The third installment to the bestselling 'Science Of Discworld' series.
Roundworld is in trouble again, and this time it looks fatal. Its creators, the wizards of Unseen University, feel responsible for its safety. The history of the planet is going haywire – Victorian England and the pace of progress has stagnated. Unless something drastic is done, there won't be time for anyone to invent spaceflight and escape the impending Big Freeze.
Why did history come adrift? Was it Sir Arthur Nightingale's dismal book about natural selection? Or was it the bestselling Theology of Species by Charles Darwin? There's no easy way to change history, as the wizards discover to their cost.