The Science of Discworld III: Darwin's Watch (Discworld)

The wizards discover to their cost that it’s no easy task to change history.Roundworld is in trouble again, and this time it looks fatal.

Having created it in the first place, the wizards of Unseen University feel vaguely responsible for its safety.

They know the creatures that lived there escaped the impending Big Freeze by inventing the space elevator — they even intervened to rid the planet of a plague of elves, who attempted to divert humanity onto a different time track.

But now it’s all gone wrong — Victorian England has stagnated and the pace of progress would embarrass a limping snail.

Unless something drastic is done, there won’t be time for anyone to invent space flight, and the human race will be turned into ice-pops.