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cases as to be mistaken when on the wing, and the difference only to be detected by close examination. As an example of the resemblance, woodcuts are given of one pair in which the colours are simple, being olive, yellow, and black, while the
very distinct neuration of the wings and form of the head and body can be easily seen.
Besides these Pieridæ, Mr. Bates found four true Papilios, seven Erycinidæ, three Castnias (a genus of day-flying moths), and fourteen species of diurnal Bombycidæ, all imitating some species of Heliconidæ which inhabited the same district; and it is to be especially noted that none of these insects were so abundant as the Heliconidæ they resembled, generally they
were far less common, so that Mr. Bates estimated the proportion in some cases as not one to a thousand. Before giving an account of the numerous remarkable cases of mimicry in other parts of the world, and between various groups of insects and of higher animals, it will be well to explain briefly the use and purport of the phenomenon, and also the mode by which it has been brought about.
How Mimicry has been Produced. The fact has been now established that the Heliconidæ possess an offensive odour and taste, which lead to their being almost entirely free from attack by insectivorous creatures; they possess a peculiar form and mode of flight, and do not seek concealment; while their colours—although very varied, ranging from deep blue-black, with white, yellow, or vivid red bands and spots, to the most delicate semitransparent wings adorned with pale brown or yellow markings— are yet always very distinctive, and unlike those of all the other families of butterflies in the same country. It is, therefore, clear that if any other butterflies in the same region, which are eatable and suffer great persecution from insectivorous animals, should come to resemble any of these uneatable species so closely as to be mistaken for them by their enemies, they will obtain thereby immunity from persecution. This is the obvious and sufficient reason why the imitation is useful, and therefore why it occurs in nature. We have now to explain how it has probably been brought about, and also why a still larger number of persecuted groups have not availed themselves of this simple means of protection.
From the great abundance of the Heliconidæ 1 all over tropical America, the vast number of their genera and species, and their marked distinctions from all other butterflies, it follows that they constitute a group of high antiquity, which in the course of ages has become more and more specialised, and owing to its peculiar advantages has now become a dominant and aggressive race. But when they first arose from some ancestral species or group which, owing to the food
1 These butterflies are now divided into two sub-families, one of which is placed with the Danaidæ ; but to avoid confusion I shall always speak of the American genera under the old term Heliconidæ.
of the larvæ or some other cause, possessed disagreeable juices that caused them to be disliked by the usual enemies of their kind, they were in all probability not very different either in form or coloration from many other butterflies. They would at that time be subject to repeated attacks by insecteaters, and, even if finally rejected, would often receive a fatal injury. Hence arose the necessity for some distinguishing mark, by which the devourers of butterflies in general might learn that these particular butterflies were uneatable ; and every variation leading to such distinction, whether by form, colour, or mode of flight, was preserved and accumulated by natural selection, till the ancestral Heliconoids became well distinguished from eatable butterflies, and thenceforth comparatively free from persecution. Then they had a good time of it. They acquired lazy habits, and flew about slowly. They increased abundantly and spread all over the country, their larvæ feeding on many plants and acquiring different habits; while the butterflies themselves varied greatly, and colour being useful rather than injurious to them, gradually diverged into the many coloured and beautifully varied forms we now behold.
But, during the early stages of this process, some of the Pieridæ, inhabiting the same district, happened to be sufficiently like some of the Heliconidæ to be occasionally mistaken for them. These, of course, survived while their companions were devoured. Those among their descendants that were still more like Heliconidæ again survived, and at length the imitation would become tolerably perfect. Thereafter, as the protected group diverged into distinct species of many different colours, the imitative group would occasionally be able to follow it with similar variations,-a process that is going on now, for Mr. Bates informs us that in each fresh district he visited he found closely allied representative species or varieties of Heliconidæ, and along with them species of Leptalis (Pieridæ), which had varied in the same way so as still to be exact imitations. But this process of imitation would be subject to check by the increasing acuteness of birds and other animals which, whenever the eatable Leptalis became numerous, would surely find them out, and would then probably attack both these and their friends the Heliconidæ in order to devour the former and reject the latter. The Pieridæ would, however, usually be less numerous, because their larvæ are often protectively coloured and therefore edible, while the larvæ of the Heliconidæ are adorned with warning colours, spines, or tubercles, and are uneatable. It seems probable that the larvæ and pupa of the Heliconidæ were the first to acquire the protective distastefulness, both because in this stage they are more defenceless and more liable to fatal injury, and also because we now find many instances in which the larvæ are distasteful while the perfect insects are eatable, but I believe none in which the reverse is the case. The larvæ of the Pieridæ are now beginning to acquire offensive juices, but have not yet obtained the corresponding conspicuous colours ; while the perfect insects remain eatable, except perhaps in some Eastern groups, the under sides of whose wings are brilliantly coloured although this is the part which is exposed when at rest.
It is clear that if a large majority of the larve of Lepidoptera, as well as the perfect insects, acquired these distasteful properties, so as seriously to diminish the food supply of insectivorous and nestling birds, these latter would be forced by necessity to acquire corresponding tastes, and to eat with pleasure what some of them now eat only under pressure of hunger; and variation and natural selection would soon bring about this change.
Many writers have denied the possibility of such wonderful resemblances being produced by the accumulation of fortuitous variations, but if the reader will call to mind the large amount of variability that has been shown to exist in all organisms, the exceptional power of rapid increase possessed by insects, and the tremendous struggle for existence always going on, the difficulty will vanish, especially when we remember that nature has the same fundamental groundwork to act upon in the two groups, general similarity of forms, wings of similar texture and outline, and probably some original similarity of colour and marking. Yet there is evidently considerable difficulty in the process, or with these great resources at her command nature would have produced more of these mimicking forms than she has done. One reason of this deficiency probably is, that the imitators, being always fewer in number, have not been able to keep pace with the variations of the much more numerous imitated form ; another reason may be the ever-increasing acuteness of the enemies, which have again and again detected the imposture and exterminated the feeble race before it has had time to become further modified. The result of this growing acuteness of enemies has been, that those mimics that now survive exhibit, as Mr. Bates well remarks, “a palpably intentional likeness that is perfectly staggering,” and also “that those features of the portrait are most attended to by nature which produce the most effective deception when the insects are seen in nature.” No one, in fact, can understand the perfection of the imitation who has not seen these species in their native wilds. So complete is it in general effect that in almost every box of butterflies, brought from tropical America by amateurs, are to be found some species of the mimicking Pieridæ, Erycinidæ, or moths, and the mimicked Heliconidæ, placed together under the impression that they are the same species. Yet more extraordinary, it sometimes deceives the very insects themselves. Mr. Trimen states that the male Danais chrysippus is sometimes deceived by the female Diadema bolina which mimics that species. Dr. Fritz Müller, writing from Brazil to Professor Meldola, says, “ One of the most interesting of our mimicking butterflies is Leptalis melite. The female alone of this species imitates one of our common white Pierida, which she copies so well that even her own male is often deceived ; for I have repeatedly seen the male pursuing the mimicked species, till, after closely approaching and becoming aware of his error, he suddenly returned."1 This is evidently not a case of true mimicry, since the species imitated is not protected; but it may be that the less abundant Leptalis is able to mingle with the female Pieridæ and thus obtain partial immunity from attack. Mr. Kirby of the insect department of the British Museum informs me that there are several species of South American Pieridæ which the female Leptalis melite very nearly resembles. The case, however, is interesting as showing that the butterflies are themselves deceived by a resemblance which is not so great as that of some mimicking species.