The issue of the conflict between evolutionism and creationism is the primary theme of "The Satanic Illusion" and motivates all of the action. A supporting theme is time travel, long a fixture in popular fiction. Without the ability to travel in time, the central characters would be unable to make their observations of different ecosystems in succeeding eras. De Camp deals with one of the conundrums of time travel—Won't killing ancient animals change history?—by saying that changes made far enough back in time will be absorbed by great evolutionary trends, and the effects will disappear long before human beings appear. This underlying idea about the effects of time travel makes evolution inevitably the winner over creationism, which does not offer a similar explanation for how ancient animals may be killed without affecting modern times.
Rivers's time traveling machine cannot move geographically, remaining in the central region of North America. This confines the succession of ecosystems the characters visit to one geographical area, with each successive ecosystem probably deriving from the ecosystem previously visited. This idea that each ecosystem evolved out of the ones previous to it implies that there would be earlier ecosystems than the earliest one the characters choose to visit, extending back in time to the moment when life first arrived in North America.
Another significant theme is a conflict between...
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