Like many great ideas, NS was one whose time had come. There were a number of people using a number of lines of evidence that seemed to point in that direction. It's true that Darwin was well-connected: nearly every page of OOS make multiple references to the work of others.
It's also true that Darwin corresponded with Wallace - and he was certainly aware of the work of Wallace when he published. However, that doesn't change the fact that Darwin developed his ideas independently. Wallace is the fellow who coined the term Darwinism.
Darwin's ideas were original, as were Leibnitz's calculus original to him, and Einstein's ideas original to him - I don't believe the urban legends about Einstein plagiarizing his work from other people, or his wife doing his math for him.
http://www.update.uu.se/~fbendz/nogod/cd-aw.htm
