It is against this backdrop that Darwin’s orchid book, On The Various Contrivances By Which British and Foreign Orchids Are Fertilised By Insects, and On The Good Effects of Intercrossing, came out in May 1862. Beyond meticulously documented studies of the pollination of orchid after fantastic orchid, the book was an argument against design. He declared his intention in the introduction: “This treatise affords me also an opportunity of attempting to show that the study of organic beings may be as interesting to an observer who is fully convinced that the structure of each is due to secondary laws, as to one who views every trifling detail of structure as the result of the direct interposition of the Creator.”31 By “secondary laws” Darwin meant laws of nature.
Darwin sent complimentary copies of the book to Gray and numerous others, from fellow naturalists to the nurserymen and amateur botanists who assisted him in various ways. His expectations were modest, but warm and enthusiastic accolades soon poured in. Most commended Darwin for his methodical and ingenious techniques in documenting the oddities of orchid pollination, and some grasped the broader evolutionary lessons taught by these plants. Wallace, for example, lavished praise on the book, declaring himself eager to get out and observe orchids himself in the new evolutionary light Darwin put them in, and, ever perceptive, further commented that he was “quite as much staggered by the wonderful adaptations [Darwin showed] to exist in them as by the Eye in animals or any other complicated organs”32 (emphases Wallace’s).
That was exactly it—those Rube Goldberg orchids were like botanical versions of the eye, complicated but clearly jury-rigged machines, parts modified this way and that to yield beautiful, intricate, often bizarre pollen delivery systems, all traceable along clear pathways of gradual modification. Is this the way an intelligent designer would operate? Unlikely, he thought. Why create a thousand mix-and-match contrivances when a single perfect one would suffice? Darwin was curious to see what his most theistically minded friend, Gray, thought of his book. In one letter Gray commented that he had read George Bentham’s recent presidential address at the Linnean, in which Bentham’s initial opposition to the Darwin-Wallace theory seemed to be weakening as a result of Darwin’s orchid work. Gray was amused, he said, “to see how your beautiful flank-movement with the Orchid-book has nearly overcome [Bentham’s] opposition to the Origin.”33 But he also said that if Darwin grants that an intelligent designer has a hand anywhere in nature, he could be sure it has much to do with his wonderful orchids. Darwin was delighted with Gray’s perceptiveness: “Of all the carpenters for knocking the right nail on the head, you are the very best: no one else has perceived that my chief interest in my orchid book, has been that it was a ‘flank movement’ on the enemy.” But as for an intelligent designer having a hand in orchid contrivances, he went on to ask Gray what he thought “about what I say in last Ch[apter] of Orchid Book on the meaning & cause of the endless diversity of means for same general purpose—It bears on design—that endless question.”34 (“Endless” indeed. It stubbornly persists today, more than 150 years after both Origin and orchid book appeared.) Gray wasn’t taking the bait: “it opens up a knotty sort of question about accident or design (his emphasis), which one does not care to meddle with much until one can feel his way further than I can.”35 The trans-Atlantic friends agreed to disagree.
Pattern, Process, and Prediction
The final chapter of Darwin’s orchid book can be read as a manifesto for the manifestly clear (to him) lessons of orchids: remarkably intricate adaptive mechanisms of pollen transfer, with flower structures that indicate evolutionary modification of the same parts in different ways in different groups. He opened with recounting homologies (a concept introduced by the antievolutionist Richard Owen but which took on new, evolutionary, significance thanks to Darwin and Wallace), then discussed gradation of orchid structures, genealogical affinities, and the importance of rudimentary traits among other topics. Amplifying key points of the Origin, he pointed out how “old” (pre-existing) structures can be repurposed by natural selection for new functions. An example would be making a machine for some special purpose, but reusing old springs, pulleys, wheels, etc. originally devised for a different purpose to do so.
In this chapter Darwin came full circle twice over, underscoring the real motivation of his orchid hobbyhorse but also concluding with the key topic that led him to examine orchids to begin with: the vital importance of cross-fertilization. Those initial investigations gave him an appreciation for how these plants promote crossing to a high art: few flowers come close to the breathtaking intricacy and diversity of structure seen in orchids, with their “almost endless diversity of beautiful adaptations,” and he marveled at “the endless diversity of structure—the prodigality of resources—for gaining the very same end, namely, the fertilisation of one flower by the pollen of another.”
But beyond documenting their elaborate pollination mechanisms Darwin was extending the new field of “evolutionary botany,” the field he inaugurated when he began to probe the secret lives of plants (Chapter 6). With orchids Darwin’s evolutionary botany was less experimental than observational, as the study of form and function often is by necessity, but no less significant for that. Seen through the lens of natural theology, exquisite adaptations are just that, while seen through Darwin’s evolutionary lens they took on a new significance: exquisite, yes, but their jury-rigged mix-and-match qualities and their anomalies such as rudimentary structures speak of history.
There is a predictive aspect to the study of evolutionary form and function too, like in any area of science where an understanding of pattern and process yields new insights. The nineteenth-century Russian chemist Dmitri Mendeleev discerned a pattern in the properties of known chemical elements and, formulating the first periodic table, used this pattern to predict the existence of elements yet to be discovered. Similarly, a generation earlier the Frenchman Urbain LeVerrier (and independently Englishman John Couch Adams) used knowledge of orbital mechanics to correctly predict the existence of a yet-unknown planet based on the orbital perturbations of Uranus. The planet subsequently named Neptune was triumphantly discovered on September 23, 1846, within a single degree of where LeVerrier predicted it would be. Darwin’s orchid work, too, led to a celebrated case of predictive insight: the mystery of the pollination of a remarkable orchid from Madagascar. Wallace in fact used the Neptune example in praise of Darwin and the case of a beautiful orchid with an appendage of puzzling proportions.
The comet orchids of the genus Angraecum, prized for their large, brilliant white star-like flowers and prominent nectary spurs, number about 220 species in Africa and the Indian Ocean islands. A great many are found on Madagascar, where, like their (very) distant relatives the lemurs, they have diversified spectacularly. One in particular took naturalists’ breath away: Angraecum sesquipedale, variously known as the star-of-Bethlehem orchid, Christmas orchid, or, now, Darwin’s orchid. The botanical name of this orchid, which grows high overhead on trees, is clear to those conversant in Latin: sesquipedale means “foot and a half”—referring to the stunning length of the nectar spur that dangles down from the shining white flower like a power cord. Many orchids and other flowers house their sweet nectar in a pouch or spur well positioned for pollen transfer when insects come calling. Typically the nectar pools deep within the nectary, so the pollinator must get up close and personal to get the treat. Darwin received specimens of this orchid in full bloom in January 1862. He excitedly wrote to Hooker: “I have just received such a box full [of orchids] from Mr Bateman with the astounding Angraecum sesquipedalia [sic] with a nectary a foot long—Good Heavens what insect can suck it.”36 Although short of the 18 inches promised by the species’ name, a foot in length is impressive nonetheless.
Darwin knew that so exaggerated a trait could hardly be functionless, and of course he also knew that long spurs must mean a pollinator with a long tongue. But there are spurs and there are SPURS . . . what insect or other animal could possibly hav
e a tongue long enough to get at the nectar of this plant? Again, let form and function be the guide. “What a proboscis the moth that sucks it must have!” he enthused to Hooker. He knew that in all likelihood it was not only a moth, but a sphinx or hawk moth, family Sphingidae. These moths are known for their large, robust bodies, long tongues, and pollination services. Darwin held that some as-yet-unknown sphinx moth must possess a nearly foot-long tongue. Such a pollinator would be a spectacular example of coevolution in Darwin’s view, with natural selection favoring, over the eons, successive generations of orchid flowers with ever-slightly-longer nectar spurs and their moth partners’ ever-slightly-longer tongues to match.
Five years later George Campbell, the eighth Duke of Argyll, long hostile to any notion of evolution, criticized Darwin’s explanation for A. sesquipedale’s long nectar spur in his book The Reign of Law. Darwin had maintained that through natural selection “we can thus partially understand how the astonishing length of the nectary may have been acquired by successive modifications.” The Duke seized on the word “partially,” sneering that “it is indeed but a partial understanding.” On the contrary, “Purpose and intention . . . are what meet us at every turn . . . We know, too, that these purposes and ideas are not our own, but the ideas and purposes of Another—of One whose manifestations are indeed superhuman and supermaterial.”37 Darwin’s use of orchids as an argument against design was not lost on Campbell any more than on Gray.
Wallace rallied to the defense. In a long critical review of the Duke’s book he marshaled a spirited counter-argument asserting that “the laws of multiplication, variation, and survival of the fittest . . . would under certain conditions necessarily lead to the production of this extraordinary nectary”38 (emphasis Wallace’s). To Wallace the coevolutionary scenario Darwin posited was clear as day, the case of A. sesquipedale’s pollinator so compelling that he commissioned an illustration of a hypothetical sphinx moth visiting comet orchids for the piece. In the illustration the moth hovers to one side, proboscis extended toward the flower like a long slender straw—aptly, as the probosces of moths and butterflies are indeed hollow drinking-straw-like tubes.
Wallace even narrowed down the best candidates for the pollinator. In the insect collection at the British Museum he scrutinized the sphinx moths with measuring stick in hand, searching for champion probosces. He found a Macrosila species from South America with a 9¼-inch proboscis, and another species in this same genus from tropical Africa measuring 7½ inches—not far from A. sesquipedale proportions. He wrote in his review, “That such a moth [with a long enough proboscis] exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune—and they will be equally successful!”39 He was more correct than he realized: at last, in 1903, a population of this very moth species was found in Madagascar boasting those crucial extra inches. Wallace knew it as Macrosila morganii, now changed to Xanthopan morganii. It was a new subspecies of X. morganii soon dubbed “praedicta” by the eccentric English naturalist and financier Walter Rothschild and his colleague Karl Jordan in their monumental treatment of the Sphingidae published in 1903. It is commonly assumed that Rothschild and Jordan were honoring Darwin’s prediction with this name, but in fact they were honoring Wallace’s more precise prediction based on his moth proboscis measurements. So, although this moth is often referred to as “Darwin’s sphinx moth” it should be called “Wallace’s sphinx moth”—a name more resonant as well as accurate: it is fitting that “Darwin’s comet orchid” should be pollinated by Wallace’s sphinx moth.
It’s remarkable to think that Darwin’s book on orchids was published just 2 years to the month after coming across an orchid on the sandwalk and deciding to look more closely at it. And his book was far from the last of his work on those endlessly fascinating flowers. Even as the book was published he continued to make observations and gather data, often aided by sons William, Frank, and George. The boys mainly helped record the kinds of insects that visited different orchids. He continued to encourage William’s botanical interests, and complimented George, then 16, for his “splendid work in watching orchids.” He also made corrections to his book: Gray pointed out that he was mistaken about lady’s slipper orchid pollination, at least for North American species. The pouched labellum (“slipper”) of these celebrated orchids has a slit-like opening along the top. Gray suggested that insects enter through the slit but cannot exit that way and are forced to escape through small holes at the rear of the structure, where they pick up and deposit pollen. Darwin confined some flies in the pouch of some lady’s slipper orchids, to no avail: “they were either too large or too stupid, and did not crawl out properly.”40 He then tried small solitary bees and, lo and behold, out they came through the small orifices, covered in pollen. He repeated the trick again and again with a number of hapless bees; this little exercise showed the value of letting the insects do the probing rather than the experimenter. These observations and more were duly reported, some in foreign-language translations of the orchid book, others in a follow-up article on orchid fertilization in the Journal of Horticulture, and most comprehensively in the next English edition of the book in 1877. Significantly, orchids took center stage in later editions of the Origin, ammunition for an argument on evidence for gradual modifications of structure in a new chapter inserted to address “miscellaneous objections to the theory of natural selection.”
As usual he forged ahead on the orchid front even as he was advancing a bevy of other botanical studies. Darwin’s “orchidelirium” may have been a hobbyhorse, but he certainly rode that horse for all it was worth. As he realized the deeper significance of their astounding pollination contrivances, orchids became object lessons in evolution by natural selection, illustrating diversification, stepwise gradualism, trait reduction or loss, co-optation, and more. All bearing on design, that endless question.
Experimentising: Orchidelirium
Prized for their extraordinary beauty, diversity, and intricacy of structure, orchids also present us with extraordinary lessons in the evolution of form and pollination contrivances, as Darwin saw. Following Darwin’s lead you can investigate these Rube Goldberg flowers yourself. Let’s take a look at representative orchids with an eye to the “beautiful adaptations” that so captivated Darwin.
A. Materials
• Forceps or tweezers
• Toothpicks
• Hand lens or dissecting microscope, if available
• Orchids
It is best to work with commercially available orchids that are propagated sustainably. Many temperate and tropical orchids are available commercially, and some, like Phalaenopsis and Dendrobium, are even common supermarket or garden center plants. Any of the following genera will do, all common in the orchid horticultural trade although availability can vary: Cattleya (tribe Epidendreae), Dendrobium (tribe Podochileae), Cymbidium (tribe Cymbidieae), or Phalaenopsis (tribe Vandeae). Others may require going to a specialist (e.g., Clown Alley Orchids: clownalleyorchids.com; Eldon Tropicals: eldontropicals.com; Flora Exotica: floraexotica.ca; Logee’s Greenhouses: logees.com). Dave’s Garden (davesgarden.com) lists many other sources.
B. Procedure
1. Orchid flower structures generally come in groups of three, the three petals and three sepals being the most obvious, arranged like two superimposed triangles inverted with respect to one another. Look closely at a Cymbidium or Cattleya flower. You can easily see the triangular petal and sepal groupings: the outermost three are sepals, at roughly 12, 4, and 8 o’clock on the flower dial, and the innermost three are petals at roughly 2, 6, and 10 o’clock. Notice that the lowermost petal looks decidedly different from the other two—sufficiently so to be given another name: the labellum, or lip. Does the lower lip seem inviting? The flower is indeed inviting company, of the insectan kind—the lip is a landing strip.
Cattleya orchid, showing anther cap (upper right)
and extracted pollinia stuck to a probe (lower right). Drawing by Leslie C. Costa.
2. Just above the lip is the column. Representing the fusion of ancestrally separate stamens and pistil(s), the column contains the sexual organs of the orchid. At the top of the column look closely to see the anther cap, which acts as a protective shield concealing the pollinia.
3. You can expose the anther cap at the end (top) of the column by removing or pushing petals and sepals back and out of the way. Remove the anther cap with a toothpick or forceps, or if you have a steady hand attempt to release the small hook-like structure at the tip of the column. This is a spring-latch of sorts that keeps the anther cap closed. The anther cap comes off very easily, exposing the pollinia beneath: look for the twin sacs of pollen housed at the tip of the column and covered by the anther cap.
Darwin's Backyard Page 28