Eight Little Piggies. Reflections in Natural History, Paperback/Stephen Jay Gould

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 book deploys this, which is one of the oddities of history.