Around two thousand years ago, a Greek doctor named Dioscorides described a plant that he considered to be medically useful. It was called ‘crocodilium’, he said, and it was supposed to help people who were splenetic. When boiled and drunk, it ‘causes copious bleeding at the nose’. Other characteristics, apart from the shape of its roots and seeds, and the fact that it grew in ‘wooded places’, were unfortunately obscure.

What exactly was crocodilium? And why should anyone care? As Anna Pavord splendidly makes plain in this elegant and scholarly history of taxonomy, a science usually regarded as even dismaller than economics, such questions are far from insignificant. Exactly which plant is which, and what its relationship is to other