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Charles Darwin was stumped by peacocks. According to his theory of evolution, some creatures were better equipped to survive in their particular environment than others. It explained a lot - but it didn't explain the peacock's brightly coloured tail feathers, which were extravagant and cumbersome. Surely such plumage made it harder for peacocks to survive?

It so happens that the life of Darwin's own grandfather offered clues to the puzzle of the peacock's tail - if only he'd known to look there...

For a full list of sources, see the show notes at timharford.com.