Other notable proto-science fiction authors and works of the early 19th century include:   The European brand of science fiction proper began later in the 19th century with the scientific romances of Jules Verne and the science-oriented, socially critical novels of H. G. Wells. Verne's adventure stories, notably "Journey to the Center of the Earth" (1864), "From the Earth to the Moon" (1865), and "Twenty Thousand Leagues Under the Sea" (1869) mixed daring romantic adventure with technology that was either up to the minute or logically extrapolated into the future. They were tremendous commercial successes and established that an author could make a career out of such whimsical material. Sprague de Camp calls Verne "the world's first full-time science fiction novelist." Wells's stories, on the other hand, use science fiction devices to make didactic points about his society. In "The Time Machine" (1895), for example, the technical details of the machine are glossed over quickly so that the Time Traveller can tell a story that criticizes the stratification of English society. The story also uses Darwinian evolution (as would be expected in a former student of Darwin's champion, Huxley), and shows an awareness of Marxism. In "The War of the Worlds" (1898), the Martians' technology is not explained as it would have been in a Verne story, and the story is resolved by a deus ex machina, albeit a scientifically explained one. The differences between Verne and Wells highlight a tension that would exist in science fiction throughout its history. The question of whether to present realistic technology or to focus on characters and ideas has been ever-present, as has the question of whether to tell an exciting story or make a didactic point. Wells and Verne had quite a few rivals in early science fiction. Short stories and novelettes with themes of fantastic imagining appeared in journals throughout the late 19th century and many of these employed scientific ideas as the springboard to the imagination. "Erewhon" is a novel by Samuel Butler published in 1872 and dealing with the concept that machines could one day become sentient and supplant the human race. Although better known for Sherlock Holmes, Sir Arthur Conan Doyle also wrote early science fiction, particularly using the character of Professor Challenger. Rudyard Kipling's contributions to the genre in the early 1900s made Campbell describe him as "the first modern science fiction writer".