Since the time of Aristotle, there had been a clear divide between the three kingdoms of animal, vegetable, and mineral. But by the eighteenth century, biological experiments, and the wide range of new creatures coming to Europe from across the world, challenged these neat divisions. Abraham Trembley found that freshwater polyps grew into complete individuals when cut.

This shocking discovery raised deep questions: was it a plant or an animal? And this was not the only conundrum. What of cora...