Mimicry: its discovery by Bates--Heliconiidæ and Pieridæ--Danaides--_Papilio merope_ and its five females--The females lead the way--Species with mimicry in both sexes--Objections--Enemies of butterflies--The immunity of the models--Poisonousness of the food-plants of immune species--Several mimics of the same immune species--Persecuted species of the same genus resemble quite different models--_Elymnias_--Degree of resemblance--Differences between the caterpillars of the model and the copy--The same resemblance arrived at by different ways--Transparent-winged butterflies--The gradually increasing resemblance points to causes operating mechanically--Rarity of the mimetic species--Danger to the existence of the species not a necessary condition of mimetic transformation--_Papilio meriones_ and _Papilio merope_--Comparison with the dimorphic caterpillars--_Papilio turnus_--'Mimicry rings' of immune species--_Danais erippus_ and _Limenitis archippus_--Marked divergence of mimetic species from their nearest relatives--Mimicry in other insects--Imitators of ants and bees. LET us now turn to the most remarkable of all protective form- and colour-adaptations, the so-called Mimicry, including all cases of the imitation of one animal by another, which we came to know first through Bates, and to a fuller understanding of which A. R. Wallace and Fritz Müller have especially contributed. While the English naturalist, Bates[2], was collecting and observing on the banks of the Amazons--as he did for twelve years--it sometimes occurred that, among a swarm of those gaily coloured, quaintly shaped butterflies, the Heliconiidæ (Pl. II, Fig. 13), he caught one which, on closer examination, proved to be essentially different from its numerous companions. It was certainly like them both in colour and form, but it belonged to quite a different family of butterflies, that of the Pieridæ or Whites (Pl. II, Fig. 19). These whites with the colours of the Heliconiidæ always occurred singly in swarms of the latter form, and Bates found that, in the different districts of the Amazon, they always resembled in a striking manner the species of Heliconiidæ there prevalent. Many of them had been previously known to entomologists, and because they diverged so far from the usual type of the Pieridæ, especially in the form of the wing, the name Dysmorphia, the 'mis-shapen,' had been given to them, although the meaning of this 'mis-shapenness' long remained a mystery. The French Lepidopterist, Boisduval, went a step further when he pointed out as something remarkable that nature sometimes makes several species of quite different families exactly alike, and called attention to three African butterflies, of which we shall have to speak later in detail. But even he was too much fettered by the old views of the immutability of species to arrive at a correct interpretation. Thus it was reserved for Bates to take the decisive step. Observing that the Heliconiidæ