Darwin never claimed credit for "
survival of the fittest" as a theory. He borrowed that phrase from Herbet Spencer {who he credited with it... cited it nicely, too}
Darwin's theories showcase the work of many previous scientists and philosophers. {Such as Aristotle} He cites them in his work, 2. He's good at that.
Also... Origin of Species {Actually... Published in 1861. Footnote in my copy. It was a paper delivered at a conference in 1859} didn't really have a lot to say about what we think of as evolution. His book, Descent of Man, has all the stuff that the creationists hate. Like the man/ape link. {Oh yeah.. he says nothing about us evolving from apes/monkeys.. just that we share a common ancestor}