Charles Darwin had a dangerous idea. Some people even speculate that it made him ill. Ever since he wrote the quiet confession, in Notebook M, that his theory might lead to materialism—the idea that the world is entirely physical, with no God, no spiritual element at all—he had been cautious about how to express himself. If the issue of materialism got in the way, it would prevent people from reading and understanding him. He wanted to introduce evolution, not kill God.
Darwin used literary techniques to resolve this problem. Not only had he drafted versions of his argument twice before by the time he sat down to write Origin, but he was, in his youth, a deep reader of literature. His published version of Origin is full of the careful use of rhetoric to make his argument as persuasive as it could be. In his own way, Darwin is an accomplished literary writer.