As well as linking the two series into a single future history, the present book served to address a criticism levelled against the largely radioactive Earth as depicted in "Pebble in the Sky" and mentioned in several other books. Though not explicitly stated, there was the clear implication the world being mostly radioactive and humans precariously surviving in the uncontaminated areas in between was the result of a nuclear war hundreds or thousands of years before the time of the plot. This would have made "Pebble in the Sky" part of the post-nuclear war subgenre common in the 1950s. It was, however, pointed out by critics that such an extensive use of nuclear weapons as to leave persistent and widespread radiation even after centuries would have completely destroyed all life on Earth at the moment when it took place. Therefore, in the present book Asimov provided a different origin for the future Earth's radioactivity.