It is most interesting to note how these mimetic species give up, more or less, the original sympathetic colouring of the under surface, and use in establishing their mimicry the marking elements which were originally directed towards concealment. According to the beautiful observations of Erich Haase on this genus _Elymnias_, the ground-colouring on the under surface must have been 'a grey, darkly mottled protective one,' as still occurs, for instance, in several mimetic species, such as _Elymnias lais_ (Pl. III, Fig. 30). This leaf-colouring disappears more and more the more perfect the mimicry of the model becomes, until, finally, the model is repeated on the under surface also. Compare, for instance, Figs. 30 and 32. From this we may conclude that a dress which makes Lepidoptera appear unpalatable morsels is a more effective protection than resemblance to a leaf. That might indeed be deduced even from the theory, for resemblance to a leaf never protects _absolutely_, and does so, in any case, only during rest, while apparent unpalatability repels assailants at all times. Those unversed in butterfly lore usually ask, when these mimetic relations are expounded to them, how we know that copies which are so like their models really belong to a different genus, or even family. There are certainly cases in which model and copy resemble each other so closely that even a zoologist cannot tell one from the other without close examination, as, for instance, in the case of certain transparent-winged Heliconiidæ of Brazil (Ithomiides) and their mimics belonging to the family of Whites. But even in such cases the likeness only extends as far as is theoretically requisite, that is, only to those characters that make the butterfly appear to the eye of its pursuer like another species, known to it to be unpalatable. The likeness does not extend to details, which can only be seen with a magnifying-glass or a microscope, and above all, it does not extend to the caterpillar, pupa, or egg. Thus, in the case cited, we may be certain that the caterpillar of _Ithomia_ is quite different from that of the mimicking White, since the former will be, in structure, of the type of _Ithomia_ caterpillar, and the other of the usual type of Whites. As yet, indeed, these two species are not known in their caterpillar stages, but other cases are known. A species belonging to the same genus as our indigenous 'kingfishers' (_Limenitis populi_), a diurnal butterfly of North America, _Limenitis archippus_ (Pl. I, Fig. 9), strongly resembles the brown-yellow, immune _Danais erippus_ (Pl. I, Fig. 8), while the caterpillars of both species are quite different, that of _Danais erippus_ possessing the remarkable, soft and flexible horn-like processes of the Danaid caterpillars (Pl. I, Fig. 10_a_), while the caterpillar of _Limenitis archippus_ (Pl. I, Fig. 11_a_) is at once recognizable by its blunt, club-shaped and spinose papillæ as a _Limenitis_ caterpillar.