He believed that strong sun made desert animals pale and acquired characteristics eventually were inheritable. Darwin wrote a book which stated that Camels had thick skin and thick pads on their knees from long years of being beasts of burden in the Desert. This, he believed, caused intense competition and resulted in a struggle for existence. The best equipped species, he wrote, would survive. Over the years this theory gained increasing popularity, especially among agnostics.
All science is theory.
The other side, of course, believes that the Biblical version of how things came to be is the correct version. The State of Tennessee had passed a law in the early 1920s prohibiting the teaching of the theory of evolution in public schools. To challenge the law, opponents urged Scopes to teach evolution in his biology class and he was charged with violation of the law. Jennings Bryan prosecuted Scopes and Clarence Darrow defended him. The media pack was led by H.L. Mencken, co-founder of the American Mercury, a magazine of satire and caustic criticism of religion, businessmen and American culture.