Sympathetically coloured caterpillars are, however, by no means the only ones; there are some with such striking, glaring colours that, far from rendering their possessors inconspicuous, they make them visible from a long way off; but this apparent contradiction of the theory of the colour-adaptation of animals that require protection has been explained by the acuteness of Alfred Russel Wallace. We know that among insects, and also among caterpillars, there are many which have a repulsive taste. In any case, certain caterpillars are rejected by many birds and lizards. Such species are, therefore, relatively safe from being devoured. If they were protectively coloured, or if, moreover, they resembled caterpillars with an agreeable taste, they would gain little advantage from their unpalatability; for the birds would at first take them for eatable, and would only discover their repulsiveness on attempting to eat them. But a caterpillar which has received a single stroke from a bird's bill is doomed to death. It must therefore be of the greatest advantage for unpalatable caterpillars, and unpalatable animals generally, to be in their colouring as conspicuously distinguishable as possible from the edible species. Hence, then, the glaring colours, which we can now refer without any further difficulty to the process of natural selection, for every individual of an ill-tasting species that is more conspicuously coloured than its fellows must have an advantage over them, and must have a better chance of surviving, because it will be less easily mistaken for a member of an edible species. [Illustration: FIG. 7. Caterpillar of a North American _Darapsa_ in its "terrifying attitude" (after Abbot and Smith).] I should like to discuss one other phenomenon, which is well calculated to give us a deeper insight into the transformation processes of organisms--I refer to the remarkable dimorphism of colour which occurs in many of the species of caterpillar just described.