Why did a peacock's tail make Darwin feel sick?

The peacock's tail bothered Darwin because his theory of natural selection said that features should help an animal survive, and a huge, shimmering train looks like the opposite of useful. "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" he wrote to a colleague. His way out was a second idea: sexual selection. Some animals leave more offspring not because they survive better, but because they are more attractive to mates, or win the fights for them.

If peahens keep choosing the most dazzling males, then dazzle gets passed on, generation after generation, even if it costs the males dearly. Darwin split this force into two kinds: rivals competing with each other, like stags clashing antlers, and one sex choosing the other, like peahens inspecting trains. The idea that females were doing the choosing shocked many Victorians.

Biologists still argue about exactly what the peahen is looking for. In the 1990s Marion Petrie found that males with more eyespots mated more, and that females lost interest when she trimmed the eyespots off. But a seven-year study in Japan found no sign that peahens preferred fancier trains. The tail that made Darwin sick is still making scientists think.