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 our ancestors had inherited a couple of genes in a different form - our canonical number, based on our fingers and toes, might be eight instead of ten. This is just one of the oddities of history that Gould deploys in this wonderfully readable book.