This is a book by Olaf Stapledon. It is not quite a novel, but it is certainly science fiction.

This is a book about two interwoven parallel worlds, each history being many hundreds of thousands of years long. The book does not include exact time scales. This is a book about both the triumph and defeat of humanity. Both facets end with the extinction of humanity, the tragic one with the end of intelligent life on earth and the worldwide dominance of the rats, the happy one with humanity creating a new species which includes what is best in us. Odd though the latter may sound, it works very well in context, and is a dramatic climax to the work.

Although this book has a plot and tension and even heroes and antagonists to an extent, it is no ordinary science fiction novel. If you enjoy only the trappings of science fiction and not strange thoughts and new ideas, this book is not for you. Some parts of it are remniscient of Stanislaw Lem for modern readers, although of course Lem read Stapledon and not the other way around.

One thing that will seem very odd to many modern readers is Olaf Stapledon's belief in communism. A surprising number of intelligent and broadly educated people were communists in the era in which he lived - many more than today. If you cannot look past it you will find this book unreadable. If you can look past it, you will eventually wonder which ideas of our own, seeming logical enought at the time, will seem absurd in the future.