mental “toolkit” may not have been conspicuous or adapted to stereotypes of “genius” – but what he built with it is a brilliant work.