feedback mechanism that kept species and varieties adapted to their environment (now called 'stabilizing", as opposed to 'directional' selection). They point to a largely overlooked passage of Wallace's famous 1858 paper, in which he likened "this principle ... [to] the centrifugal governor of the steam engine, which checks and corrects any irregularities". The cybernetician and anthropologist Gregory Bateson observed in the 1970s that, although writing it only as an example, Wallace had "probably said the most powerful thing that'd been said in the 19th Century". Bateson revisited the topic in his 1979 book Mind and Nature: A Necessary Unity, and other scholars have continued to explore the connection between natural selection and systems theory. Warning coloration and sexual selection Warning coloration was one of Wallace's contributions to the evolutionary biology of animal coloration. In 1867, Darwin wrote to Wallace about a problem in explaining how some caterpillars could have evolved conspicuous colour schemes. Darwin had come to believe that many conspicuous animal colour schemes were due to sexual selection, but he saw that this could not apply to caterpillars. Wallace responded that he and Bates had observed that many of the most spectacular butterflies had a peculiar odour and taste, and that he had been told by John Jenner Weir that birds would not eat a certain kind of common white moth because they found it unpalatable. Since the moth was as conspicuous at dusk as a coloured caterpillar in daylight, it seemed likely that the conspicuous colours served as a warning to predators and thus could have evolved through natural selection. Darwin was impressed by the idea. At a later meeting of the Entomological Society, Wallace asked for any evidence anyone might have on the topic. In 1869, Weir published data from experiments and observations involving brightly coloured caterpillars that supported Wallace's idea. Wallace attributed less importance than Darwin to sexual selection. In his 1878 book Tropical Nature and Other Essays, he wrote extensively about the coloration of animals and plants, and proposed alternative explanations for a number of cases Darwin had attributed to sexual selection. He revisited the topic at length in his 1889 book Darwinism. In 1890, he wrote a critical review in Nature of his friend Edward Bagnall Poulton's The Colours of Animals which supported Darwin on sexual selection, attacking especially Poulton's claims on the "aesthetic preferences of the insect world". Wallace effect In 1889, Wallace