Yes, people had known about Evolution and Natural Selection for years, but those people only made up a small number within the general populace as a whole. But what made Mr C. Darwin so brilliant was that he took all of those ideas from multiple sources and combined them into everything you ever wanted to know about the theories of Evolution and Natural Selection in one easy to carry volume.
We must also remember that at this time the evidence to support the theory of Evolution and that it had been happening for much longer then people thought (i.e. millions of years) was being dug up. Anyone who had read the same books that Darwin had could have written an explanation of evolution, but it was Darwin's rhetorical brilliance that got there first.
What Butler misses in his essay is not whether Darwin was original but that he struck upon the right time to explain evolution and as a result, Origin became a best-seller and educated the masses and forever changed the way in which we see ourselves.
And to those of you who called Darwin a fraud after reading Butler, I say who cares? Do you know how hard it is to come upon a completely and utterly original idea which is absolutely devoid of any influence from the ideas of those who came before you? It's damn near impossible. The only thing which Thomas Edison invented completely on his own, without any help, was the context in which we use the word "hello". Does that make him any less of a great inventor? Will you turn off the electric light bulb over your head because you are upset that it was not Edison's uniquely original idea? Would you read a book that said "here is a summary of what these various people think of evolution"? No, you would read the book that said "here is an exciting bit of prose that explains this clever scientific idea that I came up with that will blow your mind". And that is why Butler is wrong. If Darwin had written the book that Butler's essay wanted him to write, Origin would not have been a best-seller, because it would not have been written in the style of a guaranteed best-seller and evolution would not have entered mainstream thought and changed how we see life on Earth.
After all, if Butler had been right, would we be taking a course on Charles Darwin?