For at least a century before Darwin, educated people in Europe, especially England knew evolution had happened. It was well discussed, investigated and debated as to how it occurred. Darwin merely discovered the important principle that explained how evolution worked. One key to that understanding was the discovery that the earth was immensely old by people such as Charles Lyell, whose books Darwin took with him on his voyage of the Beagle. Lots of pieces of the evolution puzzle were being worked out. Which is why Wallace also put the pieces together similarly to Darwin. Demonstrating the discovering the roots of evolution was inevitable. There was so much work being done on evolution, had Darwin and Wallace never existed, somebody else would have sooner or later come up with natural selection.