Human races

For instance it might be objected that if the universe contained two human races, the one immensely long-lasting and the other short-lasting, and if these had the same population figures until AD 2150, then finding yourself before AD 2150 could give you no clue as to which human race you were in.

By entering the past we are creating whole new universes! Whole other earths, other human races, which we create and abandon.