Darwin knew that starlings looked like other starlings, of course, and moles like other moles. And Darwin also knew that within a nest or a litter, no two individuals look exactly alike. But Darwin, unlike anyone else, was absorbed in thinking about evolution, and he started to wonder about the small differences between individuals of the same kind. Why did they exist? Could they be useful?

A country gentleman by birth, Charles knew a lot about domesticated animals. He was aware that people often bred animals with desirable traits, and that over time such breeding exaggerated small differences. Horses were horses, but one bred for hunting looked very different from one bred to pull heavy loads. Dogs were dogs, but a tiny lap dog and a large, lean greyhound looked nothing alike. Somehow, Darwin thought, variation is the key to understanding how species change.