Kelvin recognized the contingent nature of his calculations, but the physics of his day included no force capable of generating new heat, and he therefore felt secure in his assumption. Early in his campaign, in calculating the sun’s age, he had admitted his crucial dependence upon no new source of energy, for he had declared his results valid “unless new sources now unknown to us are prepared in the great storehouse of creation.”
Then, in 1903, Pierre Curie announced that radium salts constantly release newly generated heat. The unknown source had been discovered. Early students of radioactivity quickly recognized that most of the earth’s heat must be continually generated by radioactive decay, not merely dissipating from an originally molten condition—and they realized that Kelvin’s argument had collapsed. In 1904, Ernest Rutherford gave this account of a lecture, given in Lord Kelvin’s presence, and heralding the downfall of Kelvin’s forty-year campaign for a young earth:
I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in for trouble at the last part of the speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me! Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the earth, provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium!
Thus, Kelvin lived into the new age of radioactivity. He never admitted his error or published any retraction, but he privately conceded that the discovery of radium had invalidated some of his assumptions.
The discovery of radioactivity highlights a delicious double irony. Not only did radioactivity supply a new source of heat that destroyed Kelvin’s argument; it also provided the clock that could then measure the earth’s age and proclaim it ancient after all! For radioactive atoms decay at a constant rate, and their dissipation does measure the duration of time. Less than ten years after the discovery of radium’s newly generated heat, the first calculations for radioactive decay were already giving ages in billions of years for some of the earth’s oldest rocks.
We sometimes suppose that the history of science is a simple story of progress, proceeding inexorably by objective accumulation of better and better data. Such a view underlies the moral homilies that build our usual account of the advance of science—for Kelvin, in this context, clearly impeded progress with a false assumption. We should not be beguiled by such comforting and inadequate stories. Kelvin proceeded by using the best science of his day, and colleagues accepted his calculations. We cannot blame him for not knowing that a new source of heat would be discovered. The framework of his time included no such force. Just as Maupertuis lacked a proper metaphor for recognizing that embryos might contain coded instructions rather than preformed parts (see next essay), Kelvin’s physics contained no context for a new source of heat.
The progress of science requires more than new data; it needs novel frameworks and contexts. And where do these fundamentally new views of the world arise? They are not simply discovered by pure observation; they require new modes of thought. And where can we find them, if old modes do not even include the right metaphors? The nature of true genius must lie in the elusive capacity to construct these new modes from apparent darkness. The basic chanciness and unpredictability of science must also reside in the inherent difficulty of such a task.
9 | For Want of a Metaphor
IN 1745, the great French savant Pierre-Louis Moreau de Maupertuis wrote a little book with a big theme and an odd title. (The original measures but 5½ by 3¼ inches and includes fewer than 200 pages of text, printed, thanks to the ample margins of a more generous age, in the even smaller space of 3¼ by 1¾ inches.) He called it Vénus physique—the “physical,” or “earthly, Venus,” or, more loosely, “physical love” (as opposed to the interpretive, spiritual, or psychological dimensions of this subject of the centuries). It presents, as the title implies, a wide-ranging account of the natural history of procreation—a primer in how various animals do it. We learn, for example, in juxtaposed contrast that
the impetuous bull, proud of his strength, does not amuse himself with caresses; he throws himself immediately upon the heifer; he penetrates deeply into her loins and squirts there, in large streams, the liquid that must make her fertile. The turtle dove, by tender calls, announces his love: a thousand caresses, a thousand pleasures precede the last pleasure.
Proceeding down the scale of being (as his century conceived it), Maupertuis reaches the hermaphroditic land snails and discusses their darts. (Many land snails develop a calcareous “arrow” with a beautifully formed tip. In the elaborate rituals that precede copulation, the snail acting as male thrusts its dart repeatedly into its partner’s muscular foot. The dart is not part of the penis, and beyond the obvious observation that it serves some role in sexual stimulation, we still do not know its precise function.) Maupertuis didn’t have an answer either, but he made an interesting, if fatuous, analogy:
What is the function of this organ? Perhaps this animal, so cold and so slow in all its operations, needs to be excited by these stings. Men made cold by age, or whose senses have become enfeebled, sometimes have recourse to equally violent means in order to awake in them the passions of love. Oh unhappy man, who tries to excite by pain the feelings that should only arise from voluptuousness!…Oh innocent snail, you are perhaps the only creature for whom these means are not criminal—because they are for you an effect of Nature’s order. Receive then, and render, a thousand times the stings of these darts that arm you.
At the bottom of the scale, Maupertuis encountered a special problem with hydras, the soft-bodied, freshwater relatives of corals. Maupertuis and his colleagues considered hydras as transitional forms between plants and animals because they reproduce either by budding new individuals off a parental stalk or by regenerating entire bodies from disarticulated fragments of the same stalk. Maupertuis, in no uncertain terms, had identified pleasure as nature’s end in the process of reproduction:
Nature has the same interest in perpetuating all species: she has inspired in each the same theme, and that theme is pleasure. It is pleasure that, in the human species, drives everything before it—that, despite a thousand obstacles opposed to the union of two hearts, a thousand torments that must follow, conducts lovers towards the purpose that nature has ordained.
But if pleasure be nature’s order, then how can the lowly hydra enjoy reproduction by having its stalk cut into pieces?
What is one to think of this strange style of reproduction, of this principle of life extended into each piece of the animal…. In other animals, nature has attached pleasure to the act that multiplies them; could it be that nature has endowed this creature with some sort of voluptuous feeling when it is cut into pieces?
Perhaps these passages inspired Maupertuis’s decision to publish anonymously, although he lived in a century so refreshingly less prudish than the one that followed (while his direct and charming words also stand in such favorable contrast to the perpetual, self-conscious analysis of our own age). Nonetheless, Vénus physique is not primarily a work about the natural history of love, whatever the value of these sections for publicity and immediate renown. It is, for most of its length, a sophisticated treatise on the science of embryology—on the most direct and enduring physical effects of love. The title, perhaps, was a come-on, but the book is a masterpiece.
Maupertuis was born in France in 1698. Although he ranged widely across the disciplinary boundaries imposed by a later age, he won his reputation for work in the physical sciences—both for his courage in introducing and expounding Newton’s work in a nation strongly wedded to Descartes’s alternatives and for directing an arduous expedition to Lapland that affirmed Newton’s prediction of an earth not perfectly spherical, but flattened at the poles. This combination of care and daring won him Voltaire’s s
upport and his star rose. In 1738, Voltaire recommended to Frederick the Great that Maupertuis might be just the man to direct his rehabilitated Academy of Sciences in Berlin. Maupertuis took the job and flourished in it for several years. But a series of involved intrigues brought him down and incurred Voltaire’s undying wrath and the deadly satire of his acerbic pen. Maupertuis was eventually exonerated, but he never recovered his health and reputation, and he died, a broken man, in 1759.
Like many general treatises, the Vénus physique had its origin in a specific problem. In a culture with deep racist traditions, human skin color has exerted a perpetual fascination, and no aspect of the subject inspired more interest than the occasional discovery of peculiar individuals who seemed to breach the boundaries. Jeremiah’s God, pessimistic about redemption among those who had fallen by the wayside, proclaimed: “Can the Ethiopian change his skin, or the leopard his spots?” But some humans did transgress beyond the limits of apparently stable categories, leading to fear that one’s own future relatives might stray or that the categories themselves might not be so comfortably fixed in their conventional statuses of relative worth. Essay 22 discusses a Caucasian woman with large patches of melanic skin, who fascinated a London physician in 1813. But her case was rare and irrelevant. A more general phenomenon was, however, fairly common and thus both threatening and fascinating—namely, albinism among black people. Albinism is well known among most, or all, dark-skinned vertebrate races and species; albino blacks, paler than any Caucasian, are not rare, and the trait is inherited in family lines.
An albino child of black parents had been on exhibit in Paris and Maupertuis’s thoughts and observations served as the inspiration for his Vénus physique. His work bears the subtitle: Dissertation physique à l’occasion du nègre blanc (“A physical dissertation inspired by the white Negro”). Vénus physique contains two parts, the much longer first section on embryology and the natural history of love and a forty-five-page closing statement on the origin of human races. (This second section contains some poorly formed evolutionary speculations and is largely responsible for Maupertuis’s reputation as a Darwinian precursor—an unfair and anachronistic assessment, based on a few fleeting passages that abstract Maupertuis from the concerns of his time. Vénus physique is a treatise on embryology and the exciting debates of his own century.)
This second section features a discussion of human biogeography and tries to explain a false pattern reconstructed from unreliable reports by travelers—a belief that blacks inhabited the tropics, while arctic regions were the exclusive preserve of giants and dwarfs. Maupertuis argues, in short, that superior white races had simply pushed all miscreants and oddballs out of the more favorable temperate regions. We can easily see how the albino child had inspired Maupertuis’s thoughts for this second section, but what influence could he have exerted over the heart of Vénus physique—the first, long section on embryology? An answer to this question provides the key to Vénus physique and a proper assessment of Maupertuis’s creative and unusual opinion in the great embryological debate of his age.
In one of the hottest arguments of eighteenth-century science, students of development lined up on both sides of an ancient dichotomy stretching back to Greek science. Aristotle had argued that embryonic development is both the greatest of all biological mysteries and the key to a deep understanding of organisms—propositions that remain as true today (for our ignorance is still profound) as when the “master of them that know” proclaimed them more than two thousand years ago. Greek scientists had envisaged two broad types of solutions, and their eighteenth-century successors continued to respect the categories. One group, the preformationists, argued that embryology must represent an unfolding of preexisting structure. A tiny homunculus must be curled up in the egg or sperm. It need not be a perfect miniature of the adult—for the relative form and position of parts may change with growth—but the structures must all be present and connected from the first. A second group including Maupertuis, the epigeneticists, argued that the visual appearance of development must be respected as a literal truth. The embryo seems to differentiate complex parts from an original simplicity, and so it must be in reality (preformationists, in response, claimed that contemporary microscopes were too poor to see preformed parts in the tiny and gelatinous early embryo). Embryology is addition and differentiation, not mere unfolding.
We must reject the silly good guy–bad guy scenario usually attached in false retrospect to this tale: namely, that preformationists were blinded by theological prejudice against change of any sort and therefore imposed upon the egg or sperm what they could not observe—while epigeneticists were heroes of empirical science and merely called it as they saw it under their microscopes.
In fact, the preformationists maintained an idea of science far closer to our own. They were the mechanists who insisted upon a material cause for all phenomena. And they were stuck in the limited knowledge of their own century. What alternative did they have? The wondrous complexity of a human body cannot develop mysteriously from an original formless nothingness; organs must therefore be present from the start. Most epigeneticists, on the other hand, were comfortable with a view of causality that we would now reject as “vitalistic”—the idea that an external, nonmaterial force could impose complex design upon a fertilized egg that began with unformed potential alone.
Maupertuis was a conspicuous oddball in this great debate, for he was both a fervent epigeneticist and a committed mechanist. Unlike his epigeneticist colleagues, therefore, he expected to find material precursors for all parts in eggs and sperm. But these parts could not form a prebuilt homunculus. They must be totally scattered and completely disaggregated. They must also exist in numbers far in excess of what the embryo needs (for if eggs and sperm included all the right parts, and these only, then Maupertuis might have been labeled an odd preformationist who advocated a disarticulated homunculus). Embryonic development must therefore represent a selection, sorting out, attraction, and creative joining of these separated parts, not a mere enlargement of structures already fixed in form, place, or number. But how could disaggregated parts come together, and how could the right ones be sorted out and joined (or the wrong ones occasionally incorporated in abnormal fetuses)? The idea of a preformed homunculus seemed to present fewer problems.
Several of Maupertuis’s arguments against preformationism were the conventional retorts of his age. Against the ovists (those who placed the homunculus in the female egg) Maupertuis raised the usual, and always troubling, issue of encapsulation. The egg cells of the homunculus must contain other, vastly smaller, homunculi and so on back to countless generations of inconceivable tininess. All of human history, in fact, must have been prefigured in the ovaries of Eve.
Eggs destined to produce males each contain only a single male. But an egg with a female contains not only that female, but also her ovaries, in which other females, already fully formed, are enclosed—the source of infinite generation. Can matter be divisible to infinitude; can the form of a fetus that will be born in a thousand years be as distinct as the one that will be born in nine months?
And why then do males exist at all? Does their semen merely release and inspire the previously lifeless homunculus? Was this, Maupertuis asks, the fire that Prometheus stole from the gods?
Against the spermaticists (those who placed the homunculi within sperm cells), Maupertuis raised the additional problem that many million cells are expelled in each ejaculation. Could nature be so profligate and endow millions of unused cells with perfect homunculi that will never enjoy life?
This little worm, swimming in the seminal fluid, contains an infinity of generations, from father to father. And each [homunculus] has his seminal fluid, full of swimming animals so much smaller than himself…. And what prodigiousness when we consider the number and tiny size of these animals. One man calculated that a single pike fish, in one generation, could produce more pikes than there are men on earth, even assuming that all the ear
th is as densely populated as Holland…. Such an immense richness, such a fecundity without limit in nature; do we not have here a prodigality of resources! May we not say that the expense and outlay are excessive!
Maupertuis ventured an alternate, and interestingly incorrect, function for the recently discovered “spermatick animalcules,” as his English colleagues called them. He imagined that they stirred and mixed the seminal fluids of male and female, thus bringing together the parts that must form the embryo.
But Maupertuis added to this great embryological contretemps some new arguments and a strikingly original perspective. For centuries, the turf of debate had been the embryo and its observable process of development. True creativity often resides in the joining of previously disparate fields, in the recognition that apparently unrelated phenomena from other disciplines may offer solutions to old dilemmas. And so, finally, we return to albinos and to Maupertuis’s creative insight.
Maupertuis was among the first European scientists to trace the pedigrees of unusual traits through family lines. He recognized that these results, apparently so unrelated to embryology, might solve the great debate in favor of epigenesis. He compiled a pedigree for polydactyly (extra fingers) through three generations of a German family and proved one cardinal fact, long touted in anecdote and folklore but never conclusively established: inheritance passes through both male and female lines; that is, extra fingers could be inherited from either fathers or mothers. Maupertuis then recognized that this feature of heredity rather than any direct aspect of embryology, might resolve the problem of development; for how could preformationism be supported if both parents can contribute to the form of their offspring. If homunculi are curled up in eggs or sperm, the noncontributing parent should not play an equal role in the form of its offspring. And what about hybrids who bear characteristic features of two parents belonging to different species? Maupertuis concluded:
The Flamingo’s Smile Page 12