进化论难题:
“There are lots of examples of animal groups that evolved penises, but I
can think of only a bare handful that subsequently lost them,” says Diane
Kelly from the University of Massachusetts in Amherst. “Ornithologists have
tied themselves in knots trying to explain why an organ that gives males an
obvious selective advantage in so many different animal species disappeared
in most birds. But it’s hard to address a question on why something
happens when you don’t know much about how it happens.”