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MIMICRY IN BUTTERFLIES

BY

REGINALD CRUNDALL PUNNETT, F.R.S.

Fellow of Gonville and Caius College
Arthur Balfour Professor of Genetics in the University of Cambridge

Cambridge:
at the University Press
1915

PREFACE

This little book has been written in the hope that it may appeal to several classes of readers.

Not infrequently I have been asked by friends of different callings in life to recommend them some book on mimicry which shall be reasonably short, well illustrated without being very costly, and not too hard to understand. I have always been obliged to tell them that I know of nothing in our language answering to this description, and it is largely as an attempt to remedy this deficiency that the present little volume has been written.

I hope also that it will be found of interest to those who live in or visit tropical lands, and are attracted by the beauty of the butterfly life around them. There are few such countries without some of these cases of close resemblance between butterflies belonging to different families and groups, and it is to those who have the opportunity to be among them that we must look for fuller light upon one of the most fascinating of all nature's problems. If this little book serves to smooth the path of some who would become acquainted with that problem, and desire to use their opportunities of observation, the work that has gone to its making will have been well repaid.

To those who cultivate biological thought from the more philosophical point of view, I venture to hope that what I have written may not be without appeal. At such a time as the present, big with impending changes in the social fabric, few things are more vital than a clear conception of the scope and workings of natural selection. Little enough is our certain knowledge of these things, and small though the butterfly's contribution may be I trust that it will not pass altogether unregarded.

In conclusion I wish to offer my sincere thanks to those who have helped me in different ways. More especially are they due to my friends Dr Karl Jordan for the loan of some valuable specimens, and to Mr T. H. Riches for his kindly criticism on reading over the proof-sheets.

R. C. P.

CONTENTS

"The process by which a mimetic analogy is brought about in nature is a problem which involves that of the origin of all species and all adaptations."—H. W. Bates, 1861.

"With mimesis, above all, it is wise, when the law says that a thing is black, first to inquire whether it does not happen to be white."—Henri Fabre.

CHAPTER I

INTRODUCTORY

It is now more than fifty years since Darwin gave the theory of natural selection to the world, and the conception of a gradual evolution has long ago become part of the currency of thought. Evolution for Darwin was brought about by more than one factor. He believed in the inherited effects of the use and disuse of parts, and he also regarded sexual selection as operating at any rate among the higher animals. Yet he looked upon the natural selection of small favourable variations as the principal factor in evolutionary change. Since Darwin's time the trend has been to magnify natural selection at the expense of the other two factors. The doctrine of the inherited effects of use and disuse, vigorously challenged by Weismann, failed to make good its case, and it is to-day discredited by the great majority of biologists. Nor perhaps does the hypothesis of sexual selection command the support it originally had. At best it only attempted to explain those features, more especially among the higher animals, in which the sexes differ from one another in pattern, ornament, and the like. With the lapse of time there has come about a tendency to find in natural selection alone a complete explanation of the process of evolution, and to regard it as the sole factor by which all evolutionary change is brought about. Evolution on this view is a gradual process depending upon the slow accumulation by natural selection of small variations, which are more or less inherited, till at last a well-marked change of type is brought about. Could we have before us all the stages through which a given form has passed as natural selection transforms it into another, they would constitute a continuous series such that even refined scrutiny might fail to distinguish between any two consecutive terms. If the slight variations are not of service they will get no favour from natural selection and so can lead to nothing. But if of use in the struggle for existence natural selection preserves them and subsequent variations in the same direction until at length man recognises the accumulation as a new form. Moreover when the perfect thing is once elaborated natural selection will keep it perfect by discouraging any tendency to vary from perfection.

Upon this view, of which the most distinguished protagonist was Weismann, natural selection is the sole arbiter of animal and plant form. Through it and it alone the world has come to be what it is. To it must be ascribed all righteousness, for it alone is the maker. Such in its extreme form is the modern development of Darwin's great contribution to philosophy.

But is it true? Will natural selection really serve to explain all? Must all the various characters of plants and animals be supposed to owe their existence to the gradual operation of this factor working upon small variations?

Of recent years there has arisen a school of biologists to whom the terms mutationist and Mendelian are frequently applied. Influenced by the writings of Bateson and de Vries, and by the experimental results that have flowed from Mendel's discovery in heredity, they have come to regard the process of evolution as a discontinuous one. The new character that differentiates one variety from another arises suddenly as a sport or mutation, not by the gradual accretion of a vast number of intermediate forms. The white flowered plant has arisen suddenly from the blue, or the dwarf plant from the tall, and intermediates between them need never have existed. The ultimate fate of the new form that has arisen through causes yet unknown may depend upon natural selection. If better endowed than the parent form in the struggle for existence it may through natural selection come to supplant it. If worse endowed natural selection will probably see to its elimination. But if, as may quite possibly happen, it is neither better nor worse adapted than the form from which it sprang, then there would seem to be no reason for natural selection having anything to do with the relation of the new form to its parent.

Between the older and the newer or mutationist point of view an outstanding difference is the rÔle ascribed to natural selection. On the one view it builds up the new variety bit by bit, on the other the appearance of the new variety is entirely independent of it. From this there follows a radical difference with regard to the meaning of all the varied characters of plants and animals. Those who uphold the all-powerfulness of natural selection are bound to regard every character exhibited by an animal or plant as of service to it in the struggle for existence. Else it could not have arisen through the operation of natural selection. In other words every character in plant or animal must be adaptive. On the mutationist view this of course does not follow. If the new character which arises independently of natural selection is neither of service nor disservice to its possessors in the struggle for existence, there seems no reason why it should not persist in spite of natural selection. In attempting to decide between the two conflicting views the study of adaptation is of the first importance.

It was perhaps in connection with adaptation that Darwin obtained the most striking evidence in support of his theory, and it is clear from his writings that it was in this field he laboured with most delight. The marvellous ways in which creatures may be adapted in structure and habit for the life they lead had not escaped the attention of the older naturalists. John Ray wrote a book^[1] upon the subject in which he pointed out that all things in the Universe, from the fixed stars to the structure of a bird, or the tongue of a chameleon, or the means whereby some seeds are wind distributed, are "argumentative of Providence and Design" and must owe their existence to "the Direction of a Superior Cause." Nor have there been wanting other authors who have been equally struck by the wonders of adaptation. But their studies generally led to the same conclusion, an exhortation to praise the infinite Wisdom of Him Who in the days of Creation had taken thought for all these things.

The advent of natural selection threw a new light upon adaptation and the appearance of design in the world. In such books as those on The Fertilization of Orchids and The Forms of Flowers Darwin sought to shew that many curious and elaborate structures which had long puzzled the botanist were of service to the plant, and might therefore have arisen through the agency of natural selection. Especially was this the case in orchids where Darwin was able to bring forward striking evidence in favour of regarding many a bizarre form of flower as specially adapted for securing the benefits of cross-fertilization through the visits of insects. In these and other books Darwin opened up a new and fascinating field of investigation, and thenceforward the subject of adaptation claimed the attention of many naturalists. For the most part it has been an observational rather than an experimental study. The naturalist is struck by certain peculiarities in the form or colour or habits of a species. His problem is to account for their presence, and as nearly all students of adaptation have been close followers of Darwin, this generally means an interpretation in terms of natural selection. Granted this factor it remains to shew that the character in question confers some advantage upon the individuals that possess it. For unless it has a utilitarian value of some sort it clearly cannot have arisen through the operation of natural selection. However when it comes to the point direct proof of this sort is generally difficult to obtain. Consequently the work of most students of adaptation consists in a description of the character or characters studied together with such details of its life-history as may seem to bear upon the point, and a suggestion as to how the particular character studied may be of value to its possessors in the struggle for existence. In this way a great body of most curious and interesting facts has been placed on record, and many ingenious suggestions have been made as to the possible use of this or that character. But the majority of workers have taken natural selection for granted and then interested themselves in shewing how the characters studied by them might be of use. Probably there is no structure or habit for which it is impossible to devise some use^[2], and the pursuit has doubtless provided many of its devotees with a pleasurable and often fascinating exercise of the imagination. So it has come about that the facts instead of being used as a test of the credibility of natural selection, serve merely to emphasise the pÆan of praise with which such exercises usually conclude. The whole matter is too often approached in much the same spirit as that in which John Ray approached it two centuries ago, except that the Omnipotency of the Deity is replaced by the Omnipotency of Natural Selection. The vital point, which is whether Natural Selection does offer a satisfactory explanation of the living world, is too frequently lost sight of. Whether we are bound or not to interpret all the phenomena of life in terms of natural selection touches the basis of modern philosophy. It is for the biologist to attempt to find an answer, and there are few more profitable lines of attack than a critical examination of the facts of adaptation. Though "mimicry" is but a small corner in this vast field of inquiry it is a peculiarly favourable one owing to the great interest which it has excited for many years and the consequently considerable store of facts that has been accumulated. If then we would attempt to settle this most weighty point in philosophy there is probably nothing to which we can appeal with more confidence than to the butterfly.

CHAPTER II

MIMICRY—BATESIAN AND MÜLLERIAN

Mimicry is a special branch of the study of adaptation. The term has sometimes been used loosely to include cases where an animal, most frequently an insect, bears a strong and often most remarkable resemblance to some feature of its inanimate surroundings. Many butterflies with wings closed are wonderfully like dead leaves; certain spiders when at rest on a leaf look exactly like bird-droppings; "looper" caterpillars simulate small twigs; the names of the "stick-" and "leaf-" insects are in themselves an indication of their appearance. Such cases as these, in which the creature exhibits a resemblance to some part of its natural surroundings, should be classified as cases of "protective resemblance" in contradistinction to mimicry proper. Striking examples of protective resemblance are abundant, and though we possess little critical knowledge of the acuity of perception in birds and other insect feeders it is plausible to regard the resemblances as being of definite advantage in the struggle for existence. However, it is with mimicry and not with protective coloration in general that we are here directly concerned, and the nature of the phenomenon may perhaps best be made clear by a brief account of the facts which led to the statement of the theory.

In the middle of last century the distinguished naturalist, H. W. Bates, was engaged in making collections in parts of the Amazon region. He paid much attention to butterflies, in which group he discovered a remarkably interesting phenomenon^[3]. Among the species which he took were a large number belonging to the group Ithomiinae, small butterflies of peculiar appearance with long slender bodies and narrow wings bearing in most cases a conspicuous pattern (cf. Pl. X, fig. 7). When Bates came to examine his catch more closely he discovered that among the many Ithomiines were a few specimens very like them in general shape, colour, and markings, but differing in certain anatomical features by which the Pierinae, or "whites," are separated from other groups. Most Pierines are very different from Ithomiines. It is the group to which our common cabbage butterfly belongs and the ground colour is generally white. The shape of the body and also of the wings is in general quite distinct from what it is in the Ithomiines. Nevertheless in these particular districts certain of the species of Pierines had departed widely from what is usually regarded as their ancestral pattern (Pl. X, fig. 1) and had come to resemble very closely the far more abundant Ithomiines among whom they habitually flew (cf. Pl. X, figs. 2 and 3). To use Bates' term they "mimicked" the Ithomiines, and he set to work to devise an explanation of how this could have come about. The Origin of Species had just appeared and it was natural that Bates should seek to interpret this peculiar phenomenon on the lines there laid down. How was it that these Pierines had come to depart so widely from the general form of the great bulk of their relations, and to mimic so closely in appearance species belonging to an entirely different group, while at the same time conserving the more deeply seated anatomical features of their own family? If the change was to be regarded as having come about through the agency of natural selection it must clearly be of advantage to the mimicking forms; otherwise natural selection could not come into operation. What advantage then have the Ithomiines over the majority of butterflies in those parts? They are small insects, rather flimsy in build, with comparatively weak powers of flight, and yet so conspicuously coloured that they can hardly be mistaken for anything else. In spite of all this they are little subject to the attacks of enemies such as birds, and Bates attributed this to the fact that the juices of their bodies are unpalatable. According to him their striking and conspicuous pattern is of the nature of a warning coloration, advertising their disagreeable properties to possible enemies. A bird which had once attempted to eat one would find it little to its taste. It would thenceforward associate the conspicuous pattern with a disagreeable flavour and in future leave such butterflies severely alone. The more conspicuous the pattern the more readily would it be noticed by the enemy, and so it would be of advantage to the Ithomiine to possess as striking a pattern as possible. Those butterflies shewing a tendency to a more conspicuous pattern would be more immune to the attacks of birds and so would have a better chance of leaving progeny than those with a less conspicuous pattern. In this way variations in the direction of greater conspicuousness would be accumulated gradually by natural selection, and so would be built up in the Ithomiine the striking warning coloration by which it advertises its disagreeable properties. Such is the first step in the making of a mimicry case—the building up through natural selection of a conspicuous pattern in an unpalatable species by means of which it is enabled to advertise its disagreeable properties effectively and thereby secure immunity from the attacks of enemies which are able to appreciate the advertisement. Such patterns and colours are said to be of a "warning" nature. The existence of an unpalatable model in considerable numbers is the first step in the production of a mimetic resemblance through the agency of natural selection.

We come back now to our Pierine which must be assumed to shew the general characters and coloration of the family of whites to which they belong (cf. Pl. X, fig. 1). Theoretically they are not specially protected by nauseous properties from enemies and hence their conspicuous white coloration renders them especially liable to attack. If, however, they could exchange their normal dress for one resembling that of the Ithomiines it is clear that they would have a chance of being mistaken for the latter and consequently of being left alone. Moreover, in certain cases these Pierines have managed to discard their normal dress and assume that of the Ithomiines. On theoretical grounds this must clearly be of advantage to them, and being so might conceivably have arisen through the operation of natural selection. This indeed is what is supposed to have taken place on the theory of mimicry. Those Pierines which exhibited a variation of colour in the direction of the Ithomiine "model" excited distrust in the minds of would-be devourers, who had learned from experience to associate that particular type of coloration with a disagreeable taste. Such Pierines would therefore have a rather better chance of surviving and of leaving offspring. Some of the offspring would exhibit the variation in a more marked degree and these again would in consequence have a yet better chance of surviving. Natural selection would encourage those varying in the direction of the Ithomiine model at the expense of the rest and by its continuous operation there would gradually be built up those beautiful cases of resemblance which have excited the admiration of naturalists.

Wallace was the next after Bates to interest himself in mimicry and, from his study of the butterflies of the Oriental region^[4], shewed that in this part of the world too there existed these remarkable resemblances between species belonging to different families. Perhaps the most important part of Wallace's contribution was the demonstration that in some species not only was it the female alone that "mimicked" but that there might be several different forms of female mimicking different models, and in some cases all unlike the male of their own species. One of the species studied by Wallace, Papilio polytes, is shewn on Plate V. We shall have occasion to refer to this case later on, and it is sufficient here to call attention to the three different forms of female, of which one is like the male while the other two resemble two other species of Papilio, P. hector and P. aristolochiae, which occur in the same localities. Instances where the female alone of some unprotected species mimics a model with obnoxious properties are common in all tropical countries. It has been suggested that this state of things has come about owing to the greater need of protection on the part of the female. Hampered by the disposal of the next generation the less protected female would be at a greater disadvantage as compared with the mimic than would the corresponding male whose obligations to posterity are more rapidly discharged. The view of course makes the assumption that the female transmits her peculiar properties to her daughters but not to her sons.

A few years later Trimen^[5] did for Africa what Bates had done for America and Wallace for Indo-Malaya. It was in this paper that he elucidated that most remarkable of all cases of mimicry—Papilio dardanus with his harem of different consorts, all tailless, all unlike himself, and often wonderfully similar to unpalatable forms found in the same localities (cf. p. 30).

We may now turn to one of the most ingenious developments of the theory of mimicry. Not long after Bates' original memoir appeared attention was directed to a group of cases which could not be explained on the simple hypothesis there put forward. Many striking cases of resemblance had been adduced in which both species obviously belonged to the presumably unpalatable groups. Instances of the sort had been recorded by Bates himself and are perhaps most plentiful in South America between species belonging respectively to the Ithomiinae and Heliconinae. On the theory of mimicry all the members of both of these groups must be regarded as specially protected owing to their conspicuous coloration and distasteful properties. What advantage then can an Ithomiine be supposed to gain by mimicking a Heliconine, or vice versÂ? Why should a species exchange its own bright and conspicuous warning pattern for one which is neither brighter nor more conspicuous? To Fritz MÜller, the well-known correspondent of Darwin, belongs the credit of having suggested a way out of the difficulty. MÜller's explanation turns upon the education of birds. Every year there hatch into the world fresh generations of young birds, and each generation has to learn afresh from experience what is pleasant to eat and what is not. They will try all things and hold fast to that which is good. They will learn to associate the gay colours of the Heliconine and the Ithomiine with an evil taste^[6] and they will thenceforward avoid butterflies which advertise themselves by means of these particular colour combinations. But in a locality where there are many models, each with a different pattern and colour complex, each will have to be tested separately before the unpalatableness of each is realised. If for example a thousand young birds started their education on a population of butterflies in which there were five disagreeable species, each with a distinct warning pattern, it is clear that one thousand of each would devote their lives to the education of these birds, or five thousand butterflies in all^[7]. But if these five species, instead of shewing five distinct warning patterns, all displayed the same one it is evident that the education of the birds would be accomplished at the price of but one thousand butterfly existences instead of five. Even if one of the five species were far more abundant than the others it would yet be to its advantage that the other four should exhibit the same warning pattern. Even though the losses were distributed pro rata the more abundant species would profit to some extent. For the less abundant species the gain would of course be relatively greater. Theoretically therefore, all of the five species would profit if in place of five distinct warning patterns they exhibited but a single one in common. And since it is profitable to all concerned what more natural than that it should be brought about by natural selection?

MÜller's views are now widely accepted by students of mimicry as an explanation of these curious cases where two or more evidently distasteful species closely resemble one another. Indeed the tendency in recent years has been to see MÜllerian mimicry everywhere, and many of the instances which were long regarded as simple Batesian cases have now been relegated to this category. The hypothesis is, of course, based upon what appears to man to be the natural behaviour of young birds under certain conditions. No one knows whether young birds actually do behave in the way that they are supposed to. In the absence of any such body of facts the MÜllerian hypothesis cannot rank as more than a plausible suggestion, and, as will appear later, it is open to severe criticism on general grounds.

Perhaps the next contribution to the subject of mimicry which must rank of the first importance was that of Erich Haase^[8], to whose book students of these matters must always be under a heavy obligation. It was the first and still remains the chief work of general scope. Since Haase's day great numbers of fresh instances of mimetic resemblance have been recorded from all the great tropical areas of the world, and the list is being added to continually. Most active in this direction is the Oxford School under Professor Poulton to whose untiring efforts are largely due the substantial increases in our knowledge of African butterflies contributed by various workers in the field during the past few years. Whatever the interpretation put upon them, there can be no question as to the value of the facts brought together, more especially those referring to the nature of the families raised in captivity from various mimetic forms. With the considerable additions from Africa^[9] during the past few years several hundreds of cases of mimicry must now have been recorded. Some of the best known and most striking from among these will be described briefly in the next two chapters.

CHAPTER III

OLD-WORLD MIMICS

The earlier naturalists who studied butterflies made use of colour and pattern very largely in arranging and classifying their specimens. Insects shewing the same features in these respects were generally placed together without further question, especially if they were known to come from the same locality. In looking through old collections of butterflies from the tropics it is not infrequent to find that the collector was deceived by a mimetic likeness into placing model and mimic together. During the last century, however, more attention was paid to the anatomy of butterflies, with the result that their classification was placed upon a basis of structure. As in all work of the sort certain features are selected, partly owing to their constancy and partly for their convenience, the insects being arranged according as to whether they present these features or not. Everybody knows that the butterflies as a group are separated from the moths on the ground that their antennae are club shaped at the end, while those of the moth are generally filamentary and taper to a fine point.

The butterflies themselves may be subdivided into five main groups or families^[10] according to the structure of the first of their three pairs of legs. In the Papilionidae or "swallow-tails," the first pair of legs is well developed in both sexes (Fig. 8). In the Pieridae or "whites," the front legs are also similar in both sexes, but the claws are bifid and a median process, the empodium, is found between them (Fig. 7). In the remaining three families the front legs differ in the two sexes. The females of the Lycaenidae or "blues" have well-developed front legs in which the tarsus is terminated by definite claws (Fig. 5), whereas in the males the terminal part of the leg, or tarsus, is unjointed and furnished with but a single small claw (Fig. 6). This reduction of the front legs has gone somewhat further in the Erycinidae (Figs. 3 and 4), a family consisting for the most part of rather small butterflies and specially characteristic of South America. In the great family of the Nymphalidae the reduction of the front legs is well marked in both sexes. Not only are they much smaller than in the other groups, but claws are lacking in the female as well as in the male (Figs. 1 and 2).

Though the structure of the fore limbs is the character specially chosen for separating these different families from one another, it is of course understood that they differ from one another in various other distinctive features. The chrysalis of the Nymphalidae for example hangs head downwards suspended by the tail, whereas in the Pieridae and Papilionidae metamorphosis takes place with the chrysalis attached by the tail but supported also by a fine girdle of silk round the middle so that the head is uppermost. The larvae also afford characters by which some of the families may be distinguished—those of the Papilionidae for example having a process on the back which can be extruded or retracted.

Owing to the great size of the family of the Nymphalidae, in which the number of species approaches 5000, it is convenient to deal with the eight sub-groups into which it has been divided. The characters serving to mark off the sub-groups from one another are various. Sometimes it is the minuter structure of the tarsus, at others the form of the caterpillar or the chrysalis, at others the arrangement of the nervures that form the skeleton of the wing. Into these systematic details, however, we need not enter more fully here^[11]. What is important from the standpoint of mimicry is that these divisions, made solely on anatomical structure, correspond closely with the separation of models from mimics. Of the eight sub-families into which the Nymphalidae are divided four, viz. the Danainae, Acraeinae, Heliconinae, and Ithomiinae, provide models and some, but far fewer, mimics; two, the Satyrinae and Nymphalinae, provide many mimics and but few models, while two groups, the Morphinae and Brassolinae, practically do not enter into the mimicry story.

Simple mimicry, explicable, at any rate in theory, on the lines laid down by Bates, is a phenomenon of not infrequent occurrence in tropical countries, though rare in more temperate lands. In each of the three great divisions of the tropical world we find certain groups of butterflies serving as models, and being mimicked by butterflies belonging as a rule to quite different groups. Speaking generally the models of any given region are confined to a few groups, while the mimics are drawn from a greater number. In Asia the principal models belong to the Danaines, the Euploeines, and to a group of swallow-tails which from the fact that their larvae feed on the poisonous Aristolochia plant are generally distinguished as the "Poison-eaters," or Pharmacophagus group. Of these the Danaines and Euploeines are closely related and have much in common. They are usually butterflies of medium size, of rather flimsy build and with a somewhat slow and flaunting flight. In spite, however, of their slight build they are toughly made and very tenacious of life. Most butterflies are easily killed by simply nipping the thorax. There is a slight crack and the fly never recovers. But the collector who treats a Danaid in a way that would easily kill most butterflies is as likely as not many hours after to find it still alive in his collecting box or in the paper to which it may have been transferred when caught. They give one the impression of being tougher and more "rubbery" in consistence than the majority of Lepidoptera. Moreover, the juices of their bodies seem to be more oily and less easily dried up. In general colour scheme they vary a great deal. Some, such as Danais chrysippus (Pl. IV, fig. 1), are conspicuous with their bright fulvous-brown ground colour and the sharp white markings on the black tips of their fore wings. Others again such as Danais septentrionis (Pl. I, fig. 3), with a dark network of lines on a pale greenish ground, are not nearly so conspicuous. Of the Euploeines some have a beautiful deep blue metallic lustre (cf. Pl. II, fig. 4), though many are of a plain sombre brown relieved only by an inconspicuous border of lighter markings (cf. Pl. I, fig. 10).

Both Danaines and Euploeines serve as models for a great variety of species belonging to different groups. Danais septentrionis (Pl. I, fig. 3) is a very abundant species in India and Ceylon, and in the same region there are several other very similar species. Flying with them in Northern India are two species of Papilio, P. macareus and P. xenocles (Pl. I, fig. 4), which resemble these Danaids fairly closely. In Southern India and Ceylon one of the two forms of Papilio clytia (Pl. I, fig. 7) is also regarded as a mimic of these Danaids. In the same part of the world there is a Pierine of the genus Pareronia, whose female is very like these Danaines on the upper surface (Pl. I, fig. 1). The male of this Pierine is quite distinct from the female (Pl. I, fig. 2).

The common Danais chrysippus (Pl. IV, fig. 1), found in this region, has been described as probably the most abundant butterfly in the world, and serves as a model for several species belonging to different groups. It and its mimics will, however, be described in more detail later on. Mention must also be made of the striking case of the Danaid, Caduga tytia and its Papilionine mimic P. agestor from Sikkim (Pl. II, figs. 2 and 3). In both species the fore wings are pale blue broken by black; while the hind wings are pale with a deep outer border of rusty red. Not only in colour but also in shape the swallow-tail bears a remarkable resemblance to the Danaid. C. tytia is also mimicked by a rare Nymphaline Neptis imitans, which exhibits the same striking colour scheme so very different from that of most of its allies.

No less remarkable are some of the cases in which the Euploeines serve as models. E. rhadamanthus, for example, is mimicked by the scarce Papilio mendax, and a glance at Figs. 8 and 9 on Plate II shews how well this butterfly deserves its name. Euploea rhadamanthus also serves as a model for one of the several forms of female of the Nymphaline species Euripus halitherses. In some Euploeines the sexes are different in appearance—a somewhat unusual thing among butterflies serving as models in cases of mimetic resemblance. Such a difference is found in Euploea mulciber, the male being predominantly brown with a beautiful deep blue suffusion, while the female is a rather lighter insect with less of the blue suffusion and with hind wings streaked with lighter markings (Pl. II, figs. 4 and 5). It is interesting to find that Elymnias malelas, a Satyrid which mimics this species, shews a similar difference in the two sexes (Pl. II, figs. 6 and 7).

It is remarkable that similar sexual difference is also shewn by the rare Papilio paradoxus, the two sexes here again mimicking respectively the two sexes of Euploea mulciber.

Many of the Euploeines, more especially those from Southern India and Ceylon, lack the blue suffusion, and are sombre brown insects somewhat relieved by lighter markings along the hinder border of the hind wings. Euploea core (Pl. I, fig. 10), a very common insect, is typical of this group. A similar coloration is found in one of the forms of Papilio clytia (Pl. I, fig. 8) from the same region as well as in the female of the Nymphaline species Hypolimnas bolina (Pl. I, fig. 6). The male of this last species (Pl. I, fig. 5) is quite unlike its female, but is not unlike the male of the allied species, H. misippus, which it resembles in the very dark wings each with a white patch in the centre, the junction of light and dark being in each case marked by a beautiful purple-blue suffusion. There is also a species of Elymnias (E. singhala) in this part of the world which in general colour scheme is not widely dissimilar from these brown Euploeas (Pl. I, fig. 9).

The third main group of models characteristic of this region belongs to the Papilionidae. It was pointed out by Haase some 20 years ago that this great family falls into three definite sections, separable on anatomical grounds (see Appendix II). One of these sections he termed the Pharmacophagus or "poison-eating" group owing to the fact that the larvae feed on the poisonous climbing plants of the genus Aristolochia. It is from this group that all Papilios which serve as models are drawn. No mimics of other unpalatable groups such as Danaines are to be found among the Oriental Poison-eaters. In the other two sections of the genus mimics are not infrequent (cf. Appendix II), though probably none of them serve as models. To the Pharmacophagus group belong the most gorgeous insects of Indo-Malaya—the magnificent Ornithoptera, largest and most splendid of butterflies. It is not a large proportion of the members of the group which serve as models, and these on the whole are among the smaller and less conspicuous forms. In all cases the mimic, when a butterfly, belongs to the Papilio section of the three sections into which Haase divided the family (cf. Appendix II). Papilio aristolochiae (Pl. V, fig. 5), for example, is mimicked by a female form of Papilio polytes, and the geographical varieties of this widely spread model are generally closely paralleled by those of the equally wide spread mimic. For both forms range from Western India across to Eastern China. Another poison-eater, P. coon, provides a model for one of the females of the common P. memnon. It is curious that in those species of the poison-eaters which serve as models the sexes are practically identical in pattern, and are mimicked by certain females only of the other two Papilio groups, whereas in the Ornithoptera, which also belong to the poison-eaters, the difference between the sexes is exceedingly striking.

Though the Pharmacophagus Papilios are mimicked only by other Papilios among butterflies they may serve occasionally as models for certain of the larger day-flying moths. Papilio polyxenus, for example, is mimicked not only by the unprotected P. bootes but also by the moth Epicopeia polydora (Pl. III, figs. 5 and 6). Like the butterfly the Epicopeia, which is comparatively rare, has the white patch and the outer border of red marginal spots on the hind wing. Though it is apparently unable to provide itself with an orthodox tail it nevertheless makes a creditable attempt at one. There are several other cases of mimetic resemblance between day-flying moths and Pharmacophagus swallow-tails—the latter in each case serving as the model. Rarely it may happen that the rÔle of butterfly and moth is reversed, and the butterfly becomes the mimic. A very remarkable instance of this is found in New Guinea where the rare Papilio laglaizei mimics the common day-flying moth Alcidis agathyrsus. Viewed from above the resemblance is sufficiently striking (Pl. III, figs. 1 and 2), but the most wonderful feature concerns the underneath. The ventral half of the moth's abdomen is coloured brilliant orange. When the wings are folded back they cover and hide from sight only the dorsal part of the abdomen, so that in this position the orange neutral surface is conspicuous. When, however, the wings of the butterfly are folded they conceal the whole of the abdomen. But the butterfly has developed on each hind wing itself a bright orange patch in such a position that when the wings are folded back the orange patch lies over the sides of the abdomen. In this way is simulated the brilliant abdomen of the moth by a butterfly, in which, as in its relations, this part is of a dark and sombre hue.

A few models are also provided in the Oriental region by the genus Delias, which belongs to the Pierines. A common form, Delias eucharis, is white above but the under surface of the hind wings is conspicuous with yellow and scarlet (Pl. II, fig. 1). It has been suggested that this species serves as a model for another and closely allied Pierine, Prioneris sita, a species distinctly scarcer than the Delias. There is some evidence that the latter is distasteful (cf. p. 115), but nothing is known of the Prioneris in this respect. Other species of Delias are said to function as models for certain day-flying moths belonging to the family Chalcosiidae, which may bear a close resemblance to them. In certain cases it may happen that the moth is more abundant than the Pierine that it resembles^[12].

Tropical Africa is probably more wealthy in mimetic analogies than Indo-Malaya, and the African cases have recently been gathered together by Eltringham in a large and beautifully illustrated memoir^[13]. The principal models of the region are furnished by the Danainae and the allied group of the Acraeinae. Of the Danaines one well-known model, Danais chrysippus, is common to Africa and to Indo-Malaya. Common also to the two regions are the mimics, Argynnis hyperbius and Hypolimnas misippus (cf. Pl. IV, figs. 3 and 7). The case of the last named is peculiarly interesting because it presents well-marked varieties which can be paralleled by similar ones in D. chrysippus. In addition to the typical form with the dark tipped fore wing relieved by a white bar there is in each species a form uniformly brown, lacking both the dark tip and the white bar of the fore wing. There is also another form in the two species in which the hind wing is almost white instead of the usual brown shade. In both species, moreover, the white hind wing may be associated either with the uniformly brown fore wing or with the typical form. There is also another common African butterfly, Acraea encedon, in which these different patterns are closely paralleled (cf. Pl. IX). Several other species of butterflies and a few diurnal moths bear a more or less close resemblance to D. chrysippus.

Danaine butterflies with the dark interlacing fines on a pale greenish-blue ground, so characteristic of the Oriental region, are represented in Africa by the species Danais petiverana (Pl. VI, fig. 1) ranging across the continent from Sierra Leone to British East Africa. A common Papilio, P. leonidas (Pl. VI, fig. 2) has a similar extensive range, and has been regarded as a mimic of the Danaine. In S. Africa P. leonidas is represented by the variety brasidas in which the white spots are reduced and the blue-green ground is lacking. Brasidas bears a strong resemblance to the tropical Danaine Amauris hyalites (Pl. VI, fig. 3) of which it has been regarded as a mimic. It must however be added that it is only over a small part of their respective ranges, viz. in Angola, that the two species are to be met with together.

The butterflies belonging to the genus Amauris are among the most abundant and characteristic Danaine models of Africa. Some of the black and white species such as A. niavius (Pl. VIII, fig. 6) are conspicuous insects in a cabinet. Others again, such as A. echeria (Pl. VIII, fig. 7), are relatively sombre-looking forms. Among the best known mimics of the genus is a species of Hypolimnas^[14]—H. dubius. This interesting form is polymorphic and mimics different species of Amauris. The variety wahlbergi, for example, is very like A. niavius, while mima strongly resembles A. echeria (Pl. VIII, figs. 8 and 9). It was at one time supposed that these two varieties of Hypolimnas dubius were different species and the matter was only definitely settled when the two forms were bred from the eggs of the same female. Other mimics of Amauris are found among the Papilios and the Nymphaline genus Pseudacraea.

But among all the mimics of Danaines in Africa and elsewhere Papilio dardanus is pre-eminent, and has been described by more than one writer as the most important case of mimicry in existence. Not only does it shew remarkable resemblances to various Danaids, but it presents features of such peculiar interest that it must be considered in more detail. Papilio dardanus in its various sub-races is spread over nearly all the African continent south of the Sahara. Over all this area the male, save for relatively small differences, remains unchanged—a lemon-yellow insect, tailed, and with black markings on fore and hind wings (Pl. VIII, fig. 1). The female, however, exhibits an extraordinary range of variation. In South Africa she appears in three guises, (1) the cenea form resembling Amauris echeria, (2) the hippocoon form like Amauris niavius, and (3) the trophonius form which is a close mimic of the common Danais chrysippus^[15]. Except that cenea does not occur on the West Coast these three forms of female are found over almost all the great continental range of dardanus and its geographical races. Northwards in the latitude of Victoria Nyanza occurs a distinct form of female, planemoides, which bears a remarkable resemblance to the common and distasteful Planema poggei, and is found only where the latter is abundant. All of these four forms are close mimics of a common Danaine or Acraeine model. Other forms of female, however, are known, of which two, dionysus and trimeni, are sufficiently distinct and constant to have acquired special names. Dionysus may be said to unite the fore wing of the hippocoon form with the hind wing of the trophonius form, except that the colour of the last part is yellow instead of bright brown. It is a western form and is unlike any model. Trimeni also is unlike any model but is of peculiar interest in that it is much more like the male with its pale creamy-yellow colour and the lesser development of black scales than occurs in most of the forms of female. At the same time the general arrangement of the darker markings is on the whole similar to that in the hippocoon and in the trophonius form. Trimeni is found on the Kikuyu Escarpment, near Mt Kenia, along with the four mimicking forms.

Continental Africa, south of the equator, has produced no female similar to the male. But in Abyssinia is found another state of things. Here, so far as is known, occur three forms, all tailed, of which one is similar in general colour and pattern to the male, while the other two, niavioides and ruspina^[16], resemble respectively a tailed hippocoon and a tailed trophonius. Lastly we have to record that Papilio dardanus is also found as the geographical race humbloti on Comoro Island, and as meriones on Madagascar. In both forms the females are tailed, and resemble the males.

From this long series of facts it is concluded that the male of P. dardanus represents the original form of both sexes. On the islands of Comoro and Madagascar this state of things still survives. But it is supposed that on the African continent existed enemies which persecuted the species more than on the islands and encouraged the development of mimetic forms in the female. The original female still lingers in Abyssinia though it is now accompanied by the two mimetic forms niavioides and ruspina. Over the rest of the area occupied by dardanus the females are always tailless and, with the exception of trimeni and dionysus, wonderfully close mimics. Trimeni, the intermediate form, provides the clue to the way in which the mimetic females have been derived from the male, viz. by the prolongation across the fore wing of the dark costal bar already found in the females of the Madagascar and Abyssinian races, by the deepening of the dark edging to the wings, and by the loss of the tail. Through the gradual accumulation of small variations trimeni came from the male-like female, and by further gradual accumulation of small favourable variations the mimetic forms came from trimeni. South of the equator the male-like form and the intermediate trimeni have disappeared owing to the stringency of selection being greater. Moreover the likeness of mimic to model is closer than in the north, a further proof of the greater stringency of natural selection in these parts. Such in brief is the explanation in terms of mimicry of the remarkable and complex case of dardanus.

Although the Euploeinae are not represented on the African continent, it is the headquarters of another distasteful family of butterflies—the Acraeinae—which is but sparingly represented in the Oriental region^[17]. Of smaller size than the Danaines they are characterised, like this group, by their tenacity of life and by the presumably distasteful character of their body juices. They are said also to possess an offensive odour apparently exuded through the thorax. The majority of the members of the group fall into the two genera Acraea and Planema. Species of Acraea are on the whole characterised by their general bright red-brown colour and by the conspicuous black spots on both fore and hind wings. A typical Acraeine pattern is that of Acraea egina (Pl. VI, fig. 7) which is mimicked remarkably closely by the Nymphaline Pseudacraea boisduvali and by the Swallow-tail Papilio ridleyanus (Pl. VI, figs. 5 and 6).

In the genus Planema the spots are as a rule fewer and clustered near the body, while on both fore and hind wings there is a tendency to develop clear wide band-like areas of orange or white (cf. Pl. VII).

Like the Acraeas the Planemas are principally mimicked by species of Pseudacraea and of Papilio. Some of the cases of resemblance between Planema and Pseudacraea are among the most striking known. Planema macarista is one of those comparatively rare instances in which a model shews a marked difference in the pattern of the two sexes. The clear area on the fore wing of the male is deep orange, whereas in the female it is somewhat different in shape, and, like the area on the hind wing, is white (cf. Pl. VII, figs. 1 and 2). Pseudacraea eurytus hobleyi (Pl. VII, figs. 6 and 7) shews a similar difference in the sexes, the male and female of this species mimicking respectively the male and female of Planema macarista. The case is made even more remarkable by the fact that both of the sexual forms of Planema macarista are mimicked by the Satyrine Elymnias phegea (Pl. VII, fig. 9), though in this species either the black and white, or the black, white, and orange form may occur in either sex. Among the best Papilionine mimics of the Planemas is Papilio cynorta whose female is extraordinarily like the common Planema epaea (Pl. VII, figs. 5 and 10). The resemblance of the planemoides female of P. dardanus to P. poggei has already been noticed.

A striking feature of the African continent is the frequency with which mimetic forms are found among the Lycaenidae. As a rule the "blues" rarely exhibit mimetic analogies, but in Africa there are several species, especially those of the genus Mimacraea, which closely resemble Acraeines. Others again bear a marked resemblance to certain small Pierines, Citronophila similis from S. Nigeria for example being extraordinarily like the common Terias brigitta, a small bright yellow Pierine with black-edged wings.

A remarkable feature of the African continent is the absence of the Pharmacophagus Swallow-tails. Of such Papilios as exhibit mimicry, and as compared with the total number of the group present the proportion is large, the majority resemble one or other of the characteristic Danaines, while a few such as P. ridleyanus and P. cynorta resemble either an Acraeoid or a Planemoid model.

As in the Oriental region the African Pierines do not offer many instances of mimetic analogies. The genus Mylothris, in which certain species are characterised by orange patches at the bases of the undersurfaces of the fore wings, is regarded by some authors as providing models for allied genera such as Belenois and Phrissura. But as neither models nor mimics offer a marked divergence in appearance from the ordinary Pierine facies it is doubtful whether much stress can be laid on these cases.

Africa also offers a few striking instances of mimicry in which day-flying moths play a part. The conspicuous Geometer Aletis helcita is an abundant form, and with its strong red colour and black wing margins broken by white it is a striking object in the preserved state. Among the forms which bear a close resemblance to it are the Nymphaline Euphaedra ruspina, and the Lycaenid Telipna sanguinea^[18].

CHAPTER IV

NEW-WORLD MIMICS

Of all the continents South America affords the greatest wealth of butterfly life, and it is in the tropical part of this region that many of the most beautiful and striking cases of mimicry are to be found. Viewed as a whole the butterfly population presents several features which serve to mark it off from that of the other two great tropical areas. In the first place the proportion of gaily coloured forms is higher. Bright red, yellow or fulvous brown contrasted with some deep shade approaching black form the dominant notes. Sombre coloured species are relatively scarcer than in the Oriental and African regions. In the second place when looking over collections from this part of the world one cannot help being struck by the frequency with which similar colour combinations occur over and over again in different as well as in the same groups. Now it is a simple scheme of black with an oblique scarlet band upon the fore wings—now an arrangement with alternating stripes of bright brown and black relieved with patches of clear yellow—now again a scheme of pure transparency and black. Gay and pleasing as are the designs turned out the palette is a small one and invention is circumscribed. Under such conditions it might well be supposed that instances of close resemblance between different species would be numerous, and this in effect is what we find.

As in Asia with its Euploeines and Danaines, and in Africa with its Danaines and Acraeines, so in S. America are the fashions set by two dominant groups of models. These are the Heliconinae and the Ithomiinae, both peculiar to this region and both characterised, like the Old-world Danaids, by slow flight and great tenacity of life. Both live on poisonous plants—the Heliconines on Passifloras and the Ithomiines on Solanaceae. In both groups, but more especially in the Ithomiinae, the species are numerous, and the number of individuals in a species often beyond computation. From the point of view of mimicry these two groups have so much in common that they may conveniently be considered together.

It was from among the Ithomiines, as already pointed out, that the models came for the Pierine mimics of the genus Dismorphia upon which Bates founded the theory of mimicry. Though the Pierine mimics are the most striking the Heliconines and Ithomiines are mimicked by members of other groups. A few Papilios (Pl. X, fig. 8), certain Nymphalines such as Protogonius (Pl. X, fig. 9), Eresia, Phyciodes and Colaenis (Pl. XI, fig. 4), together with various day-flying moths, more particularly of the genera Castnia and Pericopis, are among the well-known mimics of this group of models. The models themselves are very variable in appearance. In one locality the predominant pattern is black with a warm red-brown diagonal bar occupying rather more than a third of the fore wing (Pl. XV, fig. 5), in another it consists of parallel bands of black and fulvous brown with clear yellow patches at the tips of the fore wings (cf. Pl. X, fig. 7), while in yet another locality it is different again. Different localities often have their own peculiar pattern and this affects the various mimics as well as the Ithomiine and Heliconine models.

These groups of different species, some belonging to palatable and some to unpalatable groups, all exhibiting a close resemblance in colour and pattern, are far more strikingly developed in S. America than in either Asia or Africa, and it is not uncommon for eight or ten species to enter into such an association. A group of this sort which possesses unusual interest is the so-called "Transparency Group" from certain parts of the Amazon region. It was originally described by Bates with seven species belonging to six different genera. To-day it is said that no less than 28 species of this peculiar facies are known, though some are excessively rare. The majority are Ithomiines, but two species of the Danaine genus Ituna, the Pierine Dismorphia orise (Pl. XII, fig. 2), the Swallow-tail Papilio hahneli, and several species of diurnal moths belonging to different families (cf. Pl. XII, fig. 4) also enter into the combination. In connection with it there is a feature of peculiar interest in that the transparent effect is not always produced in the same way. In the Ithomiines such as Thyridia, where there are normally two kinds of scales, the wider ones for the most part lose their pigment, become much reduced in size and take on the shape of a stumpy V (Pl. XIV, fig. 3). Also they stand out for the most part more or less at right angles to the wing^[19], and the neck by which they are joined to the wing membrane is very short. The longer and narrow form of scales also tend to lose their pigment and become reduced to fine hairs. In Dismorphia the scales, which are of one sort, are also reduced in size though apparently not in number. Like the wider scales of the Thyridia they tend sometimes to project at right angles to the wing membrane, though not to the same extent as in the Ithomiine: possibly because the neck of the scale is not so short. As in Thyridia these reduced scales lose their pigment except in the transition region round the borders of the transparent patches. In Ituna there is a difference. The scales are not reduced to the same extent in point of size. Their necks are longer as in normal scales and they lie flat on the wing membrane. The majority of the scales, as in the preceding cases, lose their pigment, but mixed up with them is a certain proportion, about one-quarter, in which the pigment is retained. In Castnia and in Anthomysa the scales on the transparent parts which are without pigment are also somewhat reduced in size, being stumpier than the normal ones. At the same time they tend to stand out at right angles to the wing membrane^[20]. The neck here again is shorter in the transparent than in the pigmented scales. A good deal of stress has been laid upon this case by some supporters of the theory of mimicry, since it is supposed to shew that a similar effect can be brought about in a variety of ways; consequently the existence of this assembly of similar transparent forms belonging to various families cannot be put down as due to the effect of similar conditions, but must be regarded as having arisen in each instance in a different manner through the independent action of natural selection^[21]. It is doubtful, however, whether such a conclusion necessarily follows from the facts. In all of the cases the process would appear to be similar: loss of pigment, reduction in the size of the scales, and eventually a tendency for the scales to stand at right angles to the wing—this last part of the process apparently depending upon the reduction of the neck of the scale. It has been said that greater transparency is brought about by the scales standing out at right angles in this way, but as the scales themselves are already transparent there would appear to be no reason why this should be so. Of course the process has not proceeded in all of the forms to the same extent. There is least change in Ituna where the scales are not much reduced in size and where a fair proportion are still pigmented. There is probably most in an Ithomiine such as Thyridia, where the scales are not only small and entirely without pigment, but also are for the most part neckless so that they stand out at right angles to the wing. Having regard to the fact that several widely separate genera with different types of scaling formed the starting points, the final results do not seem to preclude the supposition that the transparency has arisen through a similar process in all of them.

It is somewhat remarkable that no Satyrine exhibits mimicry in S. America, in spite of the fact that transparency of the wings, as in so many of the butterflies of this region, is quite common in the group. On the other hand the relatively large number of more or less mimetic Pierines is a striking feature of S. America. For the most part they belong to the genera Dismorphia and Perrhybris, and resemble the yellow, black, and brown Heliconines and Ithomiines, though some of the former genus are mimics of the small transparent Ithomiines. Some of the species of Pereute with their dark ground colour and the bright red bar across the fore wing (Pl. XI, fig. 6) resemble Heliconius melpomene, as also does Papilio euterpinus. But some of the most interesting Pierine mimics are several forms belonging to the genus Archonias (Pl. XI, fig. 10) which exhibit the simple and striking arrangement of black, red and white so characteristic of the Swallow-tail Poison-eaters of S. America. They form one of the rare instances of a Pharmacophagus Papilio being mimicked by a butterfly which does not belong to the Swallow-tail group.

As everywhere in the tropics the Papilios of S. America supply a goodly proportion of the mimicry cases. A few, such as P. zagreus (Pl. X, fig. 8), enter into the black-brown and yellow Ithomiine-Heliconine combination; P. euterpinus resembles Heliconius melpomene (Pl. XI, fig. 5); P. pausanias is like Heliconius sulphurea (Pl. XI, figs. 1 and 2). But this practically exhausts the list of Papilios which mimic Heliconines and Ithomiines. The great majority of mimicking Swallow-tails in S. America find their models among the Poison-eaters of their own family, offering in this respect a contrast to those of Asia where the majority of models are among the Danaines and Euploeines, and of Africa where they are exclusively Acraeines or Danaines.

The Poison-eaters of S. America fall into two well-marked groups which we may call the red-spotted and the dark green groups respectively. The red spotted group form a remarkably compact and uniform assemblage. The general ground colour is a deep black-brown (Pl. XI, figs. 8 and 9), the hind wings are almost invariably marked with red near the centre or towards the outer margin, and the fore wing may or may not bear a patch which is generally whitish in the female, though often of a brilliant blue or green in the male. This simple colour scheme with variations runs throughout about three-quarters (some 40 species) of the Poison-eaters. The same general colour scheme is also found in about two dozen species of the unprotected Swallow-tails. As the total number of the unprotected species is placed by Seitz at less than 100 this means that fully one-quarter of them fall into the general colour scheme adopted by the majority of the Poison-eaters. In many cases the resemblance between mimic and model is so close as to have deceived the most expert entomologists before the structural differences between the groups had been appreciated (cf. Appendix II). The matter is further complicated by the fact that polymorphism is not uncommon, especially among the females of the mimetic forms. Papilio lysithous for instance has no less than six distinct forms of female, which differ chiefly in the extent and arrangement of the white markings on the wings, one form lacking them entirely. Several of these forms may occur together in a given locality, and may resemble as many distinct species of Poison-eaters. Thus the three forms lysithous, with white on both wings, rurik, with white on the fore wing only, and pomponius without any white, all fly together in Rio Grande do Sul and respectively mimic the three distinct Pharmacophagus species nephalion, chamissonia, and perrhebus (Pl. XIII). It is worthy of note that mimics are provided by both unprotected groups of Swallow-tails in S. America, whereas in Asia the Cosmodesmus division never provides mimics for Pharmacophagus models (cf. Appendix II).

In the second and smaller group of the Pharmacophagus Swallow-tails the general colour scheme is a more or less dark metallic blue-green with a tendency towards the obliteration of light markings. Some idea of their appearance may be got from the figure of the Central and N. American P. philenor on Pl. XVI, fig. 1. Though one or two unprotected Papilios in S. America fall more or less into this colour scheme, the group, from the point of view of mimicry, is not nearly so important as the red-spotted one.

Nevertheless the blue-green Pharmacophagus group as represented by P. philenor is supposed to play a considerable part in mimicry in N. America. P. philenor is found throughout the greater part of the Eastern United States, straggling up as far as the Canadian border. On the west it is also found reaching up to North California. Over considerable parts of its range are three other Swallow-tails, belonging to the unprotected Papilios, which are regarded by Professor Poulton and others as mimics of philenor^[22]. One of these, P. troilus, is dark brown with a dusting of blue scales over the hind wing (Pl. XVI, fig. 2). The sexes here are more or less alike. Troilus stretches up into North-west Canada some way beyond the limits reached by its model. P. glaucus is a black and yellow Swallow-tail with two forms of female. One of these resembles the male while the other is darker and is said to mimic philenor. It is known as the turnus form and is found more commonly in the southern part of the range of the species, i.e. in the country where philenor is more plentiful. The third species, P. asterius, has a more southerly distribution. Its female is darker and nearer to philenor than the male. It must, however, be admitted that none of the three species bears a very close resemblance to philenor. It is suggested that this is because P. philenor is a tropical form which has only recently invaded N. America. The crossing of philenor has, as it were, induced the three mimicking Papilios to turn dark, but the model has not been long enough in contact with them for the likeness to become a close one. The explanation, however, hardly accounts for the fact that the best mimic of the three, P. troilus, in which both sexes are dark, is found far north of philenor. Either the dark colour was established without the influence of the Pharmacophagus model, or else the species rapidly extended its range northwards after having been modified under the influence of philenor in the south. But in that case the critic may ask why it does not revert to the original pattern now that it has got beyond the model's sphere of influence. On the whole it seems at present quite doubtful whether any relation of a mimetic nature exists between P. philenor and these three species of Papilio.

P. philenor is also regarded as serving as a model for two Nymphaline butterflies in the United States. One of these is the large Fritillary Argynnis diana of which the dark female has a markedly blue tint on the upper surface (Pl. XVI, fig. 3). The other is a Limenitis^[23] related to our own White Admiral. This form, L. astyanax (Pl. XVI, fig. 5), is a dark form with a bluish iridescence on the upper surface. It is found, like P. philenor, over the greater part of the Eastern States, while to the north, near the Canadian boundary, its place is taken by L. arthemis with prominent white bar across both wings (Pl. XVI, fig. 4). There is reason for believing that where the two overlap there is occasional inbreeding, and that the hybrid is the form known as proserpina, resembling astyanax more than arthemis. It must be admitted that in general appearance L. astyanax and Argynnis diana are more like Papilio troilus than P. philenor. In explanation it has been suggested that all the mimics are on the way to resembling P. philenor, and consequently we should expect them at certain stages to shew more resemblance to one another than to the form they have all as it were set out to mimic. On this view they will all arrive at a close resemblance to philenor in time. Another explanation is that favoured by Professor Poulton on which it is assumed that we are here dealing with a case of MÜllerian Mimicry, all of the species in question being distasteful with the exception perhaps of A. diana. Thus troilus and astyanax though distasteful are less so than philenor. Hence it is of advantage to them to have even a chance of being mistaken for the more obnoxious philenor, and so the one has come from the black and yellow Swallow-tail pattern and the other from the white-banded arthemis form to what they are, i.e. more alike to one another than to philenor. They now form a MÜllerian combination for mutual protection along with the dark females of glaucus and asterius. But they are themselves still moderately distasteful so that it is to the advantage of the female of Argynnis diana to mimic them. Whether they are all on the way to resembling philenor more closely, or whether they have sufficiently vindicated their inedible properties and are now stationary, it is for the future to reveal to posterity. Lastly we have the view that these different species have attained their present coloration entirely independently of one another, and that we are not here concerned with mimicry at all. Since the sole evidence available at present is that based on general appearance and geographical distribution, the view taken of this case must rest largely upon personal inclination.

Though the cases just quoted are only very problematically mimetic, N. America has yet several examples of resemblance between distantly related forms as close as any that occur in the tropics. In this region are found two species of the genus Danais—D. archippus occurring all over the United States and reaching up northwards into Canada, D. berenice found in the South-eastern States, e.g. in Florida, where it is said to be more abundant than archippus. D. archippus (Pl. XVI, fig. 8) is very similar to the oriental D. plexippus (Pl. IV, fig. 2), from which perhaps its most notable difference lies in the extent and arrangement of the white spots near the tip of the fore wing. D. berenice is not unlike archippus in its general colour scheme but is smaller and darker (Pl. XVI, fig. 9).

We have already had occasion to mention the common Nymphaline, Limenitis arthemis (Pl. XVI, fig. 4) which is found in Canada and the Northeastern States. Widely spread over N. America is a close ally of this species, L. archippus, which, though so similar in structure and habits, is very different in external appearance. As appears from Pl. XVI, fig. 6, L. archippus is remarkably like the Danaid which bears the same specific name. In the Southern States L. archippus is replaced by a form slightly different in details of pattern and distinctly darker, L. floridensis (= eros) (Pl. XVI, fig. 7). In Florida occurs also the darker N. American Danaid, D. berenice, to which the colour of L. floridensis approximates more than to D. archippus, and it is of interest that although the last named is also found in this locality it is said to be much less abundant than D. berenice. Nevertheless it appears to be true that the range of L. floridensis is much more extensive than that of its model; in other words, that there are considerable regions where L. floridensis and D. archippus coexist, and from which L. archippus and D. berenice are wanting.

CHAPTER V

SOME CRITICISMS

The facts related in the last two chapters are sufficient to make it clear that these remarkable resemblances between species belonging as a rule to widely different groups constitute a real phenomenon, and as such demand an explanation. One explanation, that in terms of the theory of mimicry, has already been outlined, and we may now turn to consider it in more detail. Some years ago Wallace^[24], combating the suggestion that these instances of resemblance might be mere coincidences, laid down five conditions which he stated were applicable to all such cases, and rendered utterly inadequate any explanation other than in terms of natural selection. These five conditions are of historical interest and may also serve as a peg for sundry criticisms in connection with the mimicry theory. They are as follows:

(1) That the imitative species occur in the same area and occupy the very same station as the imitated.

(2) That the imitators are always the more defenceless.

(3) That the imitators are always less numerous in individuals.

(4) That the imitators differ from the bulk of their allies.

(5) That the imitation, however minute, is external and visible only, never extending to internal characters or to such as do not affect the external appearance.

In offering certain criticisms of the mimicry explanation it will be convenient to do so in connection with these five conditions which Wallace regarded as constant for all cases of mimetic resemblance.

(1) That the imitative species occur in the same area and occupy the very same station as the imitated.

This on the whole is generally true. It is well shewn in some of the most striking cases such as those of the Old-World Papilios that mimic Danaines, or of the Dismorphias and their Ithomiine models. In many of these cases the range of neither model nor mimic is a very wide one, yet the mimic is found strictly inside the area inhabited by the model. Papilio agestor, for instance, is only found where Caduga tytia occurs, nor is P. mendax known outside the area frequented by Euploea rhadamanthus. Even more striking in this respect are some of the Ithomiine-Dismorphia resemblances in the New World. The Ithomiine models are as a rule very local though very abundant. Two hundred miles away the predominant Ithomiine often bears quite a distinct pattern, and when this is the case the mimicking Dismorphia is generally changed in the same sense. But though mimic and model may be found together in the same locality, they do not always occupy the same station in the sense that they fly together. According to Seitz^[25] the Dismorphias themselves do not fly with the Ithomiines which they mimic. The occurrence of butterflies is largely conditioned by the occurrence of the plants on which the larva feeds, and this is especially true of the female, which, as has already been noticed, is more commonly mimetic than the male. The female of Papilio polytes, for instance, is found flying where are to be found the wild citronaceous plants on which its larva feeds. On the other hand, its so-called models, Papilio hector and P. aristolochiae, are generally in the proximity of the Aristolochias on which their larvae feed. The two plants are not always found together, so that one frequently comes across areas where P. polytes is very abundant while the models are scarce or absent.

Though in the great majority of cases the imitator and the imitated occur in the same locality, this is not always so. The female of the Fritillary Argynnis hyperbius (Pl. IV, fig. 3), for instance, is exceedingly difficult to distinguish from Danais plexippus when flying, although when at rest the difference between the two is sufficiently obvious. Both insects are plentiful in Ceylon but inhabit different stations. The Danaid is a low-country insect, while the Fritillary is not found until several thousand feet up. The two species affect entirely different stations and hardly come into contact with each other. Where one is plentiful the other is not found. It has been suggested that migratory birds may have come into play in such cases. The bird learns in the low country that D. plexippus is unpleasant, and when it pays a visit to the hills it takes this experience with it and avoids those females of the Fritillary which recall the unpleasant Danaine.

Migratory birds have also been appealed to in another case where the resembling species are even further removed from one another than in the last case. Hypolimnas misippus is common and widely spread over Africa and Indo-Malaya, and the male (Pl. IV, fig. 8) bears a simple and conspicuous pattern—a large white spot bordered with purple on each of the very dark fore and hind wings. The same pattern occurs in the males of two other Nymphalines allied to H. misippus, viz. Athyma punctata and Limenitis albomaculata. The two species, however, have a distribution quite distinct from that of H. misippus, being found in China. It has nevertheless been suggested by Professor Poulton^[26] that the case may yet be one of mimicry. According to his explanation, H. misippus is unpalatable, the well-known association of its female with Danais chrysippus being an instance of MÜllerian mimicry. Migratory birds did the rest. Having had experience of H. misippus in the south, on their arrival in China they spared such specimens of Athyma punctata and Limenitis albomaculata as approached most nearly to H. misippus in pattern, and so brought about the resemblance. The explanation is ingenious, but a simpler view will probably commend itself to most. Other cases are known in which two butterflies bear a close resemblance in pattern and yet are widely separated geographically. Several species of the S. American Vanessid genus Adelpha are in colour scheme like the African Planema poggei which serves as a model for more than one species. The little S. American Phyciodes leucodesma would almost certainly be regarded either as a model for or a mimic of the African Neptis nemetes, did the two occur together. Nevertheless examples of close resemblance between butterflies which live in different parts of the world are relatively rare and serve to emphasise the fact that the great bulk of these resemblance cases are found associated in pairs or in little groups.

(2) That the imitators are always the more defenceless.

In the case of butterflies "defence" as a rule denotes a disagreeable flavour rendering its possessor distasteful to birds and perhaps to other would-be devourers. Feeding experiments with birds (cf. Chapter IX) suggest that certain groups of butterflies, notably the Danaines, Acraeines, Heliconines, Ithomiines and Pharmacophagus Papilios—groups from which models are generally drawn—are characterised by a disagreeable taste, while as a rule this is not true for the mimics. This distasteful quality is frequently accompanied by a more or less conspicuous type of coloration, though this is by no means always so. Many Euploeas are sombre inconspicuous forms, and it is only some of the Ithomiines that sport the gay colours with which that group is generally associated. The members of the distasteful groups usually present certain other peculiarities. Their flight is slower, they are less wary, their bodies are far tougher, and they are more tenacious of life. The slow flight is regarded as an adaptation for exhibiting the warning coloration to the best advantage, but from the point of view of utility it is plausible to suggest that the insect would be better off if in addition to its warning coloration it possessed also the power of swift flight^[27]. It is possible that the peculiar slowness of flight of these unpalatable groups is necessitated by the peculiar tough but elastic integument which may present an insufficiently firm and resistant skeletal basis for sharp powerful muscular contraction, and so render swift flight impossible. It is stated that the flight of the mimics is like that of the model, and in some cases this is undoubtedly true. But in a great many cases it certainly does not hold good. Papilio clytia (Pl. I, figs. 7 and 8) is a strong swift flyer very unlike the Danaine and Euploeine which it is supposed to mimic. The flight of the female of Hypolimnas misippus (Pl. IV, fig. 7) is quite distinct from that of Danais chrysippus, while the mimetic forms of P. polytes fly like the non-mimetic one, a mode of flight so different from that of the two models that there is no difficulty in distinguishing them many yards away. Swift flight must be reckoned as one of the chief modes of defence in a butterfly, and on this score the mimic is often better off than the model. And of course it must not be forgotten that where the mode of flight is distinct the protective value of the resemblance must be very much discounted.

(3) That the imitators are always less numerous in individuals.

In the majority of cases this is certainly true. Probably all the Old-World Papilios that mimic Danaines are scarcer, and frequently very much scarcer, than their models. This is very evident from a study of the more comprehensive priced catalogues of Lepidoptera. The mimic is generally a more expensive insect than the model, and not infrequently it costs as many pounds as the model does shillings. But the rule is not universal. Papilio polytes is often much more common than either of its models. The remarkable Pierines, Archonias tereas and A. critias (Pl. XI, fig. 10) as a rule far outnumber the Pharmacophagus Swallow-tail which they mimic. Or again the Chalcosid moth Callamesia pieridoides^[28] is a more abundant insect than the Bornean Pierine Delias cathara which it closely resembles.

It has sometimes been suggested in explanation of the greater abundance of the mimic that in such cases we are concerned with MÜllerian mimicry, that since both of the species concerned are distasteful there is not, strictly speaking, either a mimic or a model, and consequently the relative proportions have not the significance that they possess where the mimicry is of the simple Batesian type. It is, however, very doubtful whether such an explanation is of any value, for, as will appear later, there are grave objections to accepting the current theory as to the way in which a resemblance is established on MÜllerian lines (cf. pp. 72-74).

(4) That the imitators differ from the bulk of their allies.

What importance we attach to this condition must depend upon our interpretation of the word "allies"—whether, for example, we use it for a small group of closely connected species, for a genus, for a group of genera, or in an even wider sense. Perhaps an example will serve to make the difficulty more clear. As already noticed, the S. American genus Dismorphia belongs to the family of Pieridae or "whites." Also certain species of Dismorphia bear a close resemblance to certain species of Ithomiines, a noteworthy example being D. praxinoe and Mechanitis saturata (Pl. X, figs. 3 and 7), in which the pattern, colour, and shape of the two species are all far removed from what is usually understood by a "white." It must not be forgotten, however, that these matters are generally discussed by European naturalists who have grown up in a region where the majority of the "whites" are more or less white. For this reason the statement that D. praxinoe differs from the bulk of its allies is likely to meet with general acceptance, especially as some of the species of the genus itself (e.g., D. cretacea, Pl. X, fig. 1) are regular whites in appearance. But when we come to look at the genus Dismorphia as a whole the matter assumes another complexion. Seitz^[29] recognises 75 species of which about a dozen are predominantly white. The rest present a wonderful diversity of colour and pattern. Black predominates on the fore wings, and the insect is frequently marked with gay patches of yellow, bright brown, scarlet, or blue. Forms which from their colour are clearly not mimics present nevertheless the general pattern and shape of other forms which bear a strong resemblance to some Ithomiine. Sometimes a change of colour in certain patches from blue or yellow to bright brown would make all the difference between a non-imitative and an imitative species. Moreover, the non-imitative forms frequently have the peculiar narrow wing, so unusual in a Pierine, which enhances the resemblance of the mimicking species to the Ithomiine model, and which to some extent occurs even in D. cretacea. Clearly we are not justified in saying that D. praxinoe differs from the bulk of its allies, for inside the genus there are many non-imitative species which differ from it in some particulars and are alike it in others. There is a distinct family resemblance among the bulk of the Dismorphias, including practically all the mimetic forms, and on the whole the resemblances between the imitative and the non-imitative forms are as noteworthy as the differences. Though not exhibited in so striking a fashion, the same is to a large extent true of a large proportion of the cases of mimicry. It is on the whole unusual to find cases where a single species departs widely from the pattern scheme of the other members of the genus and at the same time resembles an unrelated species. Two of the best instances are perhaps those of Limenitis archippus (p. 49) and of the Pierid Pareronia (p. 23). Of the total number of mimicry instances a high proportion is supplied by relatively few groups. In each region several main series of models and mimics run as it were parallel to one another. In Asia, for example, we have the Papilio-Danaine series where the colour-patterns of a series of Danaines, all nearly related, are closely paralleled by those of a section of the genus Papilio, and by those of the Satyrid genus Elymnias. In Africa there is a similar Papilio-Danaine series though of less extent. Africa has a group of models not found in Asia, and the Papilio-Danaine series is as it were curtailed by the Papilio-Planema series with which to some extent runs parallel the genus Pseudacraea. These phenomena of parallel series have been mentioned here as shewing that mimicry tends to run in certain groups and that in many cases at any rate little meaning can be attached to the statement that the imitators differ from the bulk of their allies.

The fifth of Wallace's conditions is clear and needs no discussion.

It is evident that at any rate a large proportion of the instances of close resemblance do not fulfil all of the conditions laid down by Wallace. Nevertheless we should expect them to do so if the resemblance has been brought about by the cumulative effect of natural selection on small favourable variations. Clearly there is a prima facie case for doubting whether we must of necessity ascribe all resemblance of the kind to natural selection, and in the next few chapters we shall discuss it in more detail from several points of view.

CHAPTER VI

"MIMICRY RINGS"

Having reviewed briefly some of the most striking phenomena of what has been termed mimicry, we may now inquire whether there are good grounds for supposing that these resemblances have been brought about through the operation of natural selection or whether they are due to some other cause. If we propose to offer an explanation in terms of natural selection we are thereby committed to the view that these resemblances are of the nature of adaptation. For unless we grant this we cannot suppose that natural selection has had anything to do either with their origin or with their survival. Granting then for the present the adaptational nature of these mimetic resemblances, we may attempt to deduce from them what we can as to the mode of operation of natural selection. In doing so we shall bear in mind what may be called the two extreme views: viz. (a) that the resemblance has been brought about through the gradual accumulation of very numerous small variations in the right direction through the operation of natural selection, and (b) that the mimetic form came into being as a sudden sport or mutation, and that natural selection is responsible merely for its survival and the elimination of the less favoured form from which it sprang.

There is a serious difficulty in the way of accepting the former of these two views. If our two species, model and would-be mimic are, to begin with, markedly different in pattern, how can we suppose that a slight variation in the direction of the model on the part of the latter would be of any value to it? Take for example a well-known South American case—the resemblance between the yellow, black, and brown Ithomiine, Mechanitis saturata (Pl. X, fig. 7) and the Pierine, Dismorphia praxinoe (Pl. X, fig. 3). The latter belongs to the family of the "whites," and entomologists consider that in all probability its ancestral garb was white with a little black like the closely allied species D. cretacea (Pl. X, fig. 1). Can we suppose that in such a case a small development of brown and black on the wings would be sufficient to recall the Ithomiine and so be of service to the Dismorphia which possessed it? Such a relatively slight approach to the Ithomiine colouring is shewn by the males of certain South American "whites" belonging to the genus Perrhybris (Pl. X, figs. 4 and 5). But the colour is confined to the under-surface and the butterflies possessing it could hardly be confused with a Mechanitis more than their white relations which entirely lack such a patch of colour. If birds regarded white butterflies as edible it is difficult to suppose that they would be checked in their attacks by a trifling patch of colour while the main ground of the insect was still white. But unless they avoided those with the small colour patch there would be an end of natural selection in so far as the patch was concerned, and it would have no opportunity of developing further through the operation of that factor. This is the difficulty of the initial variation which has been clearly recognised by most of the best known supporters of the theory of mimicry. Bates himself offered no suggestion as to the way in which such a form as a Pierid could be conceived of as beginning to resemble an Ithomiine^[30]. Wallace supposed that the Ithomiines were to start with not so distinct from many of the edible forms as they are to-day, and that some of the Pierines inhabiting the same district happened to be sufficiently like some of the unpalatable forms to be mistaken for them occasionally^[31].

The difficulty of the initial variation had also occurred to Darwin, and he discusses it in an interesting passage which is so important that we may quote it here in full:

It should be observed that the process of imitation probably never commenced between forms widely dissimilar in colour. But starting with species already somewhat like each other, the closest resemblance, if beneficial, could readily be gained by the above means; and if the imitated form was subsequently and gradually modified through any agency, the imitating form would be led along the same track, and thus be altered to almost any extent, so that it might ultimately assume an appearance or colouring wholly unlike that of the other members of the family to which it belonged. There is, however, some difficulty on this head, for it is necessary to suppose in some cases that ancient members belonging to several distinct groups, before they had diverged to their present extent, accidentally resembled a member of another and protected group in a sufficient degree to afford some slight protection; this having given the basis for the subsequent acquisition of the most perfect resemblance^[32].

Both Darwin and Wallace recognised clearly this difficulty of the initial variations, and both suggested a means of getting over it on similar lines. Both supposed that in general colour and pattern the groups to which model and mimic belonged were far more alike originally than they are to-day. They were in fact so much alike that comparatively small variations in a favourable direction on the part of the mimic would lead to its being confused with the unpalatable model. Then as the model became more and more conspicuously coloured, as it developed a more and more striking pattern warning would-be enemies of its unpleasant taste, the mimic gradually kept pace with it through the operation of natural selection, in the shape of the discriminating enemy, eliminating those most unlike the model. The mimic travelled closely in the wake of the model, coaxed as it were by natural selection, till at last it was far removed in general appearance from the great majority of its near relations.

In this way was offered a comparatively simple method of getting over the difficulty of applying the principle of natural selection to the initial variations in a mimetic approach on the part of one species to another. But it did not escape Darwin's penetration that such an argument would not always be easy of application—that there might be cases where a given model was mimicked by members of several groups of widely differing ancestral pattern, and that in these cases it would be difficult to conceive of members of each of the several groups shewing simultaneous variations which would render them liable to be mistaken for the protected model. The difficulty may perhaps be best illustrated if we consider a definite case.

It is a feature of mimetic resemblances among butterflies that a given species in a given locality may serve as a model for several other species belonging to unrelated groups. Generally such mimics belong to presumably palatable species, but other presumably unpalatable species may also exhibit a similar coloration and pattern. In this way is formed a combine to which the term "mimicry ring" has sometimes been applied. An excellent example of such a mimicry ring is afforded by certain species of butterflies in Ceylon, and is illustrated on Plate IV. It is made up in the first place of two species belonging to the presumably distasteful Danaine group, viz. Danais chrysippus and D. plexippus. The latter is a rather darker insect but presents an unmistakable general likeness to D. chrysippus. Those who believe in MÜllerian mimicry would regard it as an excellent example of that phenomenon. For those who believe only in Batesian mimicry D. plexippus, being the scarcer insect, must be regarded as the mimic and D. chrysippus as the model. In both of these species the sexes are similar, whereas in the other three members of the "ring" the female alone exhibits the resemblance. One of these three species is the common Nymphaline, Hypolimnas misippus, of which the female bears an extraordinary likeness to D. chrysippus when set and pinned out on cork in the ordinary way. The male, however (Pl. IV, fig. 8), is an insect of totally different appearance. The upper surfaces of the wings are velvety black with a large white patch bordered with purple in the middle of each^[33]. The "ring" is completed by the females of Elymnias undularis and Argynnis hyperbius. The former of these belongs to the group of Satyrine butterflies and the female is usually regarded as a mimic of D. plexippus, which it is not unlike in so far as the upper surface of the wings is concerned. Here again the male is an insect of totally dissimilar appearance. Except for a border of lighter brown along the outer edges of the hind wings the upper surface is of a uniform deep purple-brown all over (Pl. IV, fig. 6). In Argynnis hyperbius the appearance is in general that of the Fritillary group to which it belongs. But in the female the outer portion of the fore wings exhibits much black pigment and is crossed by a broad white band similar to that found in the same position on the wing of D. plexippus (Pl. IV, fig. 2).

Of the five species constituting this little "mimicry ring" in Ceylon two, on the current theory of mimicry, are to be regarded as definitely unpalatable, one (H. misippus) as doubtfully so, while the Satyrine and the Fritillary are evidently examples of simple or Batesian mimicry.

Now such examples as this of simultaneous mimicry in several species are of peculiar interest for us when we come to inquire more closely into the process by which the resemblances can be supposed to have been brought about. Take for example the case of E. undularis. The male is evidently an unprotected insect in so far as mimicry is concerned, while the female exhibits the general pattern and coloration characteristic of the warningly coloured and presumably distasteful species D. plexippus or D. chrysippus. If we are to suppose this to have been brought about by the operation of natural selection it is clear that we must regard the colour and pattern of the male as the original colour and pattern of both sexes. For natural selection cannot be supposed to have operated in causing the male to pass from a protected to an unprotected condition, or even in causing him to change one unprotected condition for another. Probably all adherents of the mimicry theory would be agreed in regarding the male of Elymnias undularis as shewing the ancestral coloration of the species, and in looking upon the female as having been modified to her own advantage in the direction of D. plexippus. The question that we have to try to decide is how this has come about—whether by the accumulation of slight variations, or whether by a sudden change or mutation in the pattern and colour of the female by which she came to resemble closely the Danaine. It is clear that if D. plexippus were what it is to-day before the mimetic approach on the part of E. undularis began, small variations in the latter would have been of no service to it. The difference between the two species would have been far too great for individuals exhibiting slight variation in the direction of D. plexippus to stand any chance of being confused with this species. And unless such confusion were possible natural selection could not work. There is, however, an immediate way out of the difficulty. We may suppose that the coloration of the male of the mimic, E. undularis, is not only the ancestral colour of its own species but also of the model. D. plexippus on this supposition was very like E. undularis, of which both sexes were then similar to what the male is to-day. The pattern is, however, an inconspicuous one, and it can be imagined that it might be to the advantage of D. plexippus to don a brighter garb for the advertisement of its unpleasant qualities. Variations in the direction of a more conspicuous pattern would for that reason tend to be preserved by natural selection, until eventually was evolved through its means the well-marked pattern so characteristic of the model to-day. If in the meantime variations in the same direction occurred among the females of E. undularis these would tend to be preserved through their resemblance to the developing warning pattern of the distasteful Danaine model. The development of model and mimic would proceed pari passu, but if the sexes of the mimic differ, as in this case, we must suppose the starting-point to have been the condition exhibited by the male of the mimicking species.

But Argynnis hyperbius is also a species in which the female mimics D. plexippus; and by using the same argument as that just detailed for Elymnias undularis we can shew that the Danaine model, D. plexippus, must also have been like the male of Argynnis hyperbius. And if the resemblance of A. hyperbius was developed subsequently to that of E. undularis, then both D. plexippus and E. undularis must at one time have been like the male of A. hyperbius, a proposition to which few entomologists are likely to assent. Further, since the female of H. misippus also comes into the plexippus-chrysippus combine we must suppose that these species must at some time or another have passed through a pattern stage like that of the misippus male.

It is scarcely necessary to pursue this argument further, for even the most devoted adherents of the theory of mimicry as brought about by the operation of natural selection on small variations are hardly likely to subscribe to the phylogenetic consequences which it must entail in cases where a model is mimicked by the females of several species whose males are widely dissimilar in appearance.

Even if we suppose the two Danaines to have been originally like the male of one of the three mimics, we must still suppose that the females of the other two originated as "sports," sufficiently near to Danaines to be confused with them. But if such sports can be produced suddenly by some mutational process not at present understood, why should not these sports be the females of the three mimicking species as we see them at present? Why need we suppose that there were intermediate stages between the mimicking female and the original hypothetical female which was like the male? If a sport occurred which was sufficiently similar to an unpalatable species to be confused with it, it is theoretically demonstrable that, although relatively scarce to start with, it would rapidly increase at the expense of the unprotected male-like female until the latter was eliminated. We shall, however, return in a later chapter (p. 96) to the argument by which this view can be supported.

So far we have discussed what we called the two extreme views as to the way in which a mimetic resemblance may be supposed to have originated. Of the two that which assumes the resemblance to have been brought about by a succession of slight variations must also assume that model and mimic were closely alike to start with, and this certainly cannot be true in many cases. On the other hand, there is so far no reason against the idea of supposing the resemblance to have originated suddenly except what to most minds will probably appear its inherent improbability.

There are writers on these questions of mimicry who adopt a view more or less intermediate between those just discussed. They regard the resemblance as having arisen in the first place as a sport of some magnitude on the part of the mimic, rendering it sufficiently like the model to cause some confusion between the two. A rough-hewn resemblance is first brought about by a process of mutation. Natural selection is in this way given something to work on, and forthwith proceeds to polish up the resemblance until it becomes exceedingly close. Natural selection does not originate the likeness, but, as soon as a rough one has made its appearance, it comes into operation and works it up through intermediate stages into the finished portrait. It still plays some part in the formation of a mimetic resemblance though its rÔle is now restricted to the putting on of the finishing touches. Those who take this view hold also that the continued action of natural selection is necessary in order to keep the likeness up to the mark. They suppose that if selection ceases the likeness gradually deteriorates owing to the coming into operation of a mysterious process called regression. This idea involves certain conceptions as to the nature of variation which we shall discuss later.

Though it is difficult to regard Batesian mimicry as produced by the accumulation of small variations through natural selection, it is perhaps rather more plausible to suppose that such a process may happen in connection with the numerous instances of MÜllerian mimicry. For since the end result is theoretically to the advantage of both species instead of but one, it is possible to argue that the process would be simplified by their meeting one another halfway, as MÜller^[34] himself originally suggested. Variations on the part of each in the direction of the other would be favourably selected, the mimicry being reciprocal.

Difficulties, however, begin to arise when we inquire into the way in which this unification of pattern may be conceived of as having come about. By no one have these difficulties been more forcibly presented than by Marshall^[35] in an able paper published a few years ago, and perhaps the best way of appreciating them is to take a hypothetical case used by him as an illustration.

Let us suppose that in the same area live two equally distasteful species A and B, each with a conspicuous though distinct warning pattern, and each sacrificing 1000 individuals yearly to the education of young birds. Further let it be supposed that A is a common species of which there are 100,000 individuals in the given area, while B is much rarer, and is represented by 5000. The toll exacted by young birds falls relatively more lightly upon A than upon B, for A loses only 1%, whereas B's loss is 20%. Clearly if some members of B varied so that they could be mistaken for A it would be greatly to their advantage, since they would pass from a population in which the destruction by young birds was 20% to one in which it would now be rather less than 1%. Moreover, as the proportion of B resembling A gradually increased owing to this advantage, the losses suffered by those exhibiting the original B pattern would be relatively heavier and heavier until the form was ultimately eliminated. In other words, it is theoretically conceivable that of two distasteful species with different patterns the rarer could be brought to resemble the more abundant.

We may consider now what would happen in the converse case in which the more numerous species exhibited a variation owing to which it was confused with the rarer. Suppose that of the 100,000 individuals of A 10,000 shewed a variation which led to their being mistaken for B, so that there are 90,000 of the A pattern and 15,000 of the B pattern of which 10,000 belong to species A. A will now lose 1000 out of the 90,000 having the A pattern, and ⅔ × 1000 out of the 10,000 of species A which exhibit the B pattern. The toll of the birds will be 1⁄90 of those keeping the original A pattern, and 2⁄30 of those of species A which have assumed the B pattern. The mortality among the mimetic members of A is six times as great as among those which retain the type form. It is clear therefore that a variation of A which can be mistaken for B is at a great disadvantage as compared with the type form^[36], and consequently it must be supposed that the MÜllerian factor, as the destruction due to experimental tasting by young birds is termed, cannot bring about a resemblance on the part of a more numerous to a less numerous species. Further, as Marshall goes on to shew, there can be no approach of one species to the other when the numbers are approximately equal. A condition essential for the establishing of a mimetic resemblance on MÜllerian lines, no less than on Batesian, is that the less numerous species should take on the pattern of the more numerous. Consequently the argument brought forward in the earlier part of this chapter against the establishing of such a likeness by a long series of slight variations is equally valid for MÜllerian mimicry^[37].

CHAPTER VII

THE CASE OF PAPILIO POLYTES

Many instances of mimicry are known to-day, but comparatively few of them have been studied in any detail. Yet a single carefully analysed case is worth dozens which are merely superficially recorded. In trying to arrive at some conception of the way in which the resemblance has come about we want to know the nature and extent of the likeness in the living as well as in the dead; the relative abundance of model and mimic; what are likely enemies and whether they could be supposed to select in the way required, whether the model is distasteful to them; whether intermediate forms occur among the mimics; how the various forms behave when bred together, etc., etc. Probably the form that from these many points of view has, up to the present, been studied with most care is that of the Swallow-tail, Papilio polytes. It is a common butterfly throughout the greater part of India and Ceylon, and closely allied forms, probably to be reckoned in the same species, reach eastwards through China as far as Hongkong. P. polytes is one of those species which exhibit polymorphism in the female sex. Three distinct forms of female are known, of which one is like the male, while the other two are very different. Indeed for many years they were regarded as distinct species, and given definite specific names. To Wallace belongs the credit of shewing that these three forms of female are all to be regarded as wives of the same type of male^[38]. He shewed that there were no males corresponding to two of the females; also that the same one male form was always to be found wherever any of the females occurred. As the result of breeding experiments in more recent years Wallace's conclusions have been shewn to be perfectly sound.

The male of polytes (Pl. V, fig. 1) is a handsome blackish insect with a wing expanse of about four inches. With the exception of some yellowish-white spots along their outer margin the fore wings are entirely dark. Similar spots occur along the margin of the hind wing also, while across the middle runs a series of six yellowish-white patches producing the appearance of a broad light band. The thorax and abdomen are full black, though the black of the head is relieved by a few lighter yellowish scales. The under surface is much like the upper, the chief difference being a series of small and slightly reddish lunules running outside the light band near the margin of the hind wing (Pl. V, fig. 1 a). In some specimens these markings are almost absent. One form of female is almost exactly like the male (Pl. V, fig. 2), the one slight difference being that the lunules on the under surface of the hind wing are generally a trifle larger. For brevity she may be called the M form. The second form of female differs in many respects from the male and the M female. Instead of being quite dark, the fore wings are marked by darker ribbed lines on a lighter ground^[39] (Pl. V, fig. 3). The hind wings shew several marked differences from those of the male. Of the series of six patches forming the cross band the outermost has nearly disappeared, and the innermost has become smaller and reddish. The middle four, on the other hand, have become deeper, reaching up towards the insertion of the wing, and are pure white. A series of red lunules occurs on the upper surface outside the white band, and the yellowish-white marginal markings tend to become red. These differences are equally well marked on the under surface (Pl. V, fig. 3 a). The colour of the body, however, remains as in the male. From the resemblance shewn by this form to another species of Swallow-tail, Papilio aristolochiae (Pl. V, fig. 5), we shall speak of it as the A form.

The third form of female is again very distinct from the other two. The fore wings are dark but are broken by an irregular white band running across the middle (Pl. V, fig. 4), and there is also an irregular white patch nearer the tips of the wing. The hind wings, on the other hand, are characterised by having only red markings. The yellowish-white band of the male is much reduced and is entirely red, while the red lunules are much larger than in the A form. The under surface (Pl. V, fig. 4 a) corresponds closely with the upper. The body remains black as in all the other forms. This type of female bears a resemblance to Papilio hector (Pl. V, fig. 6), and for that reason we shall speak of it as the H form. It should be added that these three forms of female are quite indistinguishable in the larval and chrysalis stages.

It was Wallace who first offered an explanation of this interesting case in terms of mimicry. According to this interpretation P. polytes is a palatable form. The larva, which feeds on citronaceous plants, and the chrysalis are both inconspicuous in their natural surroundings. They may be regarded as protectively coloured, and consequently edible and liable to persecution. The original coloration is that of the male and the M female. From this the other two forms of female have diverged in the direction of greater instead of less conspicuousness, although the presumed edibility of the insect might have led us to think that a less conspicuous coloration would have been more to its advantage. But these two females resemble the two species Papilio aristolochiae and Papilio hector, which, though placed in the same genus as P. polytes, belong to a very different section of it^[40]. The larvae of these two species are conspicuously coloured black and red with spiny tubercles. They feed upon the poisonous Aristolochia plants. For these reasons and also from the fact that the butterflies themselves are both conspicuous and plentiful it is inferred that they are unpalatable. In short, they are the models upon which the two polytes females that are unlike the male have been built up by natural selection.

The suggestion of mimicry in this case is supported by the fact that there is a general correspondence between the areas of distribution of model and mimic. P. hector is not found outside India and Ceylon, and the H female of P. polytes is also confined to this area. P. aristolochiae, on the other hand, has a much wider range, almost as wide indeed as that of P. polytes itself. Generally speaking the A female accompanies P. aristolochiae wherever the latter species is found. Beyond the range of P. aristolochiae, in northern China, the M female alone is said to occur. On the other hand, as the matter comes to be more closely studied exceptions are beginning to turn up. The H female, for instance, is found on the lower slopes of the Himalayas, far north of the range of P. hector, and there are indications that a careful study of the distribution in China and Japan may prove of importance.

Moreover, the investigation of a smaller area may also bring to light points of difficulty. In Ceylon, for example, P. polytes is common up to several thousand feet, while P. hector is rare at half the height to which polytes ascends. Nevertheless the H form of female is relatively just as abundant up-country where hector is rarely found as it is low down where hector is plentiful^[41]. On the other hand, P. aristolochiae may be exceedingly abundant at altitudes where hector is scarce. Yet the A form of polytes is no more relatively abundant here than elsewhere on the island. All over Ceylon, in fact, the relative proportions of the three forms of female appear to be the same, quite irrespective of the abundance or scarcity of either of the models. As, however, we shall have to return to this point later, we may leave it for the moment to consider other features of this case of P. polytes.

In collections of insects from India or Ceylon it is not unusual to find specimens of the A form of female of polytes placed with P. aristolochiae, and the H form with P. hector. When the insects are old and faded and pinned out on cork the mistake is a very natural one. But after all the enemies of polytes do not hunt it in corked cabinets, and any estimation of resemblance to be of use to us must be based upon the living insects. Are the resemblances of the mimics to the models when alive so close that they might be expected to deceive such enemies^[42] as prey upon them and have no difficulty in distinguishing the male form of polytes from P. aristolochiae or P. hector?

To answer for a bird is a hazardous undertaking. We know so little of the bird's perceptive faculties whether of taste or sight. But on general grounds, from the specialization of their visual apparatus, it is probable that the sense of sight is keen, though whether the colour sense is the same as our own is doubtful^[43]. On the other hand, the olfactory apparatus is relatively poorly developed in birds, and from this we can only argue that the senses of smell and taste are not especially acute. Really we can do little more than to describe how these mimetic resemblances appear to our own senses, and to infer that they do not appear very different to the bird. If there is any difference in keenness of perception we shall probably not be far wrong in presuming that the advantage rests with the bird. After all if there is any truth in the theory of mimicry the bird has to depend largely upon its keenness of sight in making its living, at any rate if that living is to be a palatable one. If natural selection can bring about these close resemblances among butterflies it must certainly be supposed to be capable of bringing the bird's powers of vision to a high pitch of excellence.

Returning now to the case of P. polytes, there is not the least doubt that to the ordinary man accustomed to use his eyes the A form of female is easily distinguishable from P. aristolochiae, as also is the H form from P. hector. The two models have a feature in common in which they both differ from their respective mimics. In both of them the body and head are largely of a brilliant scarlet, whereas neither of the mimics has a touch of red on the body. In the living insect when the body is swelled by its natural juices the effect is very striking^[44]. It gives at once a "dangerous" look to the insect when settled, even at a distance of several yards, and this although one may be perfectly familiar with its harmless nature. The mimics on the other hand with their sombre-coloured bodies never look otherwise than the inoffensive creatures that they are. The "dangerous" look due to the brilliant scarlet of the body and head of hector and aristolochiae is reinforced by the quality of the red on the markings of the wings. In both models it is a strong clamorous red suggestive of a powerful aniline dye, whereas such red as occurs in the mimics is a softer and totally distinct colour. The difference in quality is even more marked on the under than on the upper surface (Pl. V, figs. 3 a—6 a), and the net result is that when settled, with wings either expanded or closed, there is no possibility of an ordinarily observant man mistaking mimic for model in either case, even at a distance of several yards.

It may, however, be argued that it is not when at rest but during flight that the mimetic resemblance protects the mimic from attack. Actually this can hardly be true, for the mode of flight constitutes one of the most striking differences between model and mimic. P. hector and P. aristolochiae fly much in the same way. They give one the impression of flying mainly with their fore wings, which vibrate rapidly, so that the course of the insect, though not swift, is on the whole sustained and even. The flight of all the different forms of polytes is similar and quite distinct from that of the models. It is a strong but rather heavy and lumbering up-and-down flight. One gets the impression that all the wing surface is being used instead of principally the fore wings as appears in P. hector and P. aristolochiae. The difference is difficult to put into words, but owing to these peculiarities of flight the eye has no difficulty in distinguishing between model and mimic even at a distance of 40 to 50 yards. Moreover, colour need not enter into the matter at all. It is even easier to distinguish model from mimic when flying against a bright background, as for instance when the insect is between the observer and a sunlit sky, than it is to do so by reflected light. I have myself spent many days in doing little else but chasing polytes at Trincomalee where it was flying in company with P. hector, but I was never once lured into chasing the model in mistake for the mimic. My experience was that whether at rest or flying the species are perfectly distinct, and I find it difficult to imagine that a bird whose living depended in part upon its ability to discriminate between the different forms would be likely to be misled. Certainly it would not be if its powers of discrimination were equal to those of an ordinary civilised man. If the bird were unable to distinguish between say the A form of female and P. aristolochiae I think that it would be still less likely to distinguish between the same A form and the male or the M form of female. For my experience was that at a little distance one could easily confuse the A form of polytes with the male. Except when one was quite close the red on the A form was apt to be lost, the white markings on the hind wing were readily confused with those of the male, and one had to depend entirely on the lighter fore wing. Unless the bird were keener sighted than the man the A form would be more likely to be taken in mistake for its unprotected relative than avoided for its resemblance to the presumably unpalatable model. On the other hand, if the bird were sufficiently keen sighted never to confuse the A female with the male form its sight would be too keen to be imposed upon by such resemblance as exists between the A female and P. aristolochiae.

These, however, are not the only criticisms of the theory of mimicry which the study of this species forces upon us. Papilio polytes is one of the few mimetic species that has been bred, and in no other case of polymorphism is the relation between the different forms so clearly understood. For this result we are indebted mainly to the careful experiments of Mr J. C. F. Fryer, who recently devoted the best part of two years to breeding the different forms of this butterfly in Ceylon^[45]. Fryer came to the conclusion that an explanation of this curious case is possible on ordinary Mendelian lines. At first sight the breeding results appear complicated, for any one of the three forms of female can behave in several different ways. For the sake of simplicity we may for the moment class together the A and H females as the mimetic females, the non-mimetic being represented by the M or male-like females. The different kinds of families which each of the three females can produce may be tabulated as follows:—

(α) The M form may give either:—

(1) M only.

(2) M and mimetics in about equal numbers.

(3) Mimetics only.