Penguin Books, 1994. Some foxing to endpapers and page edges, creased spine, minor signs of secondhand wear.

This collection of essays ranges from history to the latest theories in biology, from controversies over palaeontology to the origins of language. The title is a pun and as always with Gould, the joke has a point that illustrates the largest pattern of life's history. For millennia, the animals that populated the Earth had four toes on each foot, or six. If evolution had taken a tiny shift - if man's ancestors had inherited a couple of genes in a different form - our canonical number, based on man's fingers and toes, might be eight instead of ten.