The Flamingo’s Smile (31 page)
Read The Flamingo’s Smile Online
Authors: Stephen Jay Gould
Wells explicitly denies this usual form of selection by claiming that favorable variants cannot spread through large and stable populations. His argument is incorrect and based on a false view of heredity, then current, called blending inheritance—the idea that all favorable variants will be diluted by half in offspring through intermarriage with a normal member of the population. The diluted offspring will then usually marry a normal individual (since favorable variants are so rare) and the subsequent generation will be diluted to one-quarter. Soon, the rare and favorable variant will be entirely swamped out. Heredity doesn’t work this way (although Wells couldn’t know what Mendel would discover fifty years later). Favorable traits often arise by mutation, and such features cannot be diluted by breeding with normal individuals. The mutation (if recessive) may not be expressed in the next generation, but it will not be eliminated. Wells’s belief in blending inheritance led him to deny selection by slow transformation within a population:
Those varieties [that is, favorable variations], for the most part, quickly disappear, from the intermarriages of different families. Thus, if a very tall man be produced, he very commonly marries a woman much less than himself, and their progeny scarcely differs in size from their countrymen.
How then can selection work? Wells argues that favorable variants can spread, presumably by chance rather than by selection (although he is not explicit), through small and mobile populations where vast numbers of normal individuals cannot impose dilution by backbreeding:
In districts, however, of very small extent, and having little intercourse with other countries, an accidental difference in the appearance of the inhabitants will often descend to their late posterity.
Thus, Wells conjectures that the people of Africa were initially divided into tiny, noninteracting populations. By chance, different average colors (and accompanying resistances to disease) became established among these populations. Selection then acted by competition between populations already different (for reasons unrelated to natural selection) in average skin color. Within each group, color was relatively constant and selection could only operate by sorting the groups themselves. Selection, in other words, occurs between groups, not among individuals within a group.
Of the accidental varieties of man [meaning populations this time; Wells and his contemporaries used the word
variety
both for distinct individuals and for different populations], which would occur among the first few and scattered inhabitants of the middle regions of Africa, some one would be better fitted than the others to bear the diseases of the country. This race would consequently multiply, while the others would decrease, not only from their inability to sustain the attacks of disease, but from their incapacity of contending with their more vigorous neighbors. The color of this vigorous race I take for granted, from what has been already said, would be dark.
The locus or level of selection is a “hot topic” within evolutionary theory today. While no one denies that selection works powerfully at the traditional level of differences among organisms within a population, other modes may also be important. The idea that selection may operate primarily
among
local populations—so-called intergroup selection—has long been advocated by the great geneticist Sewall Wright (who still argues eloquently for his position at age 95). Wright’s eclipse within strict Darwinian circles has recently been reversed and intergroup selection is now receiving a more favorable second look. I find it intriguing that the first formulation of natural selection advocated an intergroup process rather than the traditional focus on competing organisms.
Nonetheless, although Wells’s argument was unorthodox by later Darwinian standards, it surely states the principle of natural selection. We must therefore return to our initial question. Why were these Darwinian harbingers totally ignored, and why does Darwin deserve his status (and, I fear, Wells and Matthew theirs as well).
Loren Eiseley makes the astute point (in
Darwin’s Century
) that Wells’s argument, as stated, cannot be extended or generalized to yield the panoply of evolutionary change through life’s history. Everyone knew that organisms varied and that local breeds could be manufactured from this raw material (how else did dog and pigeon fanciers work, not to mention farmers). But variation among breeds of dogs does not automatically extrapolate to the transformation from fish to human. Perhaps species have fixed and God-given limits of variation. We may make new breeds by selecting for extremes within these limits, but we cannot transcend the boundary to construct fundamentally new creatures. Wells does not generalize his argument to encompass large-scale evolutionary change, and hindsight alone may permit us to read his speculations as harbingers to Darwin’s overturn of biology.
Still, Wells’s failure to generalize cannot be the main reason for his obscurity. (Darwin, by the way, only learned of Wells’s work from an American correspondent with antiquarian bibliographic interests.) The primary rationale is uncomplex. Ideas are cheap; mere statement counts for little or nothing. Intellectual fame accrues to people with the vision to make a good idea work in two ways: by using it to make new discoveries and by recognizing its implications as a far-ranging instrument for transforming general attitudes.
We have no reason to suspect that either Wells or Matthew recognized any of the revolutionary power behind his cleverness. Wells presented natural selection as an appendage to an essay he didn’t even bother to publish until he lay dying. Matthew buried it among his trees and saw no forest (although he, unlike Wells, did advocate evolution as the cause of life’s history). Indeed, in a second letter responding to Darwin’s apology in 1860, Matthew damns Darwin with faint praise (and inadvertently condemns himself) by arguing that he never made much of selection because, unlike Darwin who struggled so hard to formulate the principle, he had grasped it as an evident deduction from the nature of things. He regarded it as necessarily true, almost trivial in that sense, and thus unworthy of much development. Matthew therefore missed all its significance:
To me the conception of this law of Nature came intuitively as a self-evident fact, almost without an effort of concentrated thought. Mr. Darwin here seems to have more merit in the discovery than I have had—to me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly and with due caution to have made his way synthetically from fact to fact onwards; while with me it was by a general glance at the scheme of Nature that I estimated this select production of species as an a priori recognizable fact—an axiom, requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp.
Darwin, on the other hand, used natural selection as the intellectual fulcrum of an entire career. He interpreted human evolution in its light, reformulated the principles of psychology, and explained the coevolution of orchids and their pollinating insects, the biogeographic distribution of organisms, the habits and actions of worms—a rich panoply of issues from the largest enigmas of life to the smallest quirks of particular organisms. He established a workable research program for an entire profession.
I have found no documents of human thought more exciting than the notebooks that Darwin filled in London as a young man in his late twenties, just returned from five years aboard the
Beagle
. He had the key to a new view of life, and he knew it. His mind ranged over the entire intellectual landscape, from biology to psychology, morality, philosophy, and literature. Evolution by natural selection impinged on everything. Wells and Matthew had stated the same principle, but they had then either forgotten or had failed to draw any implications. Darwin sat in London, a young man rebuilding a world of thought. Consider but one statement, as a symbol of his achievement and a fitting end to this essay. Charles Darwin, cutting through two thousand years of tradition in Western philosophy with one epigrammatic note to himself:
Plato says in
Phaedo
that our “imaginary ideas” arise from the preexistence of the soul, are not derivable from experience—read monkeys for preexistence.
CHARLES DARWIN AND ABRAHAM LINCOLN
were born on the same day—February 12, 1809. They are also linked in another curious way—for both must simultaneously play, and for similar reasons, the role of man and legend. In a nation too young for mythic heroes, flesh and blood must substitute. Hence schoolchildren learn about Honest Abe, who freed the slaves single-handedly for simple justice, and who, as a young man, trudged for miles to return a few cents to a woman he had inadvertently short-changed. This legendary Lincoln may fulfill a national or psychological need, but historians must also labor to rescue the real, and wondrously complex, man from such a factually inaccurate role. Similarly, science worships no gods, and ancient sages are in strictly short supply. Historical figures must again form the stuff of needed legends. The apple beans Newton; Galileo drops his missiles from the Leaning Tower; and Darwin, alone at sea, transforms the intellectual world in splendid mental isolation.
The myth of the
Beagle
—that Darwin became an evolutionist by simple, unbiased observation of an entire world laid out before him during a five-year voyage around the world—fits all our romantic criteria for the best of legends: a young man, freed from the trammels of English society and its constraining presuppositions, face to face with nature, parrying his fresh and formidable mind with all the challenges provided by plants, animals, and rocks throughout the globe. He leaves England in 1831, planning to become a country parson upon his return. He returns in 1836, having seen evolution in the raw, understanding (albeit dimly) its implications and committed to a scientific life as evolutionary thinker. The chief catalyst: the Galápagos Islands. The main actors: tortoises, mockingbirds, and above all, thirteen species of Darwin’s finches—the finest evolutionary laboratory offered to us by nature.
We may need simple and heroic legends for that peculiar genre of literature known as the textbook. But historians must also labor to rescue human beings from their legends in science—if only so that we may understand the process of scientific thought aright. Darwin, to begin, did not become an evolutionist until several months after his return to London—probably not until March 1837 (the
Beagle
docked in October 1836). He did not appreciate the evolutionary significance of the Galápagos while he was there, and he originally misunderstood the finches so thoroughly that he was barely able to reconstruct the story later from his sadly inadequate records. The legend of the finches may persist, but it has been splendidly debunked in two recent articles by historian of science Frank Sulloway. His arguments form the basis of this essay (see bibliography).
The thirteen species of Darwin’s finches form a closely knit genealogical group of widely divergent life styles—a classic case of adaptive radiation into a series of roles and niches that would be filled by members of several bird families in more conventional, and crowded, continental situations. We get our major clues about the adaptations of these species from the shapes of their bills. Three species of ground finches have large, medium, and small beaks, while a fourth grows a sharp, pointed bill. All are adapted to eating differing seeds of appropriate size and hardness. Two species feed on cactus and another on mangroves. Four inhabit trees—of these, one is a vegetarian, while the other three eat large, medium, and small insects, respectively. A twelfth species closely resembles warblers in form and habits; while the thirteenth, the most curious of all, uses twigs and cactus spines as tools to extract insects from crevices in tree trunks.
The fine work of the great British ornithologist David Lack has taught us that the thirteen species evolved and became more distinct through a four-stage process of colonization, isolation and speciation, reinvasion, and perfecting of adaptation in competition. Lack also gave the birds their felicitous name of “Darwin’s finches,” in his 1947 book of the same title. But, contrary to anachronistic legend, this classic description of speciation is not a story that Darwin ever knew.
Darwin visited the Galápagos in September and October 1835, landing on only four of the islands. At sea, sometime during the middle of 1836, he penned a famous statement in his
Ornithological Notes
, a major source for the legend that his Galápagos experiences directly converted him to evolution and that the finches inspired his new view of life:
When I recollect, the fact from the form of the body, shape of scales and general size, the Spaniards can at once pronounce, from which Island any Tortoise may have been brought. When I see these Islands in sight of each other, and possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure and filling the same place in Nature, I must suspect that they are only varieties. The only fact of a similar kind of which I am aware, is the constant asserted difference—between the wolf-like Fox of East and West Falkland Islds.—If there is the slightest foundation for these remarks the zoology of Archipelagos will be well worth examining; for such facts would undermine the stability of Species.
First of all, the “birds” of this passage are Galápagos mockingbirds, not finches. Darwin did notice that three of the four islands he visited contained distinctly different mockingbirds. At face value, this statement seems to display a strong preference for evolution; it certainly raises the possibility. But a familiarity with nineteenth-century zoological terminology suggests an alternate interpretation. All creationists admitted that species often differentiated into mildly distinct forms in situations, as on island chains and archipelagoes, where populations could become isolated in differing circumstances of ecology and climate. These local races were called varieties, and they did not threaten the created and immutable character of a species’ essence. Properly translated from the terminology of his time, Darwin says in this famous statement that either the tortoises and mockingbirds are merely varieties—in which case they do not threaten his creationist views—or they have become separate species, in which case they do. He briefly considered evolution by admitting the second possibility, but he ultimately drew back while still at sea by tentatively deciding (incorrectly, for the mockingbirds at least) that the island forms were only varieties. Darwin’s memories as an old man confirm this view that he only briefly flirted with, and then rejected, evolution while on the
Beagle
. He wrote to the German naturalist Otto Zacharias in 1877: “When I was on board the
Beagle
I believed in the permanence of species, but, as far as I can remember, vague doubts occasionally flitted across my mind.”
A second statement, taken in conjunction with a misreading of the
Ornithological Notes
, might also be considered as evidence that Darwin became an evolutionist at sea in 1836. He wrote in his pocket journal: “In July opened first notebook on ‘Transmutation of Species’—Had been greatly struck from about Month of previous March on character of S. American fossils—and species on Galápagos Archipelago. These facts origin (especially latter) of all my views.” We know that he started the first Transmutation notebook in July 1837, and we might therefore interpret the “previous March” as 1836, about the time that he penned the
Ornithological Notes
at sea. But the previous March might as well be 1837 when, as we shall soon see, he was in London learning from specialists at the Zoological Society about the true character of his Galápagos collections—a set of phenomena that he had failed to observe during his own visit.
What, then, did Darwin see on the Galápagos, and what did he miss? Three groups of animals have come down through history as the most famous evolutionary laboratories of the Galápagos: mockingbirds, tortoises, and finches. Only for the mockingbirds did Darwin make the key observation that underlies the evolutionary tale developed later (although, as we have seen, Darwin first explicitly rejected the evolutionary reading for a different interpretation). In short, he noticed that varying forms (later recognized as true species, although Darwin originally labeled them varieties) inhabited the different islands he visited. He landed first at Chatham Island, then at Charles, and he realized that he could distinguish the Charles Island mockingbird from the form he had previously collected at Chatham. Thus, he collected more mockingbirds wherever he landed and he carefully kept the separate island collections well labeled and distinct. He could not distinguish the Albemarle mockingbird, on the third island he visited, from the Chatham form, but the James Island bird represented a third, distinct variety (by his original interpretation).
Galápagos tortoises are all of one species, but most islands feature their own recognizable subspecies. These span an impressive range of form, from smooth, dome-shaped carapaces to the peculiar saddlebacks, with a pronounced hump in the carapace just above the head. Darwin missed this story completely. He never even noted the saddlebacks. Moreover, his basic concept of these tortoises virtually guaranteed that he would not be able to make the key observation.
Nicholas Lawson, the vice-governor, told Darwin that “the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought” (although distinctions abound, this statement is overly optimistic and modern experts cannot always distinguish each island). But Darwin, by his own admission, made little of this information, writing in the 1845 edition of the
Beagle Voyage:
I did not for some time pay sufficient attention to this statement, and I had already partially mingled together the collections from two of the islands. I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under quite similar climate, rising to a nearly equal height, would have been differently tenanted.
As the result of a common error in classification, Darwin was ill-disposed to consider the differences between islands as evolutionarily (or even taxonomically) meaningful. Darwin accepted the general view that Galápagos tortoises were not taxonomically distinct but belonged to the species
Testudo indicus
, the giant land tortoise of the Aldabra Islands in the Indian Ocean. They had only recently been brought, so the false story continued, to the Galápagos by buccaneers. Hence, differences among islands, if they existed at all, could only be immediate and superficial—inspired by harsh climates at the time of introduction. Moreover, Darwin never saw live saddleback tortoises. He only observed living tortoises on James and Chatham islands, and both contain nearly indistinguishable versions of the dome-shaped carapace.
Still, Darwin cannot be entirely excused from a charge of some carelessness in observation. He did have an opportunity to observe the saddleback form but either failed to do so or recorded no impression. The Charles Island race was extinct when Darwin landed, but old carapaces were abundant at the settlement, where they served as flowerpots. Moreover, Darwin showed singularly little interest in preserving specimens for comparison among islands, a sure sign that he did not regard Lawson’s statement as significant (much to his later regret). Captain Fitzroy took thirty large Chatham tortoises on board to beef up the
Beagle
’s supply of fresh meat during the long Pacific crossing. Sulloway remarks:
But Darwin and the other crew members gradually ate their way through the evidence that eventually, in the form of hearsay, was to revolutionize the biological sciences. Regrettably, not one of the thirty Chatham Island carapaces reached England, having all been thrown overboard with the other inedible remains.
Darwin’s reaction to the Galápagos finches included even more error and misunderstanding. First, he showed no appreciation for the importance of differences between islands. In fact, he didn’t even bother to record or label the islands that had housed his specimens. Only three of his thirty-one finches are identified by island in the
Ornithological Notes
, all members of a highly distinctive species that Darwin remembered seeing only on James Island. He later wrote with regret in the
Voyage of the Beagle:
“Unfortunately most of the specimens of the finch tribe were mingled together.” Second, he failed completely to collect any finches on one of the islands he visited—Albemarle. True, he was there for only part of a day, but his own diary records an abundance of easily collectable finches at a spring near Bank’s Cove: “To our disappointment the little pits in the Sandstone contained scarcely a gallon of water and that not good. It was however sufficient to draw together all the little birds in the country; Doves and Finches swarmed around its margin.”
Third, except for cactus and warbler finches, Darwin failed to observe any distinction in diet among the species and believed erroneously that they all ate the same kinds of food. Thus, he could not have reconstructed our modern story, even if he had been inclined to evolutionary views.
Fourth, Darwin’s entire style of collection on the Galápagos strongly reflected his creationist presuppositions. Evolutionists see variation as fundamental, as the raw material of evolutionary change. Species can only be well defined by collecting many specimens and defining the spectrum of their variation. Creationists believe that each species is endowed with a fixed essence. Variation is a mere nuisance, a confusing array of environmentally induced departures from an ideal form. Creationists tend to gather a limited number of specimens from each species and to concentrate on procuring individuals closest to the essential form. Darwin collected very few specimens, generally only a male and female of each species. In all, he procured but thirty-one finches from the Galápagos. By contrast, a 1905–1906 California Academy of Sciences expedition, dispatched to study evolution explicitly, brought back more than 8,000 specimens.
Fifth, and most importantly, the finches tell no evolutionary tale unless you recognize that, despite their outward differences in form and behavior, all form a tightly knit genealogical group. But Darwin, while on the Galápagos, was fooled by the stunning diversity and failed to recognize Darwin’s finches as a taxonomic entity. He referred the cactus finch to a family of birds that includes orioles and meadowlarks, and he misclassified the warbler finch as either a wren or warbler. Those that he recognized as finches, he divided into two distantly related groups within the family. Sulloway remarks: “As for Darwin’s supposed insight into evolution by adaptive radiation while he was still in the Galápagos, the more the various species of finch exhibited this remarkable phenomenon, the more Darwin mistook them at the time for the forms they were mimicking.”