Mimicry in Butterflies/Chapter 11

From the facts recorded in the preceding chapters it is clear that there are difficulties in the way of accepting the mimicry theory as an explanation of the remarkable resemblances which are often found between butterflies belonging to distinct groups. Of these difficulties two stand out beyond the rest, viz., the difficulty of finding the agent that shall exercise the appropriate powers of discrimination, and the difficulty of fitting in the theoretical process involving the incessant accumulation of minute variations with what is at present known of the facts of heredity.

With regard to the former of these two difficulties we have seen that the supporters of the theory regard birds as the main selective agent. At the outset we are met with the fact that relatively few birds have been observed to prey habitually on butterflies, while some at any rate of those that do so shew no discrimination between what should be theoretically pleasant to eat and what should not be pleasant. Even if birds are the postulated enemies it must be further shewn that they exercise the postulated discrimination. It is required of them that they should do two things. In the first place they must confuse an incipient or "rough" mimic with a model sufficiently often to give it an advantage over those which have not varied in the direction of the model. In other words, they must be easily taken in. Secondly, they are expected to bring about those marvellously close resemblances that sometimes occur by confusing the exact mimicking pattern with the model, while at the same time eliminating those which vary ever so little from it. In other words, they must be endowed with most remarkably acute powers of discrimination. Clearly one cannot ask the same enemy to play both parts. If, therefore, birds help to bring about the resemblance we must suppose that it is done by different species—that there are some which do the rough work, others which do the smoothing, and others again which put on the final polish and keep it up to the mark. This is, of course, a possibility, but before it can be accepted as a probability some evidence must be forthcoming in its favour.

But even if the difficulty of the appropriate enemy be passed over, and it be granted that a mimetic resemblance can be built up through a number of small separate steps, which have become separately established through the agency of separate species of birds with various but distinct discriminating powers, we are left face to face with an even more serious physiological difficulty. For why is it that when the end form which is supposed to have resulted from this process is crossed back with the original form all the intermediate steps do not reappear? Why is it that when the altered germplasm is mingled with the original germplasm the various postulated stages between them are not reformed? For in various cases where we know the course of evolution this does occur. The pale pink sweet-pea has come from the wild purple by a series of definite steps, and when it is crossed back with the wild form the resulting plants give the series of stages that have occurred in the evolution of the pink. So also when the orange rabbit is crossed with the wild grey form and the offspring are inbred there are reproduced the black, the tortoiseshell, and the chocolate, forms which are stages in the evolution of the orange from the wild grey. If then, to take an example, the "aristolochiae" form of Papilio polytes has been derived from the male-like form by a series of steps, why do we not get these steps reproduced after the germplasms of the two forms have been mingled? From the standpoint of modern genetic work the inference is that these postulated intermediate steps have never existed—that the one form of polytes female came directly from the other, and was not built up gradually through a series of stages by the selective agency of birds or any other discriminating enemy.

These two objections, viz. the difficulty of finding the appropriate enemy, and the non-appearance of intermediates when the extreme forms are crossed, may, perhaps, be said to constitute the main objections to the current theory of mimicry. Others such as the relative scarcity of mimicry in the male sex and the existence of cases of polymorphism among females of a species which cannot possibly be explained on mimetic lines have already been mentioned. But while the main objections remain it is hardly necessary to insist upon these others. Looked at critically in the light of what we now know about heredity and variation the mimicry hypothesis is an unsatisfactory explanation of the way in which these remarkable resemblances between different species of butterflies have been brought about. Sometimes this is admitted by those who nevertheless embrace the theory with a mild aloofness. For they argue that even though it does not explain all the facts no other theory explains so many. Others have sought an explanation in what has sometimes been termed the hypothesis of external causes, regarding these resemblances as brought about by similar conditions of soil and climate, and so forth. It is not inconceivable that certain types of colour and pattern may be the expression of deep-seated physiological differences, which place their possessors at an advantage as compared with the rest of the species. Were this so it is but reasonable to suppose that they would become established through the agency of natural selection. But it is difficult, if not impossible, to regard this as a satisfactory solution, if for no other reason than that it offers no explanation of polymorphism. For example, each of the three forms of polytes female holds its own and all must, therefore, be regarded as equally well adapted to the circumstances under which they live. They are so distinct in colour that it is difficult on this hypothesis to suppose that they are all on the same footing in respect to their environment. Yet if one is better off than the others, how is it that these still exist?

Those who have examined long series of these cases of resemblance among butterflies find it hard to believe that there is not some connection between them apart from climatic influence. One feels that they are too numerous and too striking to be all explained away as mere coincidences engendered by like conditions. Nor is it improbable that natural selection in the form of the discriminating enemy may have played a part in connection with them, though a different one from that advocated on the current theory of mimicry. If we assume that sudden and readily appreciable variations of the nature of "sports" turn up from time to time, and if these variations happen to resemble a form protected by distastefulness so closely that the two can be confused by an enemy which has learned to avoid the latter, then there would appear to be good grounds for the mimicking sport becoming established as the type form of the species. For it has already been seen that a rare sport is not swamped by intercrossing with the normal form, but that on the contrary if it possess even a slight advantage, it must rapidly displace the form from which it sprang (cf. Chap. VIII). On this view natural selection in the form of the discriminating enemy will have played its part, but now with a difference. Instead of building up a mimetic likeness bit by bit it will merely have conserved and rendered numerically preponderant a likeness which had turned up quite independently. The function of natural selection in respect of a mimetic likeness lies not in its formation but in its conservation. It does not bring about the likeness, neither does it accentuate it: it brings about the survival of those forms which happen to shew the likeness. Why variations on the part of one species should bear a strong resemblance to other, and often distantly related, species is another question altogether.

Even a superficial survey of the facts makes it evident that cases of mimicry tend to run in series—that a closely related series of mimics, though often of very different pattern and colour, tends to resemble a closely related series of models. In Asia we have the Cosmodesmus Papilios mimicking a series of Danaines, while the true Papilios (cf. Appendix II) tend to resemble a series of the less conspicuous members of the Pharmacophagus group. In the same region the various species of Elymnias form a series resembling a series of Danaines. In Africa there stands out the Cosmodesmus group again mimicking a Danaine series, and in part also an Acraeine series. Overlapping the Acraeines again are various forms of the Nymphaline genus Pseudacraea. It is also of interest that in Danais chrysippus and Acraea encedon the Danaine and Acraeine series overlap (cf. Pl. IX). Similar phenomena occur also in South America, where closely parallel series of colour patterns are exhibited by several Ithomiines, by Heliconius, Lycorea, Dismorphia, and other genera (cf. p. 39). On the other hand such mimetic resemblances as are shewn by the South American Swallow-tails of the Papilio and Cosmodesmus groups are almost all with the Pharmacophagus group, and almost all of the red-black kind (cf. p. 43).

On the whole it may be stated that the majority of cases of mimicry fall into one or other of such series as the above. If we select a case of mimicry at random we shall generally find that there are at least several close allies of the mimic resembling several close allies of the model. Isolated cases such as the resemblance between Pareronia and Danais (p. 23), between Archonias and a Pharmacophagus Papilio (p. 43), or the extraordinary instance of Papilio laglaizei and Alcidis agathyrsus, must be regarded as exceptional.

We have before us then a number of groups of butterflies each with a series of different colour patterns. In each group a portion of the series overlaps a portion of the series belonging to another more or less distantly related group. In the light of recent discoveries connected with heredity and variation the natural interpretation to such a set of phenomena would be somewhat as follows: Each group of Lepidoptera, such as those just discussed, contains, spread out among its various members, a number of hereditary factors for the determination of colour pattern. Within the group differences of pattern depend upon the presence or absence of this or that factor, the variety of pattern being the result of the many possible permutations and combinations of these colour factors. Within the limits of each group is found a definite number of these factors—more in one group, less in another. But some factors may be common to two or more groups, in which case some of the permutations of the factors would be similar in the groups and would result in identical or nearly identical pattern. To take a simple example in illustration, let us suppose that a given group, (α), contains the eight factors A-H. Since any species in the group may exhibit any combination of one or more of these factors it follows that a considerable number of different forms are possible. Now suppose that another group, (β), distinguished from (α) by definite structural features, also contains eight factors within the group, and that these factors are F-M, F, G, and H being common to both (α) and (β). Any combination therefore in (α) lacking the factors A-E will be paralleled by any combination in (β) lacking the factors I-M. For in both cases we should be dealing only with the factors F, G, and H, which are common to each group. So again a third group might have some factors in common with (α) and some with (β), and so on for other groups. In this way certain of the series of colour patterns found in (β) would overlap certain of those in (α), while others of the groups (β) and (α) might overlap those found in different groups again. The striking resemblances not infrequently found between species belonging to quite distinct groups would on this view depend upon the hereditary factors for pattern and colour being limited in number, so that the same assortment might not infrequently be brought together even though the group whose members exhibited the resemblance might, owing to structural differences, be placed in different families.

We know from recent experimental work that something of the sort is to be found in the coat colours of different rodents. Agouti, black, chocolate, blue-agouti, blue, and fawn form a series of colours common to the rabbit, the mouse, and the guinea-pig. These colours are related to each other in the same way in these different beasts. In the rat, on the other hand, there occur of this range of colours only the agouti and the black. Each of these species again has certain colour patterns which are peculiar to itself, such as the "English" type in the rabbit, the tricolor pattern in the guinea-pig, or the "hooded" variety in the rat. The total range of colour and pattern is somewhat different for each species, but a few are common to them all. Moreover, there are others which are common to the mouse and the rabbit but are not found in the guinea-pig, and others again which may occur in the rabbit and the guinea-pig but have not been met with in the other two. In certain features the rabbit might be said to "mimic" the mouse, and in other features the guinea-pig. It is not, of course, suggested that the case of the butterflies is so simple as that of the rodents, but so far as we can see at present there would seem to be no reason why the explanation should not be sought along the same lines. On this view the various colour patterns found among butterflies depend primarily upon definite hereditary factors of which the number is by no means enormous. Many of these factors are common to several or many different groups, and a similar aggregate of colour factors, whether in an Ithomiine, a Pierid, or a Papilio, results in a similar colour scheme. The likeness may be close without being exact because the total effect is dependent in some degree on the size and relative frequency of the scales and other structural features. In so far as pattern goes Hypolimnas dubius and Amauris echeria (Pl. VIII, figs. 7 and 8) are exceedingly close. But inspection at once reveals a difference in the quality of the scaling, giving to the Hypolimnas, where the black and yellow meet, a softness or even raggedness of outline, which is distinct from the sharper and more clear-cut borders of the Amauris. It is not unreasonable to suppose that these species carry identical factors for colour pattern, and that the differences by which the eye distinguishes them are dependent upon the minuter structural differences such as occur in the scaling. So the eye would distinguish between a pattern printed in identical colours on a piece of cretonne and a piece of glazed calico. Though pattern and colour were the same the difference in material would yield a somewhat different effect.

On the view suggested the occurrence of mimetic resemblances is the expression of the fact that colour pattern is dependent upon definite hereditary factors of which the total number is by no means very great. As many of the factors are common to various groups of butterflies it is to be expected that certain of the colour patterns exhibited by one group should be paralleled by certain of those found in another group. That cases of resemblance should tend to run in parallel series in different groups is also to be expected, for in some groups the number of factors in common is likely to be greater than in other groups. In consonance with this view is the fact that where polymorphism occurs among the females of a mimicking species the models, though often widely different in appearance, are, as a rule, closely related. Some of the Asiatic Papilios, for instance, resemble Danaines, while others resemble Pharmacophagus Papilios. But although the polymorphism exhibited by the females of a given species may be very marked, we do not find one of them resembling a Danaine and another a Pharmacophagus Swallow-tail. The models of a polymorphic mimic are almost always closely related species.

In discussing the problems of mimicry more attention is naturally paid to groups which exhibit the phenomenon than to those which either do not do so, or else only do so to a very limited extent. Yet the latter may be of considerable interest. Among the Pieridae of the Old World the phenomenon of mimicry is very rare. Pareronia and Aporia agathon conform closely to the common Danaid type represented by Danais vulgaris and other species, but apart from these none of the many Pierids in Asia resemble any of the recognised models. Africa is apparently destitute of Pierids which mimic species belonging to other groups. Yet no group of butterflies is more persecuted by birds. Of all the instances of bird attacks collected together by Marshall more than one-third are instances of attacks upon this group alone. If birds are the agents by which mimetic likenesses are built up through the cumulative selection of small variations, how can the rarity or absence of mimetic Pierids in the Old World be accounted for? For the species of Pierids, like the species of other families, shew considerable variation, and if this process of selection were really at work one would expect to find many more Pierid mimics in these regions than actually occur. It is true that the white, yellow, and red pigments found in Pierids differ from those of other butterflies in being composed either of uric acid or of some substance closely allied to that body. These substances are generally found between the two layers of chitin, of which the scale is composed, whereas the black pigment is intimately associated with the chitin of the scale itself. What is perhaps the principal factor in the formation of a mimetic likeness is the distribution of the black pigment with reference to the lighter pigments; and although the latter are chemically distinct in the Pierids as compared with other butterflies, there would seem to be no reason why the same factors governing the distribution of black should not be common to members of different groups. A distribution of black pigment similar to that found in a model and its mimic may occur also in a non-mimetic ally of the mimic. Dismorphia astynome, for example, resembles the Ithomiine Mechanitis lysimnia (Pl. XV, fig. 8) both in the distribution of black as well as of yellow and bright brown pigments. A similar distribution of the black pigment is also found in Dismorphia avonia, but the yellow and bright brown of the other two species is here replaced with white. By a slight though definite alteration in chemical composition this white pigment could be changed into bright brown and yellow with the result that D. avonia would closely resemble D. astynome in its colour scheme and would in this way also become a mimic of Mechanitis lysimnia. Another good instance is that of the females of Perrhybris demophile and P. lorena, the former being black and white, whereas in the latter the white is replaced by yellow and bright brown, giving the insect a typical Ithomiine appearance. Here again a definite small change in the composition of the pigment laid down in the scales would result in the establishing of a mimetic likeness where there would otherwise be not even a suggestion of it. It is in accordance with what we know to-day of variation that such a change should appear suddenly, complete from the start. And if so there is no difficulty in supposing that it might be of some advantage to its possessor through the resemblance to an unpalatable form. Even were the advantage but a slight one it is clear from previous discussion (p. 96) that the new variety would more or less rapidly replace the form from which it had sprung. With the continued operation of natural selection the new form would entirely supplant the original one, but it is not impossible that in some cases the selecting agent may be removed before this result has been achieved. In this event the proportions of the new and the old form would fall into a condition of equilibrium as in P. polytes in Ceylon, until some other selective agent arose to disturb the balance. On this view natural selection is a real factor in connection with mimicry, but its function is to conserve and render preponderant an already existing likeness, not to build up that likeness through the accumulation of small variations, as is so generally assumed. Recent researches in heredity and variation all point to this restriction of the scope of natural selection. Hitherto an argument in favour of the older view has been that derived from the study of adaptation—of an apparent purpose, which, at first sight, appears to be behind the manner in which animals fit into their surroundings. For many the explanation of this apparent purpose has been found in the process of natural selection operating gradually upon small variations, accumulating some and rejecting others, working as it were upon a plastic organism, moulding it little by little to a more and more perfect adaptation to its surroundings. On this view adaptation is easy to understand. The simplicity of the explanation is in itself attractive. But when the facts come to be examined critically it is evident that there are grave, if not insuperable, difficulties in the way of its acceptance. To outline some of these has been the object of the present essay. Though suggestions have been made as to the lines along which an explanation may eventually be sought it is not pretended that the evidence is yet strong enough to justify more than suggestions. Few cases of mimicry have as yet been studied in any detail, and until this has been done many of the points at issue must remain undecided. Nevertheless, the facts, so far as we at present know them, tell definitely against the views generally held as to the part played by natural selection in the process of evolution.