Darwin's Backyard

by James T. Costa

  At the same time there were nagging difficulties with his favored theory—notably an absence of any marine fossils in the area whatsoever, and the absence of “roads” throughout the greater region even though topography showed that any ancient marine incursion could not have been confined to Glen Roy alone. But he argued that lack of such evidence was not the same as negative evidence. Riding high with his election to the Royal Society in January 1839 (following in the footsteps of his father and grandfather before him), Darwin decided that his inaugural paper before the premier London scientific society would be on Glen Roy, reading the first part of a hefty two-part paper barely 2 weeks after his election.

  He was very pleased with himself, but the feeling didn’t last long as it became increasingly clear that Darwin had misinterpreted Glen Roy. Just a year later, in 1840, the famous Swiss naturalist Louis Agassiz took the scientific societies of London and Glasgow by storm with papers propounding his glacial theory, a radically new way to see the landscape. Agassiz, building on the work of fellow Swiss Jean de Charpentier, first developed the theory positing a period of worldwide glaciation in 1837 based on the glaciers and associated landforms in the Swiss Alps. It was met with much skepticism at first, but steadily gained traction as geologists learned to read the evidence for the action of massive, slowly flowing rivers of ice. Seeing similar formations associated with present-day glaciers in the Alps helped them connect the dots for areas that are now ice-free. In 1840 Agassiz toured Britain in the company of Oxford geologist William Buckland (Lyell’s professor) and other notable geologists, showing them how many of the landforms and other phenomena they thought they knew could now be understood as products of glacial action. How astonishing that must have been, to come to terms with the idea of towering glaciers covering vast areas of Britain and Europe. Darwin’s marine theory held on for a few years, but the glacial theory, spurring new geological fieldwork, brought together under one explanatory umbrella a host of observations: rounded cobbles and other signs of alluvial deposition, scored bedrock, kettle lakes, erratic boulders, moraines, and more. It became clear that there was indeed a barrier damming the great Glen, and that barrier was ice.

  Darwin always considered his Glen Roy paper his “greatest blunder,” one he was thoroughly ashamed of. It makes for a case study in scientific self-deception; his successes interpreting the South American landscape had manifestly gone to his head, leading to a selective reading of evidence for and against his marine incursion idea. He soon became a convert to the glacial theory. Returning to Snowdonia’s Cwm Idwal in 1842, he realized how he (and everyone else before Agassiz) had missed the real story of that valley too: “The ruins of a house burnt by fire do not tell their tale more plainly, than do the mountains of Scotland and Wales, with their scored flanks, polished surfaces, and perched boulders, of the icy streams with which their valleys were lately filled,”48 he wrote years later. Our interest in Darwin’s blunder in Glen Roy lies not so much in the fact that he was wrong, but in how it made him more cautious in subsequent scientific studies—at least publicly. Not only must evidence be carefully and (as best we can) objectively weighed, but Darwin came to adopt something of a “shock and awe” approach to marshaling overwhelming evidence to bear on his arguments. His later so-called delay in publishing his species theory was not so much due to any simple fear of going public, but rather can be understood more as a determination to accumulate and interweave so much evidence that his case would be as unsinkable as the indomitable Beagle. And then, too, Darwin performed more and more experiments in pursuit of the evidence he needed.

  Experimentiser on the Downs

  While Darwin the geologist had his ups and downs, Darwin the closet transmutationist kicked into high evidence-gathering gear. As he commented to a correspondent in 1861, “all observation should be for or against some theory if it is to be of use.”49 Indeed, that became the working principle he applied rigorously to his “species work,” as he called it. And the theory he had in mind, well developed by the time of his ill-fated Glen Roy paper, was the idea of descent with modification driven by a process he dubbed “natural selection.” He revealed his research agenda in a breathless notebook entry in 1838: once you grant that species may change, “the whole fabric totters & falls.” And then: “Look abroad, study gradation study unity of type study geographical distribution study relation of fossil with recent . . . the fabric falls!”50 In good Whewellian consilience mode Darwin realized that evidence for this heretical idea was to be found in disparate areas, and he was determined to pursue each line, ferreting out as much information as he could for or against his theory—a theory that shone brightly to him, guiding his investigations like a navigational star guides ships.

  This brought out the experimenter in Darwin. Experimentation entails questions, and once Darwin became a transmutationist with a theory of change all sorts of new questions arose. He was suddenly looking at organisms and their structure, distribution, classification, behavior, etc. from a different angle, and his notebook reminder to “look abroad” was a call to action. We have seen how by this time Darwin had conducted a number of quick experiments, but now he became a dedicated “experimentiser” by necessity, owing to lack of data. Here too we see the beginnings of Darwin the canvasser and interlocutor pressing friends, family, and acquaintances for assistance and information—the original crowd-sourcer, to use a modern term. But there was only so much he could gain from others. Looking at the world in a wholly new way calls for asking questions about the world that no one had thought to ask before—questions that would need to be answered by careful observation in some cases, and experimentation in others.

  From very early in his transmutation notebooks we see repeated references to the need for experiments: “Many interesting experiments might be tried by comparing [zoophytes] to plants,” he noted in the A notebook; and “It would be curious experiment” to soak seeds in salt water to see if variation could be induced, and to “experimentise on land shells in salt water” he jotted in the B notebook.51 These and other notebooks are peppered with ideas for experiments, and he eventually kept one for that purpose: the “Questions & Experiments” (Q&E) notebook was started in mid-1839. Most entries in this notebook were made by 1844, the year Darwin wrote a long (over 200 page) account of his evolutionary thinking to that point (now referred to as his species Essay), but he clearly used it in the composition of the 1842 outline of his theory (now referred to as the species Sketch). Historians have pointed out that the Q&E notebook served as a storehouse of ideas and questions about breeding, variation, crossing, and pollination, key subjects for Darwin.

  Ironically, of the many animals that Darwin mentions in his Q&E notebook—indeed, in all of his transmutation notebooks—the one that was about to consume nearly all of his attention for the next 8 years merited precisely one entry: barnacles. These ubiquitous marine organisms soon became his go-to group as he grappled with the nature of variation. But Darwin’s barnacle odyssey actually started years earlier in Patagonia, with a mystery specimen that he couldn’t quite identify at first. That is the subject of the next chapter.

  Experimentising: Going to Seed

  Being an experimentiser means being an observer and an asker of questions. Testable questions—questions framed in a way that an experiment or careful observations might give a reasonably reliable answer one way or another. Those are the incremental steps of scientific progress. But, it all really starts with observing. Darwin was an inveterate observer—he noticed color, behavior, structure, and function, and often marveled at adaptation. Indeed, his life’s work became one long study to understand how adaptations arise. As he expressed it in On the Origin of Species:

  How have all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life, and of one distinct organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and missletoe; and only a little less plainly in the hu
mblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.

  Darwin’s answer to the question of how exquisite adaptations arise was natural selection. But before evolutionary process and pattern can be studied, the adaptations themselves must be understood. That’s where observation comes in. Your Darwin-inspired adventures as an experimentiser begin where his did, with observing. We’ll take a close look at “Plumed seed . . . wafted by the gentlest breeze” and other wind-dispersed seeds—object lessons in adaptive structure and function.

  A. Materials

  • Notebook and pencil

  • Hand lens or dissecting microscope, if available

  • Tape measure or meter stick

  • Fan

  • Stopwatch

  • Masking tape

  • Marker

  • Graph paper

  • Scale, if available

  • Obtain assorted wind-dispersed seeds, for example:

  * Plumed seeds, such as milkweed (Asclepias), Indian hemp (Apocynum), dandelion (Taraxacum), goldenrod (Solidago), sycamore or plane tree (Plantanus)

  * Winged seeds, such as maple (Acer), ash (Fraxinus), tuliptree (Liriodendron), basswood or linden (Tilia), pine (Pinus), or hemlock (Tsuga)

  Note on obtaining seeds: Weedy species like dandelion are widespread and produce seeds year-round. The other plants listed will produce seeds seasonally; in the northern hemisphere their seeds will be mature by late summer or early fall. You can find these plants at local nurseries and garden centers. Seeds can also be purchased from online suppliers such as Prairie Moon Nursery (www.prairiemoon.com).

  B. Procedure

  1. Observe your seeds and their wind-dispersal mechanisms carefully with the hand lens or dissecting microscope.

  2. Sketch the seeds, noting the similarities and differences in the wind-dispersal structure found in the different plumed seeds you collected. Repeat for the winged seeds.

  3. Why do you suppose the seed dispersal structure is not identical for all of the plumed seeds or all of the winged seeds, even though in each group they are adapted to disperse their seeds in basically the same way?

  (In a word, the answer is ancestry.)

  4. Carefully compare the winged structures of pine, hemlock, and ash seeds with a hand lens or dissecting microscope. Which two are the most similar to each other? If you answered “pine and hemlock” you are correct. Consider why this is true. You might say that these two are conifers while a tuliptree is not, and that explains the observed similarity and dissimilarity. That is correct too, at one level. But how would Darwin explain this?

  (He would express it in terms of ancestry. The pine and hemlock wings are homologous structures, and they are analogous with the wings of the ash seeds—the analogous structures are convergent: similar structures arising in different evolutionary lineages for the same adaptive reasons.)

  How do the different wind-dispersal mechanisms compare, in terms of their effectiveness in carrying seeds away from the parent plant? In this experiment we’ll explore the relationship between size, structure, and dispersal efficiency.

  1. Set up the fan 1 yd (or 1 m) above the floor and blowing horizontally; experiment with the fan speed setting to determine an effective speed for the available area.

  2. Extend the tape measure in the direction the fan is blowing and use masking tape to affix it to the floor.

  3. Drop a seed specimen from a standard height just in front of and above the fan. Mark with tape where the seed hits the floor, and record the distance with the tape measure. Repeat five times with the same seed type.

  4. Repeat Step 3 with each type of seed in your collection. Calculate the average distance traveled for each. Which seed type went the farthest and which the shortest distance?

  5. Weigh each seed used in the experiment, if possible.

  6. Vary the experiment by dropping each seed type from a standard height of 2 yd (~2 m) just in front of the fan.

  7. Record the time that the seed remains in the air with the stopwatch. Repeat five times with the same seed.

  8. Repeat Step 7 with each type of seed in your collection. Calculate the average time aloft for each seed type.

  9. Graph the average distance traveled and average time aloft for each seed type.

  Is there a relationship between the distance traveled and time aloft? Does size or weight play a role in efficacy of wind dispersal? Plumed seeds tend to be light and ride passively on air currents. Most winged seeds are heavier, but some can generate lift, slowing their descent and increasing their dispersal distance.

  See also:

  “Early Days” and “Doing Darwin’s Experiments” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/early-days and www.darwinproject.ac.uk/learning/11-14/doing-darwins-experiments.

  2

  Barnacles to Barbs

  January 1835 found the 26-year-old Darwin and his Beagle comrades along the west coast of South America, amid the splendid desolation of the endless rugged and densely forested islands, looming snow-capped mountains, and deep bays and channels of the submerged Chilean Coast Range. Despite being high summer the weather was far from balmy: it was a place where “it rained as if rain was a novelty,” he commented ruefully in his diary; where the rain “never seems to grow tired of pouring down.” A week in the Chonos Archipelago, January 8th through the 14th, “passed rather heavily,” he wrote; “the climate is so very bad & the country so very uniform in its character.”1 Between downpours he managed to get out on shore and collect, though, and it was there that, poking around a windswept gravelly beach, a number of thick Chilean abalone shells of the genus Concholepas caught his eye. They were riddled with tiny holes, like shot; holes he recognized were caused by the drilling of a minute organism. Intrigued, he collected some.

  Back onboard the Beagle, snug in his cabin-cum-lab, Darwin duly recorded his find as specimen no. 2495 and, under the microscope, carefully dissected adults and several early stages of what turned out to be an odd parasitic barnacle. Yellowish in color and upside-down in their holes, they were unlike any barnacle he had ever seen. “Who would recognize a young Balanus [barnacle] in this illformed little monster,” he asked in his specimen notebook, concluding that “it is manifest this curious little animal forms new genus.”2 Darwin didn’t know it then, but he had found the smallest barnacle species in the world and was to bestow, years later, the name Cryptophialus minutus upon it. He also didn’t realize that there was far more to this barnacle than met the eye; a decade later the by-then confirmed but closeted evolutionist would take another look at these specimens and make an astounding discovery, one that he believed had profound implications for his secret evolutionary ideas. As if somehow auguring his incendiary views to come, a few days after collecting his burrowing barnacle, on the night of January 19th, the towering Volcán Osorno erupted. Plainly visible to the astonished crew of the Beagle despite a distance of over 70 miles, Darwin recorded the spectacle in his diary, “a very magnificent sight.”3

  The destructive power of the eruption and the earthquake that followed was awful, but at the same time Darwin could not help but notice the astonishing geological effects of the event: the beach was uplifted some 10 feet, stranding hapless nearshore marine life high and dry and gasping, a palpable demonstration of Lyellian processes at work. Two weeks later, along the same coast, he trekked inland to see oyster beds deep in a forest at an elevation of 350 feet: evidence of uplift past that resonated deeply with the newly raised shoreline. At this point he was not many years away from applying gradual Lyellian geological and landscape change to equally gradual change in species, and would come to appreciate a profound correspondence between his mystery barnacle and the mysteries of geological change.

>   But quite a lot was to happen before then. For one thing, nearly 2 years yet remained of his Beagle voyage; 2 more years of strikingly beautiful landscapes and endless, featureless ocean, fascinating peoples, intriguing collecting, and near-constant seasickness. “I hate every wave of the ocean, with a fervor, which you, who have only seen the green waters of the shore, can never understand” he wrote from Tasmania to his cousin William Darwin Fox a year later.4 Yet there is no question that the voyage was a turning point. Darwin would declare in his diary that “nothing can be more improving to a young naturalist, than a journey in distant countries . . . the habit of comparison leads to generalization.”5 Generalization, the opportunity to see the big picture, connect disparate dots, inductively puzzle out the laws of nature. The collections and observations made at a given time and place on this or that species, fossil, or geological formation had great value, and collectively they yielded insights greater than the sum of the parts. So it was that organisms as seemingly dissimilar as cirripedes and domestic pigeons would eventually be united under a common explanatory framework: methodical analysis of structure and development within each of these groups, with all of their attendant oddities, would yield mutually reinforcing evidence for evolution by natural selection. Much later, discoveries of Hox genes would confirm the deep structural bond of invertebrates and vertebrates—of organisms as dissimilar as barnacles and barbs, the fancy pigeons of Barbary descent—in spectacular fashion that surely would have thrilled Darwin. The stage was set for this in South America, on a remote and windswept Patagonian beach.

  In that same letter to Fox, Darwin commented how he looked forward “with a comical mixture of dread & satisfaction” to the scientific work awaiting him at home. “I suppose,” he mused, “my chief place of residence will at first be Cambridge & then London”—scientific centers of choice for organizing and working up his extensive collections and notes, their museums and scientific societies lately displaying a “rapidly growing zeal for Natural History” that would be immensely useful, he was sure. That would come to pass, but as a transitional stage between his wide-ranging travels and settling down permanently at Down House—not unlike the barnacles that would come to captivate him. As Charles Kingsley, the clergyman-naturalist-historian and prolific author, wrote of them in his 1855 book on seashore life: