Darwin turned wholeheartedly to the problem of evolution. Ever since his Beagle trip he had been convinced that the difference between what naturalists called 'varieties' and what they called 'species' was much less significant than previously thought. If pigeon breeders could create varieties as different as pouters, runts, and fantails, what would prevent nature from doing the same? And, given millions of years, wasn't it possible that a pigeon could be turned into something so radically different we would no longer be willing to call it a pigeon-or even a bird?