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John Gould, the ornithologist, identified them properly — I had not realised in the field that they were all finches. I thought some were wrens, some blackbirds, some orioles. Gould set me straight in January 1837, four months after I returned to England.

Thirteen species. All descended from a single ancestor blown across 600 miles of open ocean, probably a million years ago. Each island had its own variant. The beaks were different because the food was different: seeds, insects, cactus flowers. The birds had changed to fit the available resources. No one designed the beak. The beak was the result of who survived long enough to breed.

This is the moment the theory began to crystallise for me. Not on the islands — I was not clever enough in the field. In London, looking at Gould's drawings, I began to understand what I had seen.