Charles Darwin was a known and well-respected scientist in 19th Century England. But few knew that for 20 years, he'd been holding back a theory that would radicalize our understanding of Biology and the history of life on Earth. He held it back because he knew all too well how intensely it would clash not only with the scientific thought of the day, but especially with religious belief. It has been a source of controversy from the moment he finally published his theory in his famous book, On the Origin of Species.