LECTURE V

TRUE MIMICRY

Mimicry: its discovery by Bates—Heliconiidæ and Pieridæ—Danaides—Papilio meropeand 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 merionesandPapilio merope—Comparison with the dimorphic caterpillars—Papilio turnus—'Mimicry rings' of immune species—Danais erippusandLimenitis archippus—Marked divergence of mimetic species from their nearest relatives—Mimicry in other insects—Imitators of ants and bees.

Letus 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 remaineda 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 occurred frequently, and usually in large swarms, he concluded that they must have few enemies, and as he never saw the numerous insectivorous birds and insects hunting them, he further concluded that they must have something disagreeable which secured them from the attacks of these predaceous forms. On the other hand, he found that the heliconid-like Whites were always rare, and he took this as a sign that they were much persecuted, and that they must, therefore, be palatable tit-bits for the insectivores. If it were possible, then, that a species of Whites with the usual white colour of the family should give rise to variations, which would make them in any degree resemble the Heliconiidæ, which are secure from persecution, and if, in addition, those that exhibited the profitable variation attached themselves to swarms of the mimicked form, then these variants would be to a certain extent secured from attack, and more and more so in proportion as the resemblance to the protected model increased. The great likeness of these Whites to the Heliconiidæ, Bates further argued, would depend on a process of selection, based on the fact that, in each generation, those individuals would on the average survive for reproduction which were a little more like the model than the rest, and thus the resemblance, doubtless slight to begin with, would gradually reach its present degree of perfection.

[2]Contributions to an Insect Fauna of the Amazon Valley, Trans. Linn. Soc., Vol. XXIII, 1862.

Bates's hypotheses have been subsequently confirmed in the most striking way. The Heliconiidæ do possess a disagreeable taste and odour, and are utterly rejected by birds, lizards, and other animals. It has been directly observed that puff-birds, species ofTrogon, and other insectivorous birds, looking down from the tops of trees in search of food, allowed to pass unheeded the swarms of gaily coloured Heliconiidæ which were fluttering among the leaves, and experiments with various insectivorous animals yielded the same result:the Heliconiidæ are immune. We can, therefore, not only understand that it must be advantageous to resemble them, we can also appreciate many of their peculiar characters, such as their gay coloration, which must serve as a sign of their disagreeable taste, and their slow, fluttering flight, as well as their habit of flocking together,which must make it easier for the birds to recognize them as uneatable. Everything which marks out these unpalatable morsels, and makes them more readily recognizable, must be to their advantage, and therefore must have been favoured by natural selection (Pl. II, Fig. 13).

In the same way, every increase of resemblance on the part of the mimics would increase their chances of escaping notice, and any one who is accustomed to observe butterflies in nature can well understand that even very slight resemblances may have formed the beginning of the selection process; perhaps even a mere variation in the manner of flight, combined with the habit of associating with the swarms of Heliconiidæ. I myself have many times been momentarily deceived in our own woods by a White of unusually majestic flight, so that I took it for anApaturaor aLimenitis. If, therefore, individual Whites occurred here and there in the Amazon valley, which flew somewhat after the manner of the Heliconiidæ, and associated with them, they might possibly have attained a certain degree of security through that alone, and it would be greatly increased if at the same time they varied somewhat in colour in the direction of their companions.

In any case there can be no doubt whatever that in these cases a real transformation of the species in colour and marking, and perhaps often, too, in form of wing, has taken place, and that within comparatively modern times—let us say during the distribution of a species which required protection over a large continent, or since the last breaking up of an immune species into local species. Various facts prove this; above all, the circumstance that it is often only the females which exhibit this protective mimicry; and that one and the same species may mimic a different immune species in different areas, but always the one occurring abundantly in that area, and so on.

Definite examples will make this clearer, and I will only say in advance that, since the discovery of Bates, numerous cases of mimicry in butterflies have been found, not only in South America, but in all tropical countries which have a rich Lepidopteran fauna. And it is not only between the Heliconiidæ and the Pieridæ that such relations have been evolved; many much-persecuted, unprotected species of different families everywhere mimic species which are rejected on account of their nauseous taste, and these, too, belonging to different families. The Heliconiidæ are a purely American group, but in the Old World and in Australia their place is taken by the three great families of Danaides, Euplœides, and Acræides, since, as it seems, they all taste unpleasantly, and are rejected by all, or at least by most, ofthe insectivorous birds. Numerous species of the genusDanais(Pl. I, Fig. 8),Amauris(Pl. I, Fig. 5),Euplœa(Pl. III, Fig. 25, 27), andAcræa(Pl. II, Fig. 2), and also many species ofPapilioand other genera, enjoy the advantage of unpleasant taste, if not even of poisonousness; they are, therefore, secure from pursuit, and are, in consequence, much mimicked by palatable butterflies.

As a further example, I now select a diurnal butterfly from Africa,Papilio meropeCramer[3], which was shown by Trimen in 1868 to be mimetic. The species has a wide distribution, for, if we except slight local differences in the marking of the male, its range extends over the greater part of Africa, from Abyssinia to the Cape, and from East Africa to the Gold Coast.

[3]The West African form ofPapilio meropehas been quite recently distinguished from the southern form and regarded as a distinct species, the latter being now calledPapilio cenea. The differences in the males are very slight—somewhat shorter wings, shorter wing-tail, and so on—differences which seem relatively unimportant in comparison with the differences between the males and the females.

The male is a beautiful large butterfly, yellowish white, with a touch of black, and with little tails to the posterior wings (Pl. I, Fig. 1), like our own swallowtail. A very nearly related species occurs in Madagascar, and there the female is similarly coloured, though it may be distinguished by having a little more black on the wing. On the mainland of Africa, however, the females ofPapilio meropeare so different in colour and form of wing that it would be difficult to believe them of the same species as the male had not both sexes more than once been reared from the eggs of one mother. The females (Pl. I, Fig. 6) in South Africa imitate a species ofAmauris,A. echeria(Pl. I, Fig. 7), of a dark ground-colour with white, or brownish-white, mirrors and spots, and they resemble it most deceptively. But what makes the case more interesting in its theoretical aspect is thatDanais echeriaof Cape Colony is markedly different fromDanais echeriaof Natal, and the female ofPapilio meropehas followed those two local varieties, and has likewise a Cape and a Natal local form. Even this is not all, for in Cape Colony there are two other females ofPapilio merope. One of them has a yellow ground-colour, and resemblesDanais chrysippus, which is extremely abundant there (Pl. I, Fig. 3); the other is entirely different (Pl. I, Fig. 4), for it closely mimics another Danaid occurring in the same districts of Africa, and also immune,Amauris niavius(Pl. I, Fig. 5), not only in the beautiful pure white and deep black of the wing surface, but also in the distribution of these colours to form a pattern.

We have thus in Africa four different females ofPapilio merope, each of which mimics a protected species of Danaid. They are not alwayslocally separate, so that each is exclusively restricted to a particular region, for their areas of distribution often overlap, and, at the Cape for instance, one male form and three different forms of female have been reared from one set of eggs. In addition, we have the fact that between the two local forms ofDanais echeriatransition forms occur, and that the mimetic females ofPapilio meropeshow the same transition forms locally, and we must admit that all these facts harmonize most beautifully with the selection interpretation, but defy any other. And that the last doubt may be dispelled, nature has preservedthe primitive female formon the continent of Africa—namely, in Abyssinia, where, along with the mimetic females, there are others which are tailed like the males (Pl. I, Fig. 1), and are like them in form and colour, a few minor differences excepted.

Thus we have inPapilio meropea species which, in the course of its distribution through Africa, has scarcely varied at all in the male sex, but in the female has almost everywhere lost the outward appearance of aPapilio, and has assumed that of a Danaid, which is protected by being unpalatable, and not even everywhere the appearance of the same species, but in each place that of the prevailing one, and sometimes of several in one region. These females thus show at the present day a polymorphism which consists of four chief mimetic forms, to which has to be added the primitive form—that resembling the male. This has survived in Abyssinia alone, and even there it is not the only one, but occurs along with some of the mimetic forms.

To the question why only the females are mimetic in this and other cases, Darwin and Wallace have answered that the females are more in need of protection. In the first place, the males among butterflies are considerably in the majority, and, secondly, the females must live longer in order to be able to lay their eggs. Moreover, the females, which are loaded with numerous eggs, are heavier in flight, and during the whole period of egg-laying—that is, for a considerable time—they are exposed to the attacks of numerous enemies. Whether one of the abundant males is devoured sooner or later is immaterial to the persistence of the species, since one male is sufficient to fertilize several females. The death of a single female, on the other hand, implies a loss of several hundred descendants to the species. It is, therefore, intelligible that, in species already somewhat rare, the female must first of all be protected; that is to say, that all variations tending in the direction of her protection would give rise to a process of selection resulting in an augmentation of the protective characters.

But there are also butterflies in which both sexes mimic a protected model. Thus many imitators of the unpalatable Acræides (Pl. II, Fig. 21) resemble the model in both sexes, and of the South American Whites which mimic the Heliconiidæ there are some which have the appearance of the Heliconiidæ even in the male sex (Pl. II, Fig. 18, 19), while others look like ordinary Whites (for instance,Archonias potamea). But in many of these species, which are mimetic in the female sex, we find also in the male some indications of the mimetic colouring, but in the first instance only on the under surface. Thus the females ofPerhybris pyrrha(Pl. II, Fig. 17) resemble in their black, yellow, and orange-red colour-pattern the immune American Danaid,Lycorea halia(Pl. II, Fig. 12), but their mates are, on the upper surface, like our common Whites, though they already show on the under surface the orange-red transverse stripes of theLycorea(Pl. II, Fig. 16). In other mimetic species of Whites a similar beginning is even more faintly hinted at, and in others, again, the upper surface of the male is also provided with protective colours, and only a single white spot on the posterior, or sometimes even on the anterior wing as well, shows the original white of the Pieridæ (Fig. 18).

I do not know how any one can put any other construction on these facts than that the females first assumed the protective colouring, and that the males followed later, and more slowly. Whether this is due to inheritance on the female side, and thus ensues as a mechanical necessity, in virtue of laws of inheritance still unknown to us, or whether it arose because there was a certain advantage in protection to the males—though not such a marked one—and that these, therefore, followed independently along the same path of evolution as the females, has yet to be investigated. Personally, I incline to the latter view, because there are protected mimetic species, in which the female mimics one immune model, and the male another, quite different from the female's. A case in point is that of an Indian butterfly,Euripus haliterses, and alsoHypolimnas scopas, in the latter of which the male resembles the male ofEuplœa pyrgion, and the female is like the somewhat different female of the same protected species. The IndianPapilio paradoxus, too, seems to show the independence of the processes of mimetic adaptation, for the male is like the blue male of the immuneEuplœa binotata(Pl. III, Fig. 25), while the female resembles the radially-striped female ofEuplœa midamus(Pl. III, Fig. 27), and this double adaptation is repeated in another of the persecuted butterflies,Elymnias leucocyma(Pl. III, Fig. 26, 28).

Many objections have been made to the interpretation of mimicryby selection. It has been asserted that butterflies are exposed to injury from birds only to an inconsiderable extent, not sufficient to account for such an intense and persistent process of selection, because they are not very welcome morsels, on account of the large and uneatable wings and the relatively small body. Doubt has also been raised as to the immunity of the models, which has not been proved in many of the species in regard to which it is assumed. Finally, it is maintained that the advantage which resemblance to an immune model brings is not proved, but is purely hypothetical; and that it is probable that the birds do not distinguish the colours and markings of the flying butterflies at all, but are at the most only deceived by resemblances in their manner of flight.

The last objection contains a certain amount of truth, inasmuch as the manner of flight always plays a part in the mimicry of a strange species. We shall see later how much the instincts of a species contribute to the deception in all cases of protective colouring. It is, therefore, not improbable that, in many cases, the imitation of the flight of an immune species, and a gradually increasing familiarity with the habitats of the same immune species, preceded the modification of the colour. Indeed, the slow flight of immune species (Heliconiidæ) has been unanimously emphasized by observers, as a factor in facilitating the recognition of the butterflies by the sharp-sighted birds.

That it was not only in earlier ages of the world's history that butterflies were much persecuted, as some have supposed, but that they are so still, seems to me indisputable in view of the observations of the last quarter of a century. Even in this country, where both butterflies and insect-eating birds are being more and more crowded out through cultivation, a considerable number of butterflies in flight fall victims to the birds. Kennel gives observations on this point in regard to the white-throat; Caspari for the swallows. The latter let about a hundred little tortoiseshell butterflies (Vanessa antiopa) fly from his window, 'but not ten of them reached the neighbouring wood,' all the rest being eaten by swallows, 'which congregated in numbers in front of his window.' Kathariner observed, in the highlands of Asia Minor, a flock of bee-eaters (Merops) which caught in flight and swallowed a great many individuals of a very beautiful diurnal butterfly (Thais cerisyi).

Finally, Pastor Slevogt has collected much evidence to show that our indigenous butterflies have a great deal to suffer in the way of persecution from birds. And in regard to tropical countries, the chase of butterflies by insectivorous birds has long been known.Thus Pöppig says that in the primitive forests one can easily recognize the place which has been selected by one of the Jacamars (Galbulidæ) as its favourite resting-place, for the wings of the largest and most beautiful butterflies, whose bodies alone are eaten, lie on the ground in a circle for a distance of several paces. We owe direct observations on the hunting of insects by birds of the primitive forest especially to Dr. Hahnel, who found many opportunities for observation in the course of his enthusiastic collecting journeys in Central and South America. He writes: 'No other family of butterflies suffered so much from birds as the Pieridæ (Whites), and these freebooters often snapped away the prettiest and freshest specimens from quite close to me. Every time I was amazed anew at the unfailing security of their flight, and I gladly paid for the spectacle by the loss of a few specimens.' Of the pursuit of one of the largeCaligospecies, whose leaf-like under surface, marked with eye-spots, I have already described, (Fig. 6, p. 70), he says: 'With incredible skill this fairly large insect avoided every blow of the bill of the bird which followed it in close chase, and saved itself by flying from one shrub to another, till at last it was lost to sight in the thickest tangle of branches, and the exhausted bird gave up further attempts at pursuit.'

But, in addition to the birds, the butterflies of the primitive forest have to dread the persecution of other insects, especially of the large predaceous dragon-flies, which throw themselves upon them in the midst of their flight. Hahnel often saw a specimen of the large, beautiful, blueMorpho cisseis, which was fluttering peacefully about the crown of a tree, suddenly shoot head downwards, 'like an ox with horns lowered, and then reascended apparently with difficulty, after it had torn itself free from its sudden assailant, whose jaws left distinct short scars.'

In addition to birds and predatory insects the butterflies are persecuted by the whole army of lizards. In order to entice the butterflies, Hahnel laid bait in the wood, 'sugar-cane, little sweet bananas, and such like.' Various kinds of butterfly settled on it, 'Satyrides, Ageroniæ,Adelphaand other Nymphalidæ.' He saw that they 'were persistently stalked and attacked by greedy lizards, which, in spite of their plump figure and uncouth gait, showed themselves able to spring suddenly out and snatch their prey with great adroitness. It is, however, very wonderful to see the agility such a persecuted insect displays in evading the repeated attacks of these marauders.' Thus on one occasion anAdelphawas driven off a dozen times from the exposed bait by a lizard, which pounced uponit, but it always settled down for a short time on a leaf, and soon returned to its repast, whereupon the enemy 'instantaneously rushed upon it in a fury, until at last he was obliged to give in,' abandoning the attempt to catch a creature so adept in retreat.

Many butterflies assemble at midday on sandbanks in the middle of the river, in order to drink, and there, too, the lizards are always lurking about. Hahnel gives a pretty and undoubtedly accurate description of the protective value of the long tail borne by many of the sail-like Papilios at the end of the posterior wing; they 'quite obviously' afford protection against the lizards, 'which, after snapping, often find themselves obliged to be content with the tail alone, while the rest of the animal flies away practically uninjured.'

Not only is the great persecution of the butterflies a fact, the immunity of the known species, which are models for mimicry, is also certain. For numerous species, at any rate, this has now been established. First of all—as has already been said—this is true of the Heliconiidæ, in regard to which Wallace long ago showed that, if the thorax be pressed, they exude a yellowish juice of unpleasant smell. This is probably the blood of the insect, but that does not hinder the repulsive odour of the living butterfly being perceptible at a distance of 'several paces,' as Seitz observed inHeliconius besei.

Repeated experiments have been made, which have shown that such butterflies are rejected not only by the insectivorous birds of the primitive forest, but also by tame turkeys, pheasants and partridges, usually so greedy. Hahnel has recently repeated these experiments in Brazil with hens, and he obtained the same result. The hens, 'which otherwise devoured all butterflies eagerly,' rejected all Ithomidæ, Heliconiidæ, the white Papilios, as also some of the gaily coloured Heliconiid-like moths which fly by day, such asEsthema bicolorandPericopis lycorea. Obviously, the gay or conspicuous colour of these Lepidoptera acts as a warning signal of their unpalatability, and protects them from attempts on the part of the birds to investigate their flavour. Hence we find that the under surface of these insects is coloured like the upper. Even the numbers of these species which fly about indicates that they must be little decimated, and, in point of fact, we never find the wings of Heliconiidæ lying on the ground in the forests of South America, while those of the Nymphalidæ and other butterflies are by no means uncommonly seen as the remains of birds' meals.

There is just as little room for doubt, as in the case of the Heliconiidæ and their allies, that the Danaidæ, Acræidæ, and theEuplœidæ in the tropical regions of the Old World enjoy a certain immunity on account of their repulsive odour and taste. Here, too, observation and experiment have shown that birds, lizards, and predaceous insects leave the butterflies of these families unmolested. I need only mention the observation of Trimen that, under an acacia much visited by butterflies, on which Mantides—the so-called praying-insects—caught and devoured large numbers, the wings of anAcræaor aDanaiswere never found. These unpalatable butterflies also possess a motley or at least striking dress, recognizable from afar, and alike on both surfaces; and they also have a slow flight, by which they are readily recognized. They, too, usually assemble in large swarms, and both sexes are alike, or resemble each other closely in colouring, or at least they are both equally conspicuous. But even these cases do not complete the list of butterflies which are protected by their unpalatability; among the otherwise much-persecuted and therefore palatable Pieridæ (Whites) there is an Asiatic genus,Delias, which in all probability belongs to the immune butterflies, as their gaily coloured under surface indicates, and among the nocturnal Lepidoptera of different countries and families there are isolated generations which are very gaily and conspicuously coloured, and which are rejected by birds, their unpleasant odour being perceptible at a distance of several feet (Chalcosiidæ and Eusemiidæ). The latter no longer fly under cover of night, like their relatives, but have assumed diurnal habits.

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 ofFicus, 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 genusPapilio. As far back as the sixties Wallace discovered that there were immunespecies ofPapilio, 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 inPapilio 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.

For instance, it might have been predicted from the theory alone that an immune species would often have several mimics, as, in point of fact, is frequently the case, and it would be easy to give numerous examples of this. Thus the two Danaids of South and Central Africa,Amauris echeriaandAmauris niavius, are mimicked, not only by the two female forms ofPapilio merope, as we have already described in detail, but the latter is also mimicked by Nymphalid, which requires protection,Diadema anthedon, and the former by two diurnal butterflies of different families,Diadema nuinaandPapilio echerioides.

Similarly, the black-and-red colouredHeliconius melpomenein Brazil is mimicked both by the female of a White (Archonias teuthamis), and by aPapilio, which has received the name ofP. euterpinuson account of this resemblance. Thus, too, the immuneMethona psidii, Cr. of Brazil, with its half-transparent wings marked with black bands, has five mimics, belonging to five different genera, and one of these is not a true diurnal butterfly at all,but one of the day-flying species of the genusCastnia, whose systematic position is doubtful.

Fig. 18.Upper surfaces ofA,Acræaegina, from the Gold Coast, immune.B,Papilio ridleyanus, from Gaboon, notimmune.C,Pseudacræa boisduvalii,from the Gold Coast, not immune.

The West African immune Acræid,Acræa gea(Pl. II, Fig. 21), is deceptively mimicked, both as to the narrow, long shape of the wing and its blackish-brown and white mottled markings, by a Nymphalid,Pseudacræa hirce, by the female of a Papilio (P. cynorta) whose mate is quite different, and by the female of a Satyrid (Elymnias phegea) (Pl. II, Fig. 20). In thePapiliothe resemblance extends to the peculiar pitch-black shining spot on the under side of the base of the posterior wing, and all three are like the model on both surfaces, and therefore in flight as well as in the resting attitude.

On the same West African coast occurs the strange greyish-blackAcræa egina, with brick-red spots and bands, and coal-black dots (Fig. 18,A). This immune species is deceptively mimicked in its native country by two other butterflies—a Nymphalid,Pseudacræa boisduvalii(Fig. 18,C), and by a femalePapilio(P. ridleyanus) (Fig. 18,B), by the latter not so exactly as by the former, but quite sufficiently to be confused with its model in flight.

It would have been less easy to predict with certainty from the theory that, conversely, the different species of a genus which stood in need of protection would be able to mimic quite different immune models, for who would have ventured to prophesy how far the capacity of a species for variation might go, and how many different kinds of coloration it was able to assume? But the facts teach us that there is a wide range of possibility in this respect.

Most interesting in this respect is, perhaps, the Asiatic-African genusElymnias, a Satyrid whose numerous (over thirty) species all seem to be in need of protection, for many of them mimic immune butterflies, while the rest are inconspicuous and are provided with protective colouring on the under surface. On Plates II and III some of the former are depicted beside their models. The single African species (Elymnias phegea) (Pl. II, Fig. 20) mimics, as has been already mentioned, the prevalentAcræa gea(Pl. II, Fig. 21). Many of the Asiatic Elymniidæ are mimics of the immune Euplœæ, especially the dark-brown species with steel-blue shimmer, such asE. patnain India,E. bezain Borneo, andE. penangain Borneo. In Amboina there flies anE. vitellia, the female of which mimics accurately the plain, light-brown, inconspicuousEuplœa climenawhich occurs there. The male ofElymnias leucocyma(Pl. III, Fig. 26) resembles the brown and blue shimmeringEuplœa binotata(Pl. III, Fig. 25), while the female mimics the dusky, radially-striped female ofEuplœa midamus(Pl. III, Figs. 27 and 28): the male ofElymnias cassiphoneresembles the blackish-brown and deep-blue iridescentEuplœa claudia, while the female is like the female ofEuplœa midamus. A number of species ofElymniascopy Danaids: thus both sexes ofE. laisare likeDanais vulgaris(Pl. III, Figs. 29 and 30), andE. ceryxandE. timandraare like another similar Danaid,D. tytia. The female only ofE. undularisof Ceylon mimics the brown-yellowD. genutia(Pl. II, Fig. 22) in general appearance, though not minutely, while the male (Pl. II, Fig. 24) seems to attempt an imitation of the blue Euplœæ. A rare form, not often represented in collections,Elymnias künstleri, bears a striking resemblance to the Danaid,Ideopsis daosBoisd., with its white wings spotted with black, while three species mimic the probably immune Pierid genusDelias, especially on the under surface, which is decorated with yellow and red. Perhaps the one which has diverged farthest from the original type isElymnias agondasBoisd. (Pl. II, Fig. 32) of the Papua region and the island of Waigeu, for it bears two large blue eye-spots on the posterior wings, and thus, especially in the case of the almost white female, closely resemblesTenaris bioculatus(Pl. III, Fig. 31). There are thus seven or eight types of marking and colouring differing from one another, and belonging to six different genera and a much greater number of species, which are mimicked by this one genusElymnias.

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 elementswhich were originally directed towards concealment. According to the beautiful observations of Erich Haase on this genusElymnias, 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 asElymnias 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 protectsabsolutely, 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 ofIthomiais quite different from that of the mimicking White, since the former will be, in structure, of the type ofIthomiacaterpillar, 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, immuneDanais erippus(Pl. I, Fig. 8), while the caterpillars of both species are quite different, that ofDanais erippuspossessing the remarkable, soft and flexible horn-like processes of the Danaid caterpillars (Pl. I, Fig. 10a), while the caterpillar ofLimenitis archippus(Pl. I, Fig. 11a) is at once recognizable by its blunt, club-shaped and spinose papillæ as aLimenitiscaterpillar. The adaptation of the butterfly to its protected model hasthus exercised no influence upon the caterpillar. Nor has it affected the pupa, which in both cases exhibits the very different and quite characteristic form of theDanaispupa and theLimenitispupa respectively (Pl. I, Fig. 10b, and 11b).

But even in the butterfly itself nothing is altered, except what increases the resemblance to the model. All else has remained unchanged, above all, the venation of the wings. Since the painstaking and valuable work of Herrich-Schäfer the venation has been made the basis of the whole systematic arrangement of butterflies, and it enables us, in point of fact, to distinguish with precision, not the families alone, but often even the genera, for the course of the veins in the different species of a single genus is the same, and that is true for the mimetic species as well as for others. Thus the Danaid-likeLimenitishas the usualLimenitisvenation, of the kind seen in our own indigenous species ofLimenitis, and the already describedElymniasspecies of the African and Indian forests and grassy plains have always the venation characteristic of this genus, whether they be protected only by sympathetic colouring or imitate an immuneEuplœa, aDanais, anAcræa, or aTenaris. However much the contour of the wing may vary, the venation is unaffected, and we can distinguish model from copy by this means alone, so that, even when there is the closest resemblance, no doubt is possible. In its theoretical aspect this constancy of venation is obviously important, for as nothing about the organism is incapable of variation, the veining of the wings might have varied, as indeed it has varied from genus to genus in the course of the phylogenetic history; but as changes in venation could not be detected by the butterflies' enemies, however sharp-sighted, there has been no reason in these cases for variation in this respect.

In this connexion Poulton has brought forward interesting facts showing that the mimics of one model, belonging to different genera, often secure the same effect in quite different ways. Thus the glass-like transparency of the wings in the Heliconiidæ of the genusMethonadepends on a considerable reduction of the size of the scales, which ordinarily cover both sides of the wing as thickly as the tiles on a roof, and produce the colour. In another quite similar species, also transparent-winged, the DanaidItuna ilione, the transparency is due to the absence of most of the scales, and in a third mimic,Castnia linus, var.heliconoides, the scales are not altered either in size or number, but have become absolutely unpigmented and transparent. In a fourth mimic, a Pierid,Dismorphia crise, the scales have not decreased in number, but have become quite minute, whilein a fifth case, the nocturnalHyelosia heliconoidesSwains., the same thing has happened as inCastnia, but the scales are also fewer in number. Thus in each of the mimics the changes which have taken place in the scales are quite different, but they bring about the same effect, the glass-like transparency of the wings, on which the resemblance to the model depends: what we have before us is, therefore, not a similarity of variation, but only an appearance of similarity in external features.

In the face of such facts there can be no further question of the often repeated objection, that the resemblance of model and copy depend on the similarity of external influences upon species living in the same latitude, even if that were not already sufficiently refuted by the frequent restriction of the mimicry to the female. And that mimicry should be a mere matter of chance is negatived even by the single fact that model and copy always live in the same area, and that the local varieties of the model are faithfully followed by the mimic. An interesting example of this is furnished byElymnias undularis, already mentioned, for in this case the female (Pl. II, Fig. 23) mimics the brown-yellowDanais plexippus(Pl. II, Fig. 22), not whereverE. undularisoccurs, but only in Ceylon and British India. In Burmah, where another Danaid,D. hegesippus, is common, it mimics that; and in Malacca it does not copy a Danaid at all, but resembles the male of its own species, which in India is very different from it, since there the female mimics one of the blue iridescent Euplœæ (Pl. III, Fig. 24). It cannot therefore be a matter of 'chance,' and we should have to give up all attempt at a scientific interpretation if we were not prepared to accept that of natural selection. Even the interference of a purposeful Power can hardly be seriously considered in this case, even by those who are inclined to such a view, for thegradualapproximation to the model, which is a matter of course in a process of evolution, could only appear, if referred to the benevolent intelligence of a Creator, as an unworthy trick, designed to lead humanity astray in its strivings after knowledge. On the other hand, this gradual increase of resemblance, which becomes apparent when we compare several mimetic species—this carrying over, step by step, from the female to the male—and many other facts point to the working of natural forces according to law, and, if there is to be found anywhere in living nature a complicated process of self-regulation, it certainly lies before us here, clearer and less open to objections than almost anywhere else. I do not mean to say, however, that we can verify it statistically in detail, as has been demanded by the fanatical opponents of natural selection. A direct testing ofnatural selection is, as has been already shown, nowhere possible: we can never exactly estimate how great the advantage is which a species requiring protection derives from a slight increase in the resemblance to an immune model; and I for one do not know how we could even definitely prove that a certain species needed a greater degree of protection than it had previously enjoyed in order to ensure its persistence in the struggle. It would be necessary to know the total number of individuals living on a certain area for many generations. If it appeared that there was a progressive diminution in the number of individuals, we should be justified in concluding that the species had not an adequate power of persistence, and that it therefore required a more effective protection. But it is impossible for us to collect such exact data for any species living under natural conditions, although we can often say approximately that a species is progressively decreasing in numbers. Even this, however, we can usually do only in cases which are influenced directly or indirectly by the interference of Man in nature, and in which the falling off in the species occurs so rapidly that there is no time for the slow counteractive influence of natural selection. We shall see later that in this way many species have been eliminated even within historic times.

I have just spoken of the 'need of protection,' and I have a few remarks to add on that subject. It is a mistake to believe that every 'rare' species, that is, one represented by few individuals, is already in process of disappearing. It is not the absolute number of individuals that determines the survival of a species, but the fact of the number remaining the same. It is equally mistaken to suppose that an amelioration of the conditions of existence for any species by natural selection is possible only when its persistence is already threatened; that is, when the number of individuals (the 'normal number') is steadily decreasing. On the contrary, it is of the essence of natural selection that every favourable variation which crops up is,ceteris paribus, preserved, and becomes the common possession of the species, quite independently of whether this improvement is absolutely necessary to its preservation or not. In the latter case it will simply become a commoner species instead of a rare one; and every species is, so to speak, striving to become common and widely distributed, since every advantageous variation that can possibly be produced is accumulated and made the common property of the species. But this has its limits, not only in the constitution and the structure of each species, but also in the external conditions of its life. If a species of butterfly be restricted, in the caterpillar stage, to a single, rare species of plant, its normal number will be, and must remain,a small one. But if there arise within it a variation in the food-instinct whereby a second and it may be a commoner plant becomes available, then the normal number of the species will rise, and perhaps the original number of individuals may be more than doubled. It is, however, by no means necessary to assume that the species was previously in process of decadence; on the contrary its normal number may have remained quite constant.

So, in the case of the mimetic butterflies, we do not need to assume that they all previously required protection in the sense that they would have become extinct had they not assumed a likeness to an immune species. We may indeed conclude, on other grounds, that it was the rarer species which increased their number of individuals by the mimetic protection, and in doing so they certainly enhanced at the same time their chance of survival as a species. In the more abundant species mimetic resemblance to species whose unpalatability rendered them immune could not have been evolved, as it would have been disadvantageous, not only for the model, but for the mimicking species itself, while in species less rich in individuals, such resemblance would necessarily have a protective value, no matter whether the species was in danger of extinction or not. The process of selection must have started simply because the mimetic individuals survived more frequently than the others, and the mimetic resemblance must have gone on increasing as long as the increase brought with it a more effective protection. It is, therefore, a fallacious objection to say that a species, whose existence was threatened, would, considering the slowness of the process of selection, have died out altogether before it could have acquired effective protection by mimicking an immune species. The assumption is false—the widespread, hazy idea that the process of natural selection can only begin when the existence of the species is threatened. On the contrary, every species utilizes every possibility of improvement; and every improvement for which variation supplies the necessary material is possible. The augmentation of the profitable variations follows as a necessity from the more frequent survival of the best-adapted individuals, and this 'more frequent survival' will be not only a relative one, due to the fact that the better adapted individuals will be less decimated, it will also be absolute, because more individuals of the species will survive than before. Of thisPapilio meropemay serve as an example; in Madagascar it now flies about only slightly varied from the original form, var.meriones. Here, therefore, the species is maintained, without the aid of mimetic protection. We do not know if the reason for this lies in the absenceof an immune model, or in the non-appearance of suitable mimetic variants, or in other conditions; but we know that without mimicry the species holds its own against its enemies. But if, in Abyssinia, a female of this butterfly exhibited variations which would make her resemble, in any degree, the unpalatableDanias chrysippus, these mimetic variants would be less decimated than the original form of female, and would, therefore, gain stability, and gradually increase both in mimetic resemblance and in the number of individuals. But is this any reason why the original form of the female should diminish in numbers? In itself, certainly not; the red mimetic females could increase in number without causing any decrease of the yellow ones, for the red are in no way in conflict with the yellow, and we must not think of the number of individuals as so fixed for each species that it cannot increase. On the contrary, itmustincrease, as soon as the conditions of existence are permanently improved, and this happens, in this case, through the mimetic protection of the red female. We can thus easily understand how mimetic and non-mimetic females can live side by side in Abyssinia.

In all the rest of Africa, however, there are only mimetic females ofPapilio merope, and none of the colour of the male; these last, therefore, have been crowded out by the mimetic form, not actively, but through the more frequent survival of the mimetic form, so that those like the male became gradually rarer, and finally died out—that is, ceased to occur. The matter is not so simple as it seems, and we shall best understand it by thinking of the dimorphism of the caterpillars of our hawk-moths, which we discussed before, in which the green form in the full-grown caterpillar is less well protected than the brown. In many species the brown form has crowded out the green, in others brown and green occur side by side, but the green is less abundant, and in some species very rare. This must be regarded as the simple result of the circumstance that a higher percentage of the green than of the brown caterpillars fall victims to enemies, and thus, in the course of generations, the green form becomes slowly but steadily rarer. This will be the case even if the newer and better adaptation raises the number of individuals (the 'normal number') in the species, for this increase must always be a limited one, even if it be very great, which is hardly likely in this case. For the normal number is not determined by the mortality at one stage, but by that at all the stages of life taken together. Thus a normal number always persists, notwithstanding the improved conditions for the species, and, on this assumption, the form under less favourable conditions cannot permanently hold its own with that under better conditions, but must gradually disappear. We can understand, then, that the primitive form of thePapilio meropefemale may persist even for a long time side by side with the mimetic form in certain habitats. It is, probably, not a mere chance, that this should have happened just in Abyssinia, for, in that region, the mimetic female is still tailed—that is, she has not yet reached the highest degree of resemblance to her immune model. In the whole of the rest of Africa the process of the transformation of the female has already reached its highest point, and on the east and west coasts, as well as in South Africa, the primitive form of the species is now represented only by the male.

The gradual dying out of the less favourably conditioned forms of a species is a law which follows as a logical necessity from the essence of the process of selection, but its reality may be inferred from the phenomena themselves. On it depends, as far at least as adaptations are concerned, the transformation of species.

A beautiful example of the crowding out of a less favoured form of a species by a more favoured one is afforded by a butterfly of North America, of which the two female forms have long been known, although the reason for their dimorphism was not understood. A yellow butterfly,Papilio turnus, not unlike our swallow-tail, has yellow females in the north and east of the United States, but black ones in the south and west. There was much guessing as to what the cause of this striking phenomenon might be, and it was for a time thought that this difference was directly due to the influence of climate, and, later, the black form of female was regarded as protectively coloured, because of the supposed greater persecution by birds in the south, since the female would be less easily recognized if of a dark colour, and would thus be better protected. This last explanation could hardly be looked upon as satisfactory, for a black butterfly in flight would be very easily seen by sharp-sighted birds; indeed, against a light background, it would be even more readily seen than a light one.

Since we have acquired a more exact knowledge of the immune species ofPapiliothis case has become clear to us. For on those stretches of country on which the black female ofPapilio turnuslives there occurs anotherPapiliowhich is black in both sexes,Papilio philenor, and this is one of those species which are protected by their unpleasant taste and odour. Here, therefore, we have a case of mimicry, the female ofPapilio turnusimitates the immunePapilio philenor, and thereby secures protection for itself; but as the immune model only occurs in the southern half of the distribution ofPapilioturnusa somewhat sharp separation of the two forms of female has been evolved; the black, mimetic form, being the most fit, has completely crowded the primitive yellow form out of the area inhabited byPapilio philenor, while beyond this area, to the north and west, the yellow form alone prevails. The extensive and careful studies of Edwards have shown that the two forms occur together only in a very narrow transition region.

We thus see that the facts, wherever we scrutinize them carefully, harmonize with the theory. Of course we can only penetrate to a certain depth with the theory of selection, and we are still far from having reached the fundamental causes of the phenomena. Indeed, our understanding must in the meantime stop short before the causes of variations and their accumulation, but up to that point the theory gives us clearness, and discloses the causal connexion of phenomena in the most beautiful way. Although we do not yet understand how the southern femalePapilio turnuswas able to produce the advantageous black, we do see why a black variation, when it did occur, should increase and be strengthened, until it crowded out the yellow form from the area of the immune model, and we are able in a general way to refer the whole complicated phenomena of mimicry to their proximate causes.

This is true also of other phenomena which have had no part in establishing the theory, since attention was only directed to them later, and it is true even of some which, at first sight, seem to contradict the theory altogether. To this class belongs, for instance, the phenomenon that immune species not unfrequently mimic each other, as was first observed among the Heliconiid-like butterflies of South America. In four different families, the Danaidæ, the Neotropidæ, the Heliconiidæ, and the Acræidæ, there are species, distributed over the same area, which resemble each other in their conspicuous colouring and marking, and also in the peculiar shape of the wings. After what has been said one might be inclined to regard one of these species as the unpalatable model and the others as the palatable mimics, but they are all unpalatable, and are not eaten by birds. The puzzle of this apparent contradiction was solved by Fritz Müller[4], who pointed out that the aversion to non-edible butterflies is not innate in birds, but must be acquired. Each young bird has to learn from experience which victim is good to eat, and which bad. If every inedible species had its particular and distinctive colour-dress a considerable number of individuals of each species would fall victims to the experiments of young birds in each generation, for a butterflywhich has once been pecked at, or squeezed by the bill of a bird, is doomed to die. But if two inedible species which resemble each other inhabit the same area they will be regarded by the birds as one and the same, and if five or more inedible species resemble each other all five will present the same appearance to the bird, and it will not require to repeat on the other four the experience of unpalatability it has gained from one. Thus the total of five species will be no more severely decimated by the young birds than each of them would have been if it had occurred alone; the same number of victims of experiment, which are necessary every year in the education of the young birds, will, when all five species look alike, be divided among the whole 'mimicry ring,' as we may say. The advantage of the resemblance is thus obvious, and we can understand why a process of selection should develop among such inedible species which should result in their being readily mistaken for one another; we can understand why, in the neighbourhood of Fritz Müller's home, Blumenau, in the province of Santa Catarina in South Brazil, the Danaidæ, species ofLycorea; the Heliconiidæ,Heliconius eucrateandEueides isabella; and the Neotropinæ,Mechanitis lysimniaand species ofMelinæa, should all exhibit the same colours, brown, black and yellow, in a similar pattern, on similarly shaped wings. The agreement is by no means perfect in detail, but it can be noticed in all parts of South America inhabited by species of these genera, and the same differences which distinguish, for instance, the two species ofHeliconiusflying in two different regions, also distinguish the two species ofEueidesand the two species ofMechanitis. In Honduras we find the same mutually protective company of inedible genera as in Santa Catarina, but represented by other species, which all differ from the species in Santa Catarina in the same characters, as, for instance, that they have two instead of one pale yellow cross-stripe on the anterior wings. The species are:Lycorea atergatis,Heliconius telchinia,Eueides dynastes,Mechanitis doryssus, andMelinæa imitata[5]. In the environs of Bahia this mimicry ring consists of the following species:Heliconius eucrate,Lycorea halia,Mechanitis lysimnia, andMelinæa ethra, as figured onPl. II, Fig. 12, iv, and such a mutual assurance society has always one or other edible species as mimic. The larger the mimetic assurance company is, the less harm can mimics do to it. In the case figured it is two Pieridæ already known to us that have fairly well assumed the Heliconiid guise, namely,Dismorphia astynome(Pl. II, Figs. 18 and 19) andPerhybris pyrrha(Pl. II, Figs. 16 and 17). In the latter of these the male still has, on the upper surface, just the appearance of one of our common Garden-whites, while the female is coloured quite like the Heliconiidæ, but without having lost the form of wing of the Whites. The larger the mimetic company is the greater will be the protection afforded to its palatable mimics, since they will be the more rarely seized by way of experiment. It is, of course, obvious that in this kind of mimicry—that is, in the imitation of an unpalatable and rejected species for protection—it is presupposed as a general postulate that the edible mimics are considerably in the minority, as Darwin showed; for if it were otherwise their enemies would soon discover that among the apparently unpalatable species there were some which were pleasant to taste. Here, too, the facts bear out the theory, although exceptions can easily be imagined, and do seem to occur.

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