Creation by Evolution/Butterflies and Moths As Evidence of Evolution
BUTTERFLIES AND MOTHS AS EVIDENCE OF EVOLUTION
By Edward Bagnall Poulton
Hope Professor of Zoölogy in the University of Oxford
On the wings of butterflies “nature writes, as on a tablet, the story of the modifications of species. . . . As the laws of nature must be the same for all beings, the conclusions furnished by this group of insects must be applicable to the whole organic world.” (H. W. Bates.)
In spite of my title and my quotation from the great naturalist of the Amazon I must at the outset consider for a moment the evidence by which the belief in any scientific theory is justified. Why do we believe that the theory of the movements of the planets and satellites in our solar system is true? We believe it because by the light of this theory astronomers can predict the future, and we know from experience that their predictions will be verified. Farseeing people years ago made arrangements for observations on June 29, 1927, because they had been told by astronomers that on that day the moon would come between the sun and the earth, that its shadow would sweep across England from Southport to Hartlepool, and that every place in succession on that line would be, for about 25 seconds, buried in the darkness of total eclipse. And at the precise moment we saw, just as predicted, its sudden onset and swift passing away, while, in favored places where fortunately the astronomers had erected their instruments, the clouds cleared and unveiled the glorious spectacle of the Corona, drowned at all times except during total solar eclipse in the overpowering light of the sun.
We cannot expect to find such clear-cut evidence of evolution in the story of life on the earth, but we have abundant opportunities of applying a test that is in principle the same as that which justifies our belief in the astronomical theory.
If the theory of the movements of the solar system is correct an eclipse will take place at such a time and place; if the theory of evolution is true then certain organs or parts should be found in the ancestors of animals that do not now possess them or perhaps still exist in the early stages of animals of to-day. We search in the rocks, we find and dissect the early stages, and there the missing parts are revealed. The prediction founded on the theory of evolution is verified.
In order to convince others it is necessary to be convinced oneself, and nothing is so convincing as personal experience. Therefore I will tell of a prediction founded on evolution and its verification nearly forty years ago.
Few people, I suppose, realize that their teeth are among the most ancient of the parts that make up the human body. They are, for example, older than the hair, which was derived from the horny scales of ancestors much nearer to our own times than those which gave us our teeth. Teeth have come by direct descent from a remote ancestor that was covered with scales like a shark—scales which in this ancestor, as in the shark, passed over the lips into the mouth and, without any essential modification of structure, were used as teeth. The scales and the teeth of sharks are composed, like our own teeth, of hard dentine, developed from cells below the epidermis, or surface skin, covered with the much harder enamel, developed, at any rate mainly, from the epidermis itself. Scales of this kind and teeth have the same structure, the same development, and in the shark the one passes over the lips into the other.
Gradually, through long ages, body-scales like those of the shark were replaced by scales of different structure, and these directly or indirectly gave rise to reptilian scales, birds' feathers, and mammalian hair. But all through these changes the ancient scales in the mouth—the teeth—have remained essentially the same. Existing birds have lost them, but they were possessed by their ancestors, as we know from the fossil birds of both the Old World and the New.
The story of the mammals—our own branch—is more completely represented by animals alive to-day because of the preservation in Australia, cut off by sea from the stress and rush of life in other great land areas, of the duck-billed platypus (Ornithorhynchus) and the echidna, which is found also in New Guinea. Thus preserved from extinction, these remarkable animals have come down to us, descended from a link that connects the mammals with some primitive reptilian or pre-reptilian ancestor. Their temperature is much lower than that of mammals; their skeleton shows strong reptilian affinity; above all, they lay eggs like reptiles and birds, but they suckle their young by a primitive form of mammary gland. Yet these ancient forms, descendants of an ancestor through which the mammals received their teeth, are now, both of them, toothless. The echidna, feeding by means of its tongue, like the true ant-eaters of South America, is entirely toothless; the platypus has hard, toothlike plates for crushing the insects and mollusks of the streams in which it lives. These plates are really hardened gums; they have nothing of the structure of teeth. So the evidence looked for was wanting just where we should chiefly expect to find it. The evolutionist was nevertheless confident that these animals or their immediate ancestors originally possessed teeth but had lost them, and he believed that this inference from the known to the unknown might at some time be verified.
In 1888 I was working on the hair of the platypus and was able to show that it retained scale-like features which have been lost in the higher mammals. It was desirable to examine a young specimen, and knowing that Professor Kitchin Parker possessed one I asked if he would lend me some sections of the head prepared by his son, Professor Newton Parker. Just as I was about to examine them the thought flashed through my mind, “Perhaps at this young stage the platypus has not lost its true teeth.” I looked, and there they were, complete, with dentine and enamel, lying beneath the gum. The prediction was verified.
I cannot refrain from saying a few words about the generous treatment I received from that great man. I had borrowed his sections not to look for teeth but for hair, and he might well have said that I had anticipated the study he intended to make and must not publish the discovery. Far from it, he wrote full of enthusiasm and kindness, offering himself to communicate my paper to the Royal Society. It was a splendid thing for a young man to meet with so much kindness from one more than twice his age. I shall never forget it, and I hope the memory of it has enabled me to help on my younger comrades.
Later on Professor Charles Stewart found that the teeth cut the gum and are used for a time by the young platypus, but that they soon fall out and are replaced by the horny plates which invade their sockets.
Before considering the evidences of evolution furnished by butterflies and moths, I will attempt to answer the objection that nobody has ever seen one species turn into another and that nobody has brought convincing proof that species do change in this way. Just such an objection might be raised by one who paid a short visit to this planet and was assured that children became men and women. “I have been here for a whole week,” the visitor might well say, “and I have looked everywhere for this transformation, but I have never seen a child turn into a man or woman.” But a week is a far greater part of the period of human growth than is the time of human observation in the life of a species. Furthermore, if the visitor prolonged his stay indefinitely he would still never see a child “turn into” a man or woman, for between the two intervenes a growth so gradual that no difference is perceptible from day to day or from week to week. So is it with evolution. One species does not “turn into” another: it becomes another species through a series of gradual changes, and at no time would it be possible to say—“Now the change has come; what was species A yesterday is species B to-day.”
To prove that species A, known to us only from remains in the rocks, had become species B of to-day it would be necessary to restore to life the animals of innumerable past generations of beings and to show that, whereas those of adjacent strata could interbreed, their ancestors (species A) could not interbreed and produce fertile offspring with their living successors (species B). As this is manifestly impossible, we infer from the gradual changes of form or structure preserved in the rocks that A is a different species from B, which has apparently sprung from A by direct descent.
If, however, we cannot witness the transformation of one species into another any more than we can witness the sudden transformation of a child into a man or woman, we are able to witness the results of a series of changes in living forms in adaptation to the conditions of life—to what is called environment—and it is here that the butterflies and moths provide excellent illustration of evolution.
When I was a boy the common peppered moth was known to produce a rare black variety. The growth of the manufacturing districts of Lancashire and Yorkshire has greatly increased the volume of the smoke there, which, carried by the prevalent southwest winds, has done deadly work over a wide area, killing the gray lichens and leaving the treetrunks dark and sooty. Resting on bark like this the peppered moth would be more conspicuous to the eye of a bird seeking food than the black variety, and accordingly for many years this black form has entirely replaced the other form in these northern tracts. The others, being more easily seen, have been eaten. And the peppered moth is not the only species that shows change; several other bark-haunting moths have also become much darker in the same strip of country and during the same short period. Furthermore, similar changes have been observed in the moths of other smoke-producing areas in this country and on the Continent. Harrison has recently shown that some of these moths have become dark after their caterpillars have been fed for many generations on plants contaminated with salts of manganese, such as are contained in smoke. Inasmuch as the effects were transmitted in Mendelian proportions we must conclude that the salts acted upon the germ cells.
A still better but less well-known example of change in colour is found among the butterflies of tropical America. In each district these insects and some of the day-flying moths form groups that are of similar pattern and colouring but that are composed of species having very different degrees of relationship. Among the groups of any locality one species is generally predominant in numbers and is among the most conspicuous. But as the naturalist passes from one district to another he sees that the pattern of the groups becomes different, “as if at the touch of an enchanter’s wand,” to use the words by which Bates describes the change that sweeps over all the diverse members of a group. In Central America and Venezuela the chief group is made conspicuous by a tawny and black-barred pattern; in the Guianas the members of the corresponding group have much darker hind wings—the wings of some are almost entirely black; across the Amazon, in eastern Brazil, the hind wings change in the opposite direction, gaining a bright yellow stripe; high up the Amazon, at Ega (now Teffé), Bates’ headquarters for many years, the general colour of both wings changes from tawny to deep chestnut.
How shall we explain the advantages of these local colours and patterns, which run through many distantly related species? Let us see.
The insect-eating animals of each district, especially the birds, learn by experience that insects having certain conspicuous colours and patterns have an unpleasant taste or smell or are indigestible. So if a number of different noxious species bear the same pattern the birds easily learn to avoid them, with little waste of insect life in experimental tasting. Those so marked survive. Hence the great advantage to the butterflies of a combined advertisement or announcement that they are unpalatable, instead of each distasteful species having its own warning pattern, requiring to be tested separately. It is probable, too, that among the members of a large group there are many degrees of distastefulness and some also that are not distasteful at all, but that flaunt a false advertisement and live on the reputation (or rather the disreputation) of the others. This false advertisement was interpreted by H. W. Bates and is spoken of as Batesian mimicry. The combined advertisement was interpreted some years later by Fritz Müller and is called Müllerian mimicry.
I hope that the above paragraph makes clear the advantage that would be gained by a species whose pattern resembled that of another species which was not eaten by birds. Probably we should have an excellent opportunity of actually witnessing evolutionary progress if we could compare one imperfect mimic in such a group with what it was a hundred years ago. A century is only a drop in the ocean of time, but even in this short period some notable change might be evident. Well, we are fortunate enough to be able to make this comparison.
Between the years 1825 and 1830 the great naturalist William John Burchell was travelling in eastern Brazil, making extensive collections of animals and plants, concerning which he made the most accurate and detailed notes. I only wish that all naturalists to-day would do their work as well. His whole collection of insects is in the Oxford University Museum, and among them are many butterflies belonging to the great yellow-banded group of which I have spoken. In a certain species known as Lycorea halia, the yellow band along the hind wing is not so bright as in other species of the group, and the eight specimens in the Burchell collection show that this characteristic feature was on the average even less bright a hundred years ago. It may be suggested that the colours have become darker with age; but with age butterfly pigments generally become paler rather than darker. Besides, among the eight there is one that is fairly bright, though by no means equal to the brightest of to-day, whereas among specimens recently caught a small proportion resemble the seven that are not so bright. The evidence that the colour has changed slightly in a hundred years is, I think, unquestionable.
The naturalist does not require such evidence of evolution as that described above. If future enquiries should prove that it and similar evidence are based on insufficient material and are hence illusory his faith in evolution would remain unshaken. Why is this? Because during the whole of his life’s work he is always meeting with facts which, on the theory of evolution, receive a clear and fascinating interpretation, but which without it are meaningless. Accept evolution and they fit into their place in the scheme of things; reject it and they are isolated and devoid of interest. An instance or two will make this clear.
The female of the common vapourer moth has lost the power of flight. The brown male, which has a white spot on each forewing, flies actively by day and may be seen dashing through the streets and squares of London seeking the female where she sits quietly on the outside of the cocoon from which she emerged. Later on she will lay her eggs on this cocoon and die without leaving it. The little caterpillars that are hatched out eat many kinds of plants and are in no danger of starvation, for a short journey will bring them to food. Therefore the female does not require wings in order to seek the plant that provides food for her offspring and lay eggs on it, and she does not have to seek her mate. All the seeking is done by him. He will even enter a house and creep under a door to enter a room where a freshly emerged female is being kept in a box. Now the evolutionist knows that this nearly wingless female is descended from ancestors that possessed wings like other moths and that her rudimentary wings have become what they are by gradual degeneration. Why are the rudiments there? No theory except evolution can give a reasonable answer. To believe that by an arbitrary act of creation one moth was given useful and another useless wings is a childish creed—an unworthy conception of a Creator. And such an assumption becomes even grotesque when the facts are examined a little more closely. The wings of ordinary moths are developed within the much smaller wing envelopes of the chrysalis and expand to their full size only after emergence. Before this they can fit into the narrow space only by a complicated system of pleating. But the rudimentary wing of the female vapourer lies within a chrysalis wing far larger than itself._p183_rudimentary_wings_of_vapourer_moth.png)
To the evolutionist these facts mean that the useless wings, being probably a source of danger to the moth as well as a waste of material, have been gradually reduced by natural selection until they became first no longer and finally much smaller than the chrysalis wing cases. These chrysalis wing cases are also themselves reduced but, concealed in the cocoon and less subject to selection, their shrinkage is not nearly so great.
The most wonderful instance of butterfly mimicry that we now know is the “swallowtail”—Papilio dardanus—of Africa and the neighboring islands. (See Fig. 2.) The male is a pale-yellow black-marked butterfly, having, like most swallowtails, long tails to the hind wing. A male from Madagascar and one from Uganda are shown in the accompanying illustration. In Madagascar and the Comoro Islands the pattern of the females is very like that of the males, and they also have tails. On the mainland of Africa, however, male-like females are known only in Abyssinia. In other parts of the continent the males are pale-yellow black-marked butterflies with long tails, but the females are entirely different, resembling quite different tailless butterflies that have an unpleasant taste and that bear conspicuous “warning” patterns. The commonest of these mimicking females is a black-and-white tailless form (4), and the butterfly which it resembles and is therefore called its “model” is represented in 5.
Now the evolutionist felt confident that these tailless mimicking females were derived from females that had tails like the males, and his confidence has received a three-fold verification.
The first verification was obtained about twelve years ago, when Dr. W. A. Lamborn discovered that the female chrysalises have pockets for the tails, although no tails are developed within them.
The second verification is found in the fact that underfeeding the caterpillar or subjecting the chrysalis to cold may result in the production of rudimentary wing-tails.
The last and most convincing verification is provided by the Abyssinian race of the swallowtail, in which the females are generally male-like, but some comparatively rare females have gained the mimetic pattern yet have not lost their tails. An example is shown in 6. The right tail is well developed, although the left one has been torn off, perhaps as a result of attack by some enemy. The specimen figured is one of five—two in the Prague Museum, one in Lord Rothschild’s Museum at Tring, and two at Oxford, the second having unfortunately lost both its tails. The figured specimen was
_p184_face_swallow-tailed_butterfly_metamorphosis.png)
Metamorphosis of swallow-tailed butterfly: a, larva; b, chrysalis; c, imago, or perfect insect.
“The change from the caterpillar to the chrysalis and from this to the butterfly is in reality less rapid than might at first sight be supposed. The internal organs all metamorphose very gradually, and even the sudden and striking change in external form (from the chrysalis to the perfect insect) is very deceptive, consisting merely of a throwing off of the outer skin—the drawing aside, as it were, of a curtain—and the revelation of a form which, far from being new, has been in preparation for days, or even for months.”—Sir John Lubbock.
“The winged butterfly has come such a long distance from its wormlike ancestor that we ordinarily would never connect the two. But if we wish to visualize the far ancestors of the butterflies we have but to look at their caterpillars. What an interesting revelation of evolution at work!”—Vernon Kellogg.
Why, except as answered by evolution, does a butterfly pass through the stages of a crawling grub and a quiescent chrysalis to the full-fledged “imago,” with wings?
Editor.
_p185_face_butterfly_wing_evolution.jpg)
Fig. 2.—Record of evolution on the wings of butterflies.
1, a male, and 2, a female, of the Madagascar race of Papilio dardanus, 3, a male, and 4, the commonest female form of the Uganda and West Coast race of the same species of butterfly. The model resembled by the female and inhabiting the same area is very like 5, but differs from it in having a rather smaller white patch on the hind wing. The other female forms of Papilio dardanus in Uganda and elsewhere mimic other unpalatable models. 5, the model, Amauris niavius, and 6, the mimicking female of Papilio dardanus, from southwestern Abyssinia. Both were taken, together with three more of the model, by Mr. Arnold Hodson on November 15, 1925. Only four mimetic females, like 6, have been taken in Abyssinia, the ordinary form of female being male-like and much resembling 2. The left tail of the female shown in 6 has been torn off.
The acquisition of the mimetic pattern in the Abyssinian race is so recent that the females have not lost their tails, as they have in races in other parts of Africa (compare 6 and 4).
Photograph by Alfred Robinson. The figures are much below natural size.
captured by Mr. Arnold Hodson in southwestern Abyssinia on November 15, 1925. Four of the models, of which one is represented in 5, were taken in the same locality on the same day.
It should be added that other nauseous butterflies are mimicked by other female forms of the same swallowtail, as well as by very different butterflies and by day-flying moths. Four out of the five Abyssinian mimetic females resemble 6, but the fifth, at Prague, exhibits the very different colouring of another model.
Examples could be multiplied indefinitely, but I believe that those here described afford sufficient evidence that predictions based on evolution are verifiable and have been verified, and that natural history becomes in the light of evolution a living and inspiring study.
Although there are widely different opinions about the causes of evolution, it is probable that no living student of nature has any doubt about the truth of Evolution.
BIBLIOGRAPHY
- Poulton, E. B. Colours of Animals. International Science Series. London, 1890.
- Poulton, E. B. Essays on Evolution. Oxford, 1908.
- Pycraft, W. P. Camouflage in Nature (London, 1926). (A good general account of the whole subject.)
- Thayer, A. H. Concealing-coloration in the Animal Kingdom. Macmillan Co., 1918, New York. A beautifully illustrated book on this aspect of the subject, containing a detailed account of Thayer’s great discovery of the meaning of the underside coloration of animals.
- Wallace, A. R. Darwinism. London, 1889.