time of the Byzantine invasion of the Ostrogoths. De Camp's time traveler, Martin Padway, is depicted as making permanent historical changes and implicitly forming a new time branch, thereby making the work an alternate history. In William Tenn's short story Brooklyn Project (1948), a tyrannical US Government brushes aside the warnings of scientists about the dangers of time travel and goes on with a planned experiment - with the result that minor changes to the prehistoric past cause Humanity to never have existed, its place taken by tentacled underwater intelligent creatures - who also have a tyrannical government which also insists on experimenting with time-travel. Time travel as the cause of a point of divergence (POD), which can denote either the bifurcation of a historical timeline or a simple replacement of the future that existed before the time-travelling event, has continued to be a popular theme. In Ward Moore's Bring the Jubilee, the protagonist lives in an alternate history in which the Confederacy has won the American Civil War. He travels backward through time and brings about a Union victory at the Battle of Gettysburg. When a story's assumptions about the nature of time travel lead to the complete replacement of the visited time's future, rather than just the creation of an additional time line, the device of a "time patrol" is often used where guardians move through time to preserve the "correct" history. A more recent example is Making History by Stephen Fry in which a time machine is used to alter history so that Adolf Hitler was never born. That ironically results in a more competent leader of Nazi Germany and results in the country's ascendancy and longevity in the altered timeline. Cross-time stories H.G. Wells' "cross-time" or "many universes" variant (see above) was fully developed by Murray Leinster in his 1934 short story "Sidewise in Time", in which sections of the Earth's surface begin changing places with their counterparts in alternate timelines. Fredric Brown employed this subgenre to satirize the science fiction pulps and their adolescent readers—and fears of foreign invasion—in the classic What Mad Universe (1949). In Clifford D. Simak's Ring Around the Sun (1953), the hero ends up in an alternate earth of thick forests in which humanity never developed but a band of mutants is establishing a colony; the story line appears to frame the author's anxieties regarding McCarthyism and the Cold War. Quantum