In “On the Origin of Species,” Darwin relied heavily on the analogy between artificial selection by humans for desirable traits of livestock and natural selection for traits in the wild. Now Bates had a whole new body of evidence for natural selection in the wild.

Bates explained to Darwin that he had found many instances in which a completely harmless and potentially edible animal resembled a distasteful, inedible, noxious or poisonous species. He observed flies that looked like bees, beetles that looked like wasps, even caterpillars that looked like pit vipers. He referred to these as “analogous resemblances” or “mimetic analogies.”

Bates deduced that defenseless mimics gained an advantage by resembling well-defended species. He concluded that the many cases he had observed were not mere coincidences, as the mimicking forms only occurred in the same geographical area as the species they imitated. He offered the phenomenon, still referred to today as Batesian mimicry, as “a most beautiful proof of the theory of natural selection.”

Some less scientific, more sentimental naturalists of the day were inclined to view these resemblances among species as merely nature’s proclivity for beauty and ornamentation, not the consequence of the battle of nature. Bates countered by pointing to other kinds of imitations, like moths and caterpillars that resembled bird droppings. Where was the beauty in that, he asked?

There is good experimental evidence for Batesian mimicry and the advantages gained by innocuous animals that resemble well-defended animals. But, until recently, there were few tests of how the imitation of bird waste, thorns, twigs or stones actually works. The challenge is to distinguish whether the ruse is a matter of concealment, with the predator failing to detect the animal, or a case of “masquerade,” with the predator actually detecting the imitator, misidentifying it as something inedible, and then ignoring it.

Recently, the biologists John Skelhorn and Graeme D. Ruxton at the University of Glasgow and their collaborators Hannah M. Rowland and Michael P. Speed at the University of Liverpool devised such a test using the twig-imitating caterpillars of the brimstone moth and early thorn moth as prey .

For masquerade to work, a predator must have had some experience with the objects being imitated. So the scientists divided young chickens into several groups  one group was exposed to a hawthorn branch, which is a common home for the caterpillars, another group was exposed to a hawthorn branch that was wound in colored thread in order to alter its appearance, and a third group was exposed only to an empty testing cage.