which renders it almost impossible for a naturalist to detect the animal when living in its natural environment. [Illustration: FIG. 110.--Imitative forms and colours in insects. Drawn from nature (_R. Coll. Surg. Mus._).] _Warning Colours._ In strange and glaring contrast to all these cases of protective colouring, stand other cases of conspicuous colouring. Thus, for example, although there are numberless species of caterpillars which present in an astonishing degree the phenomena of protective colouring, there are numberless other species which not only fail to present these phenomena in any degree, but actually go to the opposite extreme of presenting colours which appear to have been developed for the sake of their conspicuousness. At all events, these caterpillars are usually the most conspicuous objects in their surroundings, and therefore in the early days of Darwinism they were regarded by Darwin himself as presenting a formidable difficulty in the way of his theory. To Mr. Wallace belongs the merit of having cleared up this difficulty in an extraordinarily successful manner. He virtually reasoned thus. If the _raison d'être_ of protective colouring be that of concealing agreeably flavoured caterpillars from the eye-sight of birds, may not the _raison d'être_ of conspicuous colouring be that of protecting disagreeably flavoured caterpillars from any possibility of being mistaken by birds? Should this be the case, of course the more conspicuous the colouring the better would it be for the caterpillars presenting it. Now as soon as this suggestion was acted upon experimentally, it was found to be borne out by facts. Birds could not be induced to eat caterpillars of the kinds in question; and there is now no longer any doubt that their conspicuous colouring is correlated with their distastefulness to birds, in the same way as the inconspicuous or imitative colouring of other caterpillars is correlated with their tastefulness to birds. Here then is yet another instance, added to those already given, of the verification yielded to the theory of natural selection by its proved competency as a guide to facts in nature; for assuredly this particular class of facts would never have been suspected but for its suggestive agency.