It is to be supposed that the repulsiveness of such 'unpalatable' butterflies is associated with the food-plant on which the caterpillar lives. Acrid, nauseous, astringent, and actually poisonous substances are produced in many plants, and we shall see later that this is to their own advantage; these substances pass into the insect, and they do so probably in part unaltered, in part certainly altered, but still they are protective, perhaps even in an increased degree. This is borne out by the fact that many caterpillars of immune butterflies live on more or less poisonous plants: the Acræidæ and Heliconiidæ on Passiflores, which contain nauseous substances; the Danaidæ on the poisonous Asclepiadæ, which are rich in milky juice or latex; the Euplœæ on the poisonous species of _Ficus_, the Neotropinæ on the Solanaceæ, and so on. But there are many genera, rich in species, and distributed over the whole earth, the caterpillars of which live on plants of very various families and characters, and of these the majority of species are palatable, though a few are repulsive in taste and odour, and therefore immune. This is the case in the genus _Papilio_. As far back as the sixties Wallace discovered that there were immune species of _Papilio_, and that these were mimicked by other species. Later it was shown that these immune species live chiefly on poisonous plants (in the wide sense), on various Aristolochiæ; and Haase has recently grouped these together as poison-eaters (Aristolochia-butterflies or Pharmacophagæ). They are distinguished by a conspicuous red on the body. In some of them, as in _Papilio philoxenus_, a repulsive odour as of decomposing urine has been detected in the living animal. We see, then, that the much-persecuted and easily injured butterflies make use of a poisonous substance (in the widest sense), prepared in the plant for its own protection, and, wherever their own metabolism makes it possible, they use it to protect themselves. We need not wonder, therefore, that so many butterflies are immune, nor that among the numerous palatable species a small proportion have endeavoured to become like the protected species, as far as natural selection was able to bring such a resemblance about. There is hardly any adaptation phenomenon so widely distributed and diverse in its manifestations, which has been at the same time so much observed and followed out into all its details, as Mimicry; and it must surely be regarded as a justification of the validity of interpreting it in terms of Natural Selection that all the observed phenomena tally so beautifully with the deductions from the theory. I at least know of no facts which contradict the theory, but of many which might have been predicted from it.