The giveaway comes on page 289: "Discworld does not have science as such." So, as with all the best science fiction, we are not really dealing with the wizardly denizens of Pratchett's intentionally flat world, but with our own planet, masquerading here as Roundworld.
And this time, in the series that weaves scientific commentary around a novelette, the terrific trio are taking on those latterday flat-earthers, the creationists.
The wizards have to put Charles Darwin and history back on course: his bestselling Theology of Species is the wrong book (in this universe, it is the Rev Richard Dawkins who has written The Origin of Species).
So be amused at the antics of the academics of Unseen University. Chuckle as cutting-edge research is hogtied by collegiate conservatism — "it's the pushing that matters, not the boundary". Then cheer as our heroes describe how the theory of evolution has been tested and found good in many scientific fields, whereas the argument for intelligent design stated by clergyman William Paley, who died three years after Darwin was born, cannot be sustained.
Paley told of a maker for an iconic watch found on a heath, a powerful allusion to divine purpose even today. But what you see in nature is not always what you get. A stone, for example, zirconium, can tell a story of the passage of geological time that makes nonsense of the brief span of history allowed by some evangelists for creationism.
The wizards also grapple with the meaning of infinity. I once spent an evening arguing whether there were limits to infinity. Had this book been to hand I would have realised that my case that the universe was boundless founders because it has been expanding for about 13bn years since the Big Bang. Perhaps the creationists, too, would reconsider in the face of the evidence.