Bully for Brontosaurus

by Stephen Jay Gould

  Abbott Thayer, a native of Boston, began his artistic career in the maelstrom of New York City but eventually retreated to a hermitlike existence in rural New Hampshire, where his old interests in natural history revived and deepened. As a committed Darwinian, he believed that all form and pattern must serve some crucial purpose in the unremitting struggle for existence. He also felt that, as a painter, he could interpret the colors of animals in ways and terms unknown to scientists. In 1896, Thayer published his first, landmark article in The Auk: “The Law Which Underlies Protective Coloration.”

  Of course, naturalists had recognized for centuries that many animals blend into their background and become virtually invisible—but scientists had not properly recognized how and why. They tended to think, naively (as I confess I did before my research for this essay), that protection emerged from simple matching between animal and background. But Thayer correctly identified the primary method of concealment as countershading—a device that makes creatures look flat. Animals must indeed share the right color and pattern with their background, but their ghostly disappearance records a loss of dimensionality, not just a matching of color.

  In countershading, an animal’s colors are precisely graded to counteract the effects of sunlight and shadow. Countershaded animals are darkest on top, where most sunlight falls, and lightest on the bottom (Thayer thereby identified the adaptive significance of light bellies—perhaps the most universal feature of animal coloration). The precise reversal between intensity of coloration and intensity of illumination neatly cancels out all shadow and produces a uniform color from top to bottom. As a result, the animal becomes flat, perfectly two-dimensional, and cannot be seen by observers who have, all their lives, perceived the substantiality of objects by shadow and shading. Artists have struggled for centuries to produce the illusion of depth and roundness on a flat canvas; nature has simply done the opposite—she shades in reverse in order to produce an illusion of flatness in a three-dimensional world.

  Contrasting his novel principle of countershading with older ideas about mimicry, Thayer wrote in his original statement of 1896: “Mimicry makes an animal appear to be some other thing, whereas the newly discovered law makes him cease to appear to exist at all.”

  Thayer, intoxicated with the joy of discovery, attributed his success to his chosen profession and advanced a strong argument about the dangers of specialization and the particular value of “outsiders” to any field of study. He wrote in 1903: “Nature has evolved actual art on the bodies of animals, and only an artist can read it.” And later, in his 1909 book, but now with the defensiveness and pugnacity that marked his retreat:

  The entire matter has been in the hands of the wrong custodians…. It properly belongs to the realm of pictorial art, and can be interpreted only by painters. For it deals wholly in optical illusion, and this is the very gist of a painter’s life. He is born with a sense of it; and, from his cradle to his grave, his eyes, wherever they turn, are unceasingly at work on it—and his pictures live by it. What wonder, then, if it was for him alone to discover that the very art he practices is at full—beyond the most delicate precision of human powers—on almost all animals.

  So far, so good. Thayer’s first articles and outdoor demonstrations won praise from scientists. He began with relatively modest claims, arguing that he had, elucidated the basis for a major principle of concealment but not denying that other patterns of color displayed quite different selective value. Initially, he accepted the other two traditional categories—revealing coloration and mimicry—though he always argued that concealment would gain a far bigger scope than previously admitted. In his most technical paper, published in the Transactions of the Entomological Society of London (1903), and introduced favorably by the great English Darwinian E. B. Poulton, Thayer wrote:

  Every possible form of advantageous adaptation must somewhere exist…. There must be unpalatability accompanied by warning coloration…and equally plain that there must be mimicry.

  Indeed, Thayer sought ways to combine ideas of concealment with other categories that he would later deny. He supported, for example, the ingenious speculation of C. Hart Merriam that white rump patches are normally revealing, but that their true value lies in a deer’s ability to “erase” the color at moments of danger—a deer “closes down” the patch by lowering its tail over the white blotch and then disappears, invisible, into the forest. In his 1909 book, however, Thayer explicitly repudiated this earlier interpretation and argued for pure concealment—the white patch as “sky mimicking” when seen from below.

  Thayer’s pathway from insight to ridicule followed a distressingly common route among intellectuals. Countershading for concealment, amidst a host of alternatives, was not enough. Thayer had to have it all. Little by little, plausibly at first, but grading slowly to red wings in the sunset, Thayer laid his battle plans (not an inappropriate metaphor for a father of camouflage). As article succeeded article, Thayer progressively invaded the categories of mimicry and revealing coloration to gain, or so he thought, more cases for concealment. Finally, nothing else remained: All patterns of color served to conceal. He wrote in his book: “All patterns and colors whatsoever of all animals that ever prey or are preyed upon are under certain normal circumstances obliterative.”

  Thayer made his first fateful step in his technical article of 1903. Here, he claimed a second major category of concealing coloration—what he called “ruptive” (we now call them “disruptive”) bars, stripes, splotches, and other assorted markings. Disruptive markings make an animal “disappear” by a route different from countershading. They break an animal’s coherent outline and produce an insubstantial array of curious and unrelated patches (this principle, more than countershading, became important in military camouflage). A zebra, Thayer argues, does not mimic the reeds in which it hides; rather, the stripes break the animal’s outline into bars of light and darkness—and predators see no coherent prey at all.

  Again, Thayer had proposed a good idea for some, even many, cases (though not for zebras, who rarely venture into fields of reeds). His 1903 article argues primarily that butterflies carry disruptive pictures of flowers and background scenery upon their wings: “The general aspect of each animal’s environment,” Thayer wrote, “is found painted upon his coat, in such a way as to minimize his visibility, by making the beholder think he sees through him.”

  But, amidst his good suggestions, Thayer had made his first overextended argument. Countershading could scarcely be mistaken for anything else and offered little scope for claiming too much. But the principle of “ruptive” concealment permitted enormous scope for encompassing other patterns that actually serve to reveal or mimic. Color patches and splotches—the classic domain of warning and revealing patterns (consider the peacock’s tail)—could, for an overenthusiast like Thayer, become marks of disruptive concealment. Thus, to cite just one example of overstatement, Thayer argued in a 1909 article, adversarily entitled “An Arraignment of the Theories of Mimicry and Warning Colors,” that white patches on a skunk’s head mimic the sky when seen by mice from below:

  Such…victims as can see would certainly have much more chance to escape were not what would be a dark-looming predator’s head converted, by its white sky-counterfeiting, into a deceptive imitation of mere sky.

  Still, by 1903, Thayer was not yet ready to claim concealment for all colors. He admitted one category of obvious conspicuousness: “Only unshiny, bright monochrome is intrinsically a revealing coloration.”

  Now we can finally understand why Thayer was eventually driven to his absurd argument about flamingos and the sunset. (Divorced from the context of Thayer’s own personal development, the idea sounds like simple disembodied craziness—as professors always present it for laughs in introductory classes.) Once Thayer decided to go for broke, and to claim that all color works for concealment, flamingos became his crucial test, his door-die attempt at exclusivity. As a last shackle before the final plunge, Thayer ha
d admitted that stark monochromes—animals painted throughout with one showy color—were “intrinsically revealing.” If he could now show that such monochromes also served for concealment, then his triumph would be complete. Flamingos occupied the center of his daring, not a curious diversion. He had to find a way to fade bright red into ethereal nothingness. Hence the sunset—his as well as the flamingo’s.

  So Thayer visited the West Indies, got down on his belly in the sulfurous muds, and looked at flamingos—not comfortably down from above (as he always accused lazy and uncritical zoologists of doing), but from the side as might a slithering anaconda or a hungry alligator. And he saw red wings fading into the sunset—the entire feeding flock became a pink cloud, a “sky-matching costume”:

  These birds are largely nocturnal, so that the only sky bright enough to show any color upon them is the more or less rosy and golden one that surrounds them from sunset till dark and from dawn until soon after sunrise. They commonly feed in immense, open lagoons, wading in vast phalanxes, while the entire real sky above them and its reflected duplicate below them constitute either one vast hollow sphere of gold, rose, and salmon, or at least glow, on one side or the other, with these tones. Their whole plumage is a most exquisite duplicate of these scenes…. This flamingo, having at his feeding time so nearly only sunrise colors to match, wears, as he does, a wonderful imitation of them.

  Thayer had finally gone too far and exasperated even his erstwhile supporters. His exaggerations—particularly his flamingos—now brought down a storm of criticism, including Roosevelt’s hundred-page barrage. Critics pointed out Thayer’s errors in every particular: Flamingos do not concentrate their feeding at dawn and dusk, but are active all day; anacondas and alligators do not inhabit the thin films of saline ponds that flamingos favor; flamingos eat by filtering tiny eyeless animals that cannot enjoy the visual pleasures of sunset.

  Most sadly, Thayer’s argument even failed in its own terms—and Thayer, who was overenthusiastic to a fault, but neither dishonest nor dishonorable, had to confess. Any object viewed against the fading light will appear dark, whatever its actual color. Thayer admitted this explicitly by painting a dark palm tree against the sunset in his infamous and fanciful painting of fading flamingos (reproduced here, for unfortunate practical reasons, in inappropriate black and white). Thus, he could only claim that flamingos looked like the sunset in the opposite side of the sky: red clouds of sunset in the west, red masses of flamingos in the east. Would any animal be so confused by two “sunsets,” with flamingos showing dark against the real McCoy? Thayer admitted in his 1909 book:

  White (top) and red (bottom) flamingos fade to invisibility against the sky at sunrise and sunset. From Thayer’s 1909 book. NEG. NO. 2A13239. COURTESY DEPARTMENT OF LIBRARY SERVICES, AMERICAN MUSEUM OF NATURAL HISTORY.

  Of course a flamingo seen against dawn or evening sky would look dark, like the palm in the lower left-hand figure, no matter what his colors were. The…right-hand figures, then, represent the lighted sides of flamingos at morning or evening, and show how closely these tend to reproduce the sky of this time of day; although always, of course, in the opposite quarter of the heavens [Thayer was good enough to underline his admission] from the sunset or dawn itself.

  As a final, and feeble, parting salvo, Thayer added: “but the rosy hues very commonly suffuse both sides of the sky, so that…the flamingos’ illuminated ruddy color very often has a true ’background’ of illuminated ruddy sky.”

  Teddy Roosevelt was particularly perturbed. As an old big-game hunter, he knew that most of Thayer’s “ruptive” patterns did not conceal quarries. How could Thayer have it both ways—how could a lion be concealed in the desert, a zebra amongst the reeds when, in fact, they share the same habitat, often to the zebra’s fatal disadvantage? Thus, Roosevelt decided to counterattack and wrote his scientific magnum opus during some spare time amidst other chores. He saved his best invective for the poor flamingos. Writing on February 2, 1911, to University of California biologist Charles Kofoid, he stated:

  [Thayer’s] book shows such a fantastic quality of mind on his part that it is a matter of very real surprise to me that any scientific observer…no matter how much credit he may give to Mr. Thayer for certain discoveries and theories, should fail to enter the most emphatic protest against the utter looseness and wildness of his theorizing. Think of being seriously required to consider the theory that flamingos are colored red so that fishes (or oysters for that matter—there is no absurdity of which Mr. Thayer could not be capable) would mistake them for the sunset!

  The debate between Roosevelt and Thayer developed into an interesting discussion of scientific methodology, not merely some rhetorical sniping about specifics. To grasp Roosevelt’s primarily methodological (and cogent) objections to Thayer’s work, consider Thayer’s most remarkable painting of all—the frontispiece to his 1909 book, showing a peacock obliterated in the foliage. Here, Thayer argues that every nuance of a peacock’s coloration increases his concealment in a particular bit of habitat—the combined effect adding to invisibility. Given the usual interpretation of a peacock’s color as revealing, and the gaudy impression that he makes both upon us and, one must assume, the peahen, Thayer’s interpretation represents quite a departure from tradition and common sense:

  A peacock in the woods, showing how, in at least one highly peculiar position, each “showy” feature can help blend the bird into invisibility. From Thayer’s 1909 book. NEG. NO. 2A13238. COURTESY DEPARTMENT OF LIBRARY SERVICES, AMERICAN MUSEUM OF NATURAL HISTORY.

  The peacock’s splendor is the effect of a marvelous combination of “obliterative” designs, in forest-colors and patterns…. All imaginable forest-tones are to be found in this bird’s costume; and they “melt” him into the scene to a degree past all human analysis.

  Thayer then positions his bird so precisely that all features blend with surroundings. He paints the blue neck against a gap in the foliage, so that it may mimic “blue sky seen through the leaves.” He matches the golden greens and browns of the back to forest tones. He depicts the white cheek patch as a “ruptive” hole that disaggregates the face. He paints the celebrated ocelli (eye-spots) of the tail feathers as leaf mimics. He also notes that ocelli are smallest and dimmest near the body, grading to larger and brighter toward the rear end: “They inevitably lead the eye away from the bird, till it finds itself straying amid the foliage beyond the tail’s evanescent border.” The spread tail, he claims, may impress the peahen, but it “looks also very much like a shrub bearing some kind of fruit or flower.” Finally, he argues that the tail’s coppery brown color represents perfectly “the bare ground and tree-trunks seen between the leaves.”

  What a tour de force, but what can we possibly make of such special pleading? Who would doubt that some conceivable habitat might conceal almost any animal? Note how precisely the peacock must choose his spot to receive the cryptic benefit that Thayer wishes to confer upon him. In particular, he must always place his shimmering blue neck in a gap amidst the foliage where it will vanish against a clear sky (but what does he do on a cloudy day, or in a bush so dense that no holes exist, as seen from all relevant directions at once?). Peacocks, in any case, live primarily in open fields. Their spreading display is a glory to behold—and the very opposite of invisible.

  Thayer, of course, knew all this. He didn’t claim (as his critics sometimes charged) that a habitat offering protection by concealment must be the usual, or even a common, haunt of its invisible beneficiary. Thayer simply argued that such protection might be important at critical moments occurring only once or twice in an animal’s lifetime—at crucial instants of impending death from a stalking predator.

  But how could odd and improbable moments shape such complex and intricate patterns as the innumerable details of a peacock’s design? With this question, we finally arrive at the key theoretical issue of this debate—the power of natural selection itself. In order to believe that complex designs might be construct
ed by such rare and momentary benefits as sunsets or particular positions in trees outside an animal’s normal habitat, one must have an overarching faith in the power of natural selection. Selection must be so potent that even the rarest of benefits will eventually be engraved into the optimal designs of organisms. Thayer had this faith; Roosevelt, and most biologists then and since, did not. Thayer wrote in 1900: “Of course, to any one who feels the inevitability of natural selection, it is obvious that each organ or structural detail, and likewise each quality of organic forms, owes its existence to the sum of all its uses.” Thayer then laid it on the line in stark epitome—patterns of color are built by natural selection, “pure, simple, and omnipotent.”

  Roosevelt and other acute critics correctly identified the central flaw in Thayer’s science—not in his numerous factual errors, but in his methodology. Thayer found a hiding place for all his animals but with a method that made his theory untestable and therefore useless to science. Thayer insisted that he had proved his point simply by finding any spot that rendered an animal invisible. He didn’t need to show that the creature usually frequented such a place or that the location formed part of a natural habitat at all. For the animal might seek its spot only in the rarest moments of need. But how then could we disprove any of Thayer’s claims? We might work for years to show that an animal never entered its domain of invisibility, and Thayer would reply: Wait till tomorrow when urgent need arises. Scientists are trained to avoid such special pleading because it exerts a chilling and stupefying effect upon hypotheses, by rendering them invulnerable to test and potential disproof. Doing is the soul of science and we reject hypotheses that condemn us to impotence.