Darwin

As a biologist I can say that this man is with David Attenbourgh on my list of great men.  Not forgetting Wallace of course (who also formed the theory of evolution at the same time as Darwin, but didn't want to publish).  Darwin took many years to publish his work and he was treated to a great deal of abuse by the Victorian church.  To me he’s formed a theory which was one of the great scientific “keys” to our understanding of the world we live in.