The Panda’s Thumb

by Stephen Jay Gould

  I don’t wish to downplay the pivotal influence of the Beagle voyage on Darwin’s career. It gave him space, freedom and endless time to think in his favored mode of independent self-stimulation. (His ambivalence towards university life, and his middling performance there by conventional standards, reflected his unhappiness with a curriculum of received wisdom.) He writes from South America in 1834: “I have not one clear idea about cleavage, stratification, lines of upheaval. I have no books, which tell me much and what they do I cannot apply to what I see. In consequence I draw my own conclusions, and most gloriously ridiculous ones they are.” The rocks and plants and animals that he saw did provoke him to the crucial attitude of doubt—midwife of all creativity. Sydney, Australia—1836. Darwin wonders why a rational God would create so many marsupials on Australia since nothing about its climate or geography suggests any superiority for pouches: “I had been lying on a sunny bank and was reflecting on the strange character of the animals of this country as compared to the rest of the World. An unbeliever in everything beyond his own reason might exclaim, ‘Surely two distinct Creators must have been at work.’”

  Nonetheless, Darwin returned to London without an evolutionary theory. He suspected the truth of evolution, but had no mechanism to explain it. Natural selection did not arise from any direct reading of the Beagle’s facts, but from two subsequent years of thought and struggle as reflected in a series of remarkable notebooks that have been unearthed and published during the past twenty years. In these notebooks, we see Darwin testing and abandoning a number of theories and pursuing a multitude of false leads—so much for his later claim about recording facts with an empty mind. He read philosophers, poets, and economists, always searching for meaning and insight—so much for the notion that natural selection arose inductively from the Beagle’s facts. Later, he labelled one notebook as “full of metaphysics on morals.”

  Yet if this tortuous path belies the Scylla of inductivism, it has engendered an equally simplistic myth—the Charybdis of eurekaism. In his maddeningly misleading autobiography, Darwin does record a eureka and suggests that natural selection struck him as a sudden, serendipitous flash after more than a year of groping frustration:

  In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work.

  Yet, again, the notebooks belie Darwin’s later recollections—in this case by their utter failure to record, at the time it happened, any special exultation over his Malthusian insight. He inscribes it as a fairly short and sober entry without a single exclamation point, though he habitually used two or three in moments of excitement. He did not drop everything and reinterpret a confusing world in its light. On the very next day, he wrote an even longer passage on the sexual curiosity of primates.

  The theory of natural selection arose neither as a workmanlike induction from nature’s facts, nor as a mysterious bolt from Darwin’s subconscious, triggered by an accidental reading of Malthus. It emerged instead as the result of a conscious and productive search, proceeding in a ramifying but ordered manner, and utilizing both the facts of natural history and an astonishingly broad range of insights from disparate disciplines far from his own. Darwin trod the middle path between inductivism and eurekaism. His genius is neither pedestrian nor inaccessible.

  Darwinian scholarship has exploded since the centennial of the Origin in 1959. The publication of Darwin’s notebooks and the attention devoted by several scholars to the two crucial years between the Beagle’s docking and the demoted Malthusian insight has clinched the argument for a “middle path” theory of Darwin’s creativity. Two particularly important works focus on the broadest and narrowest scales. Howard E. Gruber’s masterful intellectual and psychological biography of this phase in Darwin’s life, Darwin on Man, traces all the false leads and turning points in Darwin’s search. Gruber shows that Darwin was continually proposing, testing, and abandoning hypotheses, and that he never simply collected facts in a blind way. He began with a fanciful theory involving the idea that new species arise with a prefixed life span, and worked his way gradually, if fitfully, towards an idea of extinction by competition in a world of struggle. He recorded no exultation upon reading Malthus, because the jigsaw puzzle was only missing a piece or two at the time.

  Silvan S. Schweber has reconstructed, in detail as minute as the record will allow, Darwin’s activities during the few weeks before Malthus (The Origin of the Origin Revisited, Journal of the History of Biology, 1977). He argues that the final pieces arose not from new facts in natural history, but from Darwin’s intellectual wanderings in distant fields. In particular, he read a long review of social scientist and philosopher Auguste Comte’s most famous work, the Cours de philosophie positive. He was particularly struck by Comte’s insistence that a proper theory be predictive and at least potentially quantitative. He then turned to Dugald Stewart’s On the Life and Writing of Adam Smith, and imbibed the basic belief of the Scottish economists that theories of overall social structure must begin by analyzing the unconstrained actions of individuals. (Natural selection is, above all, a theory about the struggle of individual organisms for success in reproduction.) Then, searching for quantification, he read a lengthy analysis of work by the most famous statistician of his time—the Belgian Adolphe Quetelet. In the review of Quetelet, he found, among other things, a forceful statement of Malthus’s quantitative claim—that population would grow geometrically and food supplies only arithmetically, thus guaranteeing an intense struggle for existence. In fact, Darwin had read the Malthusian statement several times before; but only now was he prepared to appreciate its significance. Thus, he did not turn to Malthus by accident, and he already knew what it contained. His “amusement,” we must assume, consisted only in a desire to read in its original formulation the familiar statement that had so impressed him in Quetelet’s secondary account.

  In reading Schweber’s detailed account of the moments preceding Darwin’s formulation of natural selection, I was particularly struck by the absence of deciding influence from his own field of biology. The immediate precipitators were a social scientist, an economist, and a statistician. If genius has any common denominator, I would propose breadth of interest and the ability to construct fruitful analogies between fields.

  In fact, I believe that the theory of natural selection should be viewed as an extended analogy—whether conscious or unconscious on Darwin’s part I do not know—to the laissez faire economics of Adam Smith. The essence of Smith’s argument is a paradox of sorts: if you want an ordered economy providing maximal benefits to all, then let individuals compete and struggle for their own advantages. The result, after appropriate sorting and elimination of the inefficient, will be a stable and harmonious polity. Apparent order arises naturally from the struggle among individuals, not from predestined principles or higher control. Dugald Stewart epitomized Smith’s system in the book Darwin read:

  The most effective plan for advancing a people…is by allowing every man, as long as he observes the rules of justice, to pursue his own interest in his own way, and to bring both his industry and his capital into the freest competition with those of his fellow citizens. Every system of policy which endeavors…to draw towards a particular species of industry a greater share of the capital of the society than would naturally go to it…is, in reality, subversive of the great purpose which it means to promote.

  As Schweber states: “The Scottish analysis of society contends that the combined effect of individual actions results in the institutions upon which society is based, and that such a society is a stable and evol
ving one and functions without a designing and directing mind.”

  We know that Darwin’s uniqueness does not reside in his support for the idea of evolution—scores of scientists had preceded him in this. His special contribution rests upon his documentation and upon the novel character of his theory about how evolution operates. Previous evolutionists had proposed unworkable schemes based on internal perfecting tendencies and inherent directions. Darwin advocated a natural and testable theory based on immediate interaction among individuals (his opponents considered it heartlessly mechanistic). The theory of natural selection is a creative transfer to biology of Adam Smith’s basic argument for a rational economy: the balance and order of nature does not arise from a higher, external (divine) control, or from the existence of laws operating directly upon the whole, but from struggle among individuals for their own benefits (in modern terms, for the transmission of their genes to future generations through differential success in reproduction).

  Many people are distressed to hear such an argument. Does it not compromise the integrity of science if some of its primary conclusions originate by analogy from contemporary politics and culture rather than from data of the discipline itself? In a famous letter to Engels, Karl Marx identified the similarities between natural selection and the English social scene:

  It is remarkable how Darwin recognizes among beasts and plants his English society with its division of labor, competition, opening up of new markets, ‘invention,’ and the Malthusian ‘struggle for existence.’ It is Hobbes’ bellum omnium contra omnes (the war of all against all).

  Yet Marx was a great admirer of Darwin—and in this apparent paradox lies resolution. For reasons involving all the themes I have emphasized here—that inductivism is inadequate, that creativity demands breadth, and that analogy is a profound source of insight—great thinkers cannot be divorced from their social background. But the source of an idea is one thing; its truth or fruitfulness is another. The psychology and utility of discovery are very different subjects indeed. Darwin may have cribbed the idea of natural selection from economics, but it may still be right. As the German socialist Karl Kautsky wrote in 1902: “The fact that an idea emanates from a particular class, or accords with their interests, of course proves nothing as to its truth or falsity.” In this case, it is ironic that Adam Smith’s system of laissez faire does not work in his own domain of economics, for it leads to oligopoly and revolution, rather than to order and harmony. Struggle among individuals does, however, seem to be the law of nature.

  Many people use such arguments about social context to ascribe great insights primarily to the indefinable phenomenon of good luck. Thus, Darwin was lucky to be born rich, lucky to be on the Beagle, lucky to live amidst the ideas of his age, lucky to trip over Parson Malthus—essentially little more than a man in the right place at the right time. Yet, when we read of his personal struggle to understand, the breadth of his concerns and study, and the directedness of his search for a mechanism of evolution, we understand why Pasteur made his famous quip that fortune favors the prepared mind.

  6 | Death Before Birth, or a Mite’s Nunc Dimittis

  CAN ANYTHING BE more demoralizing than parental incompetence before the most obvious and innocent of children’s questions: why is the sky blue, the grass green? Why does the moon have phases? Our embarrassment is all the more acute because we thought we knew the answer perfectly well, but hadn’t rehearsed it since we ourselves had received a bumbled response in similar circumstances a generation earlier. It is the things we think we know—because they are so elementary, or because they surround us—that often present the greatest difficulties when we are actually challenged to explain them.

  One such question, with an obvious and incorrect answer, lies close to our biological lives: why, in humans (and in most species familiar to us), are males and females produced in approximately equal numbers? (Actually, males are more common than females at birth in humans, but differential mortality of males leads to a female majority in later life. Still, the departures from a one to one ratio are never great.) At first glance, the answer seems to be, as in Rabelais’s motto, “plain as the nose on a man’s face.” After all, sexual reproduction requires a mate; equal numbers imply universal mating—the happy Darwinian status of maximal reproductive capacity. At second glance, it isn’t so clear at all, and we are drawn in confusion to Shakespeare’s recasting of the simile: “A jest unseen, inscrutable, invisible, as a nose on a man’s face.” If maximal reproductive capacity is the optimal state for a species, then why make equal numbers of males and females. Females, after all, set the limit upon numbers of offspring, since eggs are invariably so much larger and less abundant than sperm in species familiar to us—that is, each egg can make an offspring, each sperm cannot. A male can impregnate several females. If a male can mate with nine females and the population contains a hundred individuals, why not make ten males and ninety females? Reproductive capacity will certainly exceed that of a population composed of fifty males and fifty females. Populations made predominantly of females should, by their more rapid rates of reproduction, win any evolutionary race with populations that maintain equality in numbers between the sexes.

  What appeared obvious is therefore rendered problematical and the question remains: why do most sexual species contain approximately equal numbers of males and females? The answer, according to most evolutionary biologists, lies in a recognition that Darwin’s theory of natural selection speaks only of struggle among individuals for reproductive success. It contains no statement about the good of populations, species, or ecosystems. The argument for ninety females and ten males was framed in terms of advantages for populations as a whole—the usual, congenial, and dead wrong, way in which most people think of evolution. If evolution worked for the good of populations as a whole, then sexual species would contain relatively few males.

  The observed equality of males and females, in the face of obvious advantages for female predominance if evolution worked upon groups, stands as one of our most elegant demonstrations that Darwin was right—natural selection works by the struggle of individuals to maximize their own reproductive success. The Darwinian argument was first framed by the great British mathematical biologist R.A. Fisher. Suppose, Fisher argued, that either sex began to predominate. Let us say, for example, that fewer males than females are born. Males now begin to leave more offspring than females since their opportunities for mating increase as they become rarer—that is, they impregnate more than one female on average. Thus, if any genetic factors influence the relative proportion of males born to a parent (and such factors do exist), then parents with a genetic inclination to produce males will gain a Darwinian advantage—they will produce more than an average number of grandchildren thanks to the superior reproductive success of their predominantly male offspring. Thus, genes that favor the production of males will spread and male births will rise in frequency. But, this advantage for males fades out as male births increase and it disappears entirely when males equal females in number. Since the same argument works in reverse to favor female births when females are rare, the sex ratio is driven by Darwinian processes to its equilibrium value of one to one.

  But how would a biologist go about testing Fisher’s theory of sex ratio? Ironically, the species that confirm its predictions are no great help beyond the initial observation. Once we frame the basic argument and determine that the species we know best have approximately equal numbers of males and females, what do we achieve by finding that the next thousand species are similarly ordered? Sure, it all fits, but we do not gain an equal amount of confidence each time we add a new species. Perhaps the one to one ratio exists for another reason?

  To test Fisher’s theory, we must look for exceptions. We must seek unusual situations in which the premises of Fisher’s theory are not met—situations that lead to a specific prediction about how sex ratio should depart from one to one. If change of premises leads to a definite and successful prediction of a
ltered outcome, then we have an independent test that strongly boosts our confidence. This method is embodied in the old proverb that “the exception proves the rule,” although many people misunderstand the proverb because it embodies the less common meaning of “prove.” Prove comes from the Latin probare—to test or to try. Its usual, modern meaning refers to final and convincing demonstration and the motto would seem to say that exceptions establish indubitable validity. But in another sense, closer to its root, “prove” (as in “proving ground” or printer’s “proof”) is more like its cognate “probe”—a test or an exploration. It is the exception that probes the rule by testing and exploring its consequences in altered situations.

  Here nature’s rich diversity comes to our aid. The stereotyped image of a birder assiduously adding the rufous-crowned, peg-legged, speckle-backed, cross-billed and cross-eyed towhee to his life list gives, in unwarranted ridicule, a perverted twist to the actual use made by naturalists of life’s diversity. It is nature’s richness that permits us to establish a science of natural history in the first place—for the variety virtually guarantees that appropriate exceptions can be found to probe any rule. Oddities and weirdnesses are tests of generality, not mere peculiarities to describe and greet with awe or a chuckle.

  Fortunately, nature has been profligate in providing species and modes of life that violate the premises of Fisher’s argument. In 1967, British biologist W.D. Hamilton (now at the University of Michigan) gathered the cases and arguments into an article entitled “Extraordinary sex ratios.” I will discuss in this essay only the clearest and most important of these probing violations.

  Nature rarely heeds our homilies in all cases. We are told, and with good reason, that mating of brothers and sisters should be avoided, lest too many unfavorable recessive genes gain an opportunity to express themselves in double dose. (Such genes tend to be rare, and chances are small that two unrelated parents will both carry them. But the probability that two sibs carry the same gene is usually fifty percent.) Nonetheless, some animals never heard the rule and indulge, perhaps exclusively, in sib mating.