The mistletoe paradox: Natural selection and the fatal embrace of parasitism

“ But the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. How have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird ”

“ In the case of the mistletoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself. ”

“ In the introductory parts Darwin explained that “the facts which kept me longest scientifically orthodox are those of adaptation—the pollen-masses in asclepias—the mistletoe, with its pollen carried by insects, and seed by birds—the woodpecker, with its feet and tail, beak and tongue, to climb the tree and secure insects. To talk of climate or Lamarckian habit producing such adaptations to other organic beings is futile. ”

“ What the theory does show is, that adaptation to environment is necessary for life, and that organisms unable to adapt themselves pass away. Adaptation to environment will thus be implied in, or be an essential means towards, self-preservation and race-preservation, self-development and race-development. And should this preservation or development be looked upon as the end of conduct, the adaptation to environment it implies may help to define and characterise the end. ”

“ A plant which annually produces a thousand seeds, of which only one on an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites grow on the same tree, it languishes and dies. ”

“ A plant which annually produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. ”

“ Some grow strong and lusty in the clinging clasp of the mistletoe, and are only saved from being smothered in its tender embraces by the pruning-knife, which cuts down and strews the ground with all such pleasant parasites as would otherwise sap the strength and destroy the life of the strong young oaks. ”

“ Their self-assertion is limited by the organic process, or what would at one time have been called the law of design, the equilibrium and comity of tissues being secured by a self-restraint that is inherent in them, that was inherent in the vital impulse that called them into being, a restraint on the nutrition and reproduction of each to secure the nutrition and reproduction of all, a restraint that when from any cause it is broken down leads to disease, as in the overgrowth of cancer. ”

“ There are many sites around the world that chronicle this war -- memorials that tell stories of courage and heroism; graves and empty camps that echo of unspeakable depravity. Yet in the image of a mushroom cloud that rose into these skies, we are most starkly reminded of humanity’s core contradiction; how the very spark that marks us as a species -- our thoughts, our imagination, our language, our tool-making, our ability to set ourselves apart from nature and bend it to our will -- those very things also give us the capacity for unmatched destruction.
How often does material advancement or social innovation blind us to this truth. ”

“ THUS the view that Balder’s life was in the mistletoe is entirely in harmony with primitive modes of thought. It may indeed sound like a contradiction that, if his life was in the mistletoe, he should nevertheless have been killed by a blow from the plant. But when a person’s life is conceived as embodied in a particular object, with the existence of which his own existence is inseparably bound up, and the destruction of which involves his own, the object in question may be regarded and spoken of indifferently as his life or his death, as happens in the fairy tales. ”

“ Haws, rose-hips, and holly-berries will wither and wilt on the tree in mild winters, because they can't drop off of themselves without the aid of birds, while the birds are too well supplied with other food to care for them. One of the strangest cases of all, however, is that of the mistletoe, which, living parasitically upon the forest-boughs and apple-trees, would of course be utterly lost if its berries dropped their seeds on to the ground beneath it. ”

“ Persons in Sweden afflicted with epilepsy carry with them a knife having a handle of oak mistletoe, which plant they call Thunder-besom, connecting it with lightning and fire. The thrush is the great disseminator of the parasite. He devours the berries eagerly, and soils, or "missels" his feet with their viscid seeds, conveying them thus from tree to tree, and getting thence the name of missel thrush. ”

“ But by far the most interesting point about the missel-thrush is that curious connection between the bird and the mistletoe which was observed so long since even by our prehistoric ancestors as to have given the species its vernacular name in all European languages. Turdus viscivorus—the mistletoe-eating thrush—is Linnæus’s scientific Latin title for the creature, and he well deserves it. He is almost or altogether the only bird that will eat the mistletoe berries, and on him accordingly the mistletoe depends for the dispersal of its seeds and the propagation of its mystic parasitic seedlings. ”

“ In the winter, when the flow of food is very slow in the host, it is likely that the mistletoe does some of its own food building in its yellow-green leaves, which would be exposed to the full light, as the host’s leaves would have fallen away. The mistletoe has soft, white fruits which are scattered by birds, and as they are very sticky, they hold for some time on to the branch where they are dropped, and there the seedling sprouts and fastens itself on to the tissues of the host, growing every year with its growth. ”

“ As the mistletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience's sake the general term of Struggle for Existence. ”

“ Some plants are absolutely dependent on birds for their diffusion, and a kind of mutualism exists between the two—one of those remarkable phenomena in which very different kinds of beings are reciprocally advantageous and adapted to one another. A carious example of such adaptation is seen in the case of the mistletoe and the mistle-thrush. ”

“ In answer to your correspondent Ache, I may add, that the opinion of recent botanists is contrary to Sir Thomas Browne's notion with reference to the propagation of the seed; for it is known that the seeds, in germinating, send their radicles into the plant to which they are attached; and grow afterwards as true parasites, selecting certain chemical ingredients in preference to others. The mistletoe has never been known to grow in Ireland; but its frequency in various parts of the world—in France, Italy, Greece, and parts of Asia—has been remarked by travellers. ”

“ Parasitic Plants, of which there are various kinds, strike their roots, or what answer to roots, into the tissue of foster plants, or form attachments with their surface, so as to prey upon their juices. Of this sort is the Mistletoe, the seed of which germinates on the bough where it falls or is left by birds; and the forming root penetrates the bark and engrafts itself into the wood, to which it becomes united as firmly as a natural branch to its parent stem; and indeed the parasite lives just as if it were a branch of the tree it grows and feeds on. ”

“ The facts of variability, of the struggle for existence, of adaptation to conditions, were notorious enough; but none of us had suspected that the road to the heart of the species problem lay through them, until Darwin and Wallace dispelled the darkness, and the beacon-fire of the 'Origin' guided the benighted.
Whether the particular shape which the doctrine of evolution, as applied to the organic world, took in Darwin's hands, would prove to be final or not, was, to me, a matter of indifference. ”

“ Darwin's great achievement to have explained. The struggle for existence, the survival of the fittest, the adaptation to new environments by the use and disuse of parts, the changes induced by change of climate and food, or by the action of new organisms in the environment, all these considerations open out to the astonished and admiring [Pg 22] gaze of man vast and interesting histories of changes such as a discerning mind like Mr. Grant Allen revels in in his rambles through the English fields. ”

“ Year after year, the wild animals with which man never interferes are, on the average, neither more nor less numerous than they were; and yet we know that the annual produce of every pair is from one to perhaps a million young,—so that it is mathematically certain that, on the average, as many are killed by natural causes as are born every year, and those only escape which happen to be a little better fitted to resist destruction than those which die. The individuals of a species are like the crew of a foundered ship, and none but good swimmers have a chance of reaching the land. ”

“ Darwin that he professes to have discovered the existence and the modus operandi of this natural selection, as he terms it; and, if he be right, the process is perfectly simple and comprehensible, and irresistibly deducible from very familiar but well nigh forgotten facts.
Who, for instance, has duly reflected upon all the consequences of the marvellous struggle for existence which is daily and hourly going on among living beings? Not only does every animal live at the expense of some other animal or plant, but the very plants are at war. ”

“ Of course, the carnivore plays a great part in the selective process; but he is the selector only; the real competition is between the selected. Now, let us take the case of the plant. A thousand seedlings occupy the space where few alone can ultimately grow; and between these seedlings the struggle is fierce, the strongest and best adapted ultimately surviving. To take Darwin's own example, the mistletoe, which is a parasite, cannot truly be said to struggle with the apple tree on which it fastens ”

“ Scarcely thirty years have elapsed [71] since Darwin first propounded the principles of his theory of evolution. [Pg 217] Yet, already we see his ideas firmly rooted in every branch of human thought, however remote. Everywhere, in history, in philosophy, even in the physical sciences, we hear the watchwords: heredity, adaptation, selection. We speak of the struggle for existence among the heavenly bodies and of the struggle for existence in the world of molecules. [72]
The impetus given by Galileo to scientific thought was marked in every direction ”

“ In fact, for a voluntarist like Schopenhauer, a theory so sanely and cautiously empirical and rational as that of Darwin left out of account the inward force, the essential motive, of evolution. For what is, in effect, the hidden force, the ultimate agent, which impels organisms to perpetuate themselves and to fight for their persistence and propagation? Selection, adaptation, heredity, these are only external conditions. ”

“ Now it is undeniable that to a very large number of the more thoughtful the old form of the argument from design received its death blow from the Darwinian doctrine of natural selection. In the light of this theory there was no greater need to argue that intelligence was necessary to produce animal adaptations than there was to assume intelligence for the sifting of sand by the wind. As the lighter grains are carried farthest because they are lightest, so natural selection, operating upon organic variations, favoured the better adapted specimens by killing off the less favoured ones. ”

“ The idea entertained by Darwin, that the development of species may be explained by a natural choice—Selection—which operates through the struggle of individuals for existence, cannot permanently satisfy the spirit of inquiry. Even the factors of variability, heredity, and adaptation, which are essential to the transformation of species, do not offer an exact explanation. ”

“ Some of you will be shocked at this account of what we are in the habit of describing in very different language, for the same emotional reason that we all admire the tiger and the tiger-beetle and loathe the tape-worm and the louse, namely, because our instinctive horror of the parasites to which our species is so constantly exposed, prevents us even as twentieth-century zoologists from appreciating the extent to which all life, ourselves included, is saturated with parasitic proclivities. ”

“ The real fact is, as you may learn for yourself if you will look at nature instead of merely reading about it at second or third hand, that mistletoe on an oak-tree is extremely rare; the Druids prized it because they thought it the life or soul of the oak, which was the sacred tree of Celtic mythology. I notice, indeed, that missel-thrushes very seldom perch on oaks; and even when they do by chance dislodge a stray seed of mistletoe on one, it has difficulty in fixing its young suckers on the alien bark, and draining the tree’s nutritious juices. ”

“ So long as the mistletoe, in which the flame of the lightning smouldered, was suffered to remain among the boughs, so long no harm could befall the good and kindly god of the oak, who kept his life stowed away for safety between earth and heaven in the mysterious parasite; but when once that seat of his life, or of his death, was torn from the branch and hurled at the trunk, the tree fell—the god died—smitten by a thunderbolt.760
The King of the Wood and the Golden Bough. ”