With the acclaim for Darwin’s postulate of evolution through natural selection came the excitement and the challenge of explaining ever-more complicated natural phenomena in Darwinian terms. And as biological explorers continued to describe new observations, particularly from tropical habitats, the number of challenges grew rapidly. An avalanche of letters among these naturalists exchanged ideas and hypotheses, with Darwin’s correspondence itself revealing extensive musing on an array of patterns and their possible emergence from natural selection on individuals.

Few observations were as intriguing as those on mimicry and camouflage. While some observations seemed easy to explain – caterpillars that blended with their leafy backgrounds would be less likely to be preyed upon than caterpillars that contrasted with their backgrounds – others were more difficult. Mimicry was one of those more difficult challenges. The early history of Darwinian evolution, as a science, is tightly entwined with the arguments about whether mimicry could be readily explained in Darwinian terms (see Essay 15, “Mimicry and Camouflage”).

It is important to distinguish two distinct phenomena of organismal coloration and pattern (Ruxton, Speed, and Kelly 2004). Crypsis occurs when it is difficult to distinguish an organism from its background. This can happen when an organism’s color or pattern causes it to blend visually into its background, when its shape and color make it resemble an object in its background, or when its pattern and color break the outline of its shape against its natural background and make it difficult to recognize. Our usual understanding of “camouflage” embraces one or the other of these descriptions ( Figs. 40.1 and 40.2 ). Mimicry occurs when the features of one species resemble those of another and, through that resemblance, confer some survival advantage on the mimic.