With "wonderfully logical detail . . . exciting storytelling and moving characterization" (Anthony Boucher), science fiction master Poul Anderson explores what happens when the next stage of evolution is thrust upon humanity and animals. As Earth passes out of a magnetic field that has suppressed intelligence for eons, the mental capacity for all mammals increases exponentially, radically changing the structures of society.

A mentally impaired farm worker finds himself capable of more delicate and intelligent thoughts than he ever dreamed. A young boy on holiday manages to discern the foundations of calculus before breakfast. Animals that were seen as livestock and pets can now communicate clearly with their owners and one another. And an already brilliant physics researcher now uses his boundless intellect to bring humankind to the stars--even as his wife plunges into an existential crisis. For all of them, the world will never be the same . . .