It is seldom difficult to see some such possibility of gain in the numerous well-known cases of insect mimicry. The wasp tribe—formidable with their stings—are often “mimicked”; the unpalatable Heliconid butterflies are “imitated” by members of edible families, and some insects are such exact imitations of leaves that the all-devouring army ants have been seen to run over them without discovering the imposition. “Mimicry” is an unfortunate term inasmuch as it seems to imply intentional imitation; “protective resemblance” is better. It is generally accounted for by the action of “natural selection” upon random variations. No two members of a brood are exactly alike; slight variations in form, size, colour, etc., are constantly occurring, and when the variation is a useful one the animal possessing it has a slightly better chance of surviving and rearing progeny, some of whom will probably possess the same peculiarity, perhaps even in a more marked degree, and will be better equipped than their neighbours in the struggle for life. The happy possessors of such favourable variations are thus in a sense “selected” by nature, and this selection, acting through countless generations, is thought to be the chief agent in bringing about the remarkable phenomenon of protective resemblance. The theory has, no doubt, been pushed too far; fanciful resemblances have been detected and advantages of which there is no proof are sometimes asserted, and moreover other possible ways of accounting for the facts have been too much overlooked. But however it has come about, there is a case of “mimicry” among crab-spiders which deserves more than a passing mention. The name of the spider in question is _Phrynarachne decipiens_, and it was accidentally discovered by Forbes when butterfly-hunting in Java. It spins a white patch of silk on the upper side of a leaf on which it places itself back-downwards, clinging to the web by means of spines on its legs. It then folds its legs closely and lies absolutely still. In this position the spider and web look precisely like the dropping of some bird upon the leaf; such droppings are frequently seen, and seem to be particularly attractive to butterflies. It was not until Forbes tried to catch a butterfly settled on a leaf that he found that what looked like excrement was really a spider which held the butterfly in its grasp. Even after this experience he was again deceived by the same species in Sumatra. There are several extremely ant-like spiders, and it is remarkable that some of the imitators belong to widely different spider families:—that is to say the resemblance has arisen independently from quite different starting points.