Purpose and Desire Read online

  An enduring legacy of this logic was Cuvier’s famous theory of the correlation of parts,* to which the modern science of allometry traces its roots.* The correlation of parts was the operational system for the conditions for existence: the meat on the bones, so to speak. If all parts of an animal had to function well together to meet the conditions for existence, a change of one part by itself would violate those conditions. There would therefore have to be specific correlated changes of multiple parts to ensure that the organism’s conditions for existence could be met as the body changed. Furthermore, not just any correlation would do. If the upper limb bones—the femur and humerus—of, say, leopards elongated over time to enable their descendants to run faster and become cheetahs, a correlated reduction of the length of the bones of the lower limb would not help the leopard’s cheetahlike descendants to run faster: the conditions for existence for fast running would not be met, because the total lengths of the limbs would be unchanged (Figure 5.3). Only a correlated elongation of both upper and lower limb bones would meet the conditions for fast-running cheetahs to exist.

  This notion fits neatly into Cuvier’s interpretation of the fossil record as intermittent bouts of catastrophic extinction, interspersed with long periods of stasis in which the form of lineages would not change. Cuvier took this as evidence of how difficult it was to change from one set of conditions for existence—those that made leopards possible, for example—to another set of conditions for existence—those that made cheetahs possible. One would expect, therefore, to see these correlations in the fossil record as generations of leopards became cheetahs. It was only when things were mixed up precipitously at an extinction catastrophe that a new set of conditions for existence among the survivors could begin to be negotiated and settled upon. Once settled upon, it was difficult to nudge lineages out of their new accommodations, sustaining the long periods of stasis Cuvier had claimed in the fossil record.

  Figure 5.3

  Cuvier’s correlation of parts model for adaptation. For a leopard to be transformed into a fast-running cheetah, there must be a coordinated elongation of both the upper limb (u) and the lower limb (l).

  Cuvier worked out these particular correlations in exhaustive and exquisite detail for a variety of animals, both living and extinct, to the point that he could boast that he could reconstruct an entire animal from a single tooth. The shape of a tooth was related by a particular correlation to the jaw. Know the tooth, and you will know the jaw. The jaw, for its part, was correlated in a particular way to the skull, which was correlated to the spine . . . and so on. Know the tooth, and if you know the correlations, you will ultimately know the tiniest hairs on the tip of the animal’s tail. Behind the braggadocio, there was an important point to Cuvier’s paleontological prestidigitation: the correlation of parts is a clear example of the organism’s “many little lives” working out their mutual accommodation within the context of the well-functioning organism. Indeed, you can see in Cuvier’s thought the seeds for Bernard’s own speculations about the constancy of the milieu intérieur: they are drawn from the same philosophical well.

  There’s another similarity that has bedeviled our understanding of Cuvier’s evolutionary thought. Just as Bernard saw homeostasis as a fundamental property of living things, Cuvier saw the same in his conditions for existence. The specific correlations that had to exist for the animal to be well-adapted betokened a fundamental self-knowledge that permeated the organism. All the parts had to know, in a deep sense, how to fit in with one another, and to be capable, again in some deep sense, of working out an accommodation with the other parts that carried their own sense of knowledge and striving toward a destiny. At root, the “many little lives” metaphor was a statement of life as fundamentally a cognitive and intentional phenomenon. Purpose and desire, in a nutshell.

  What of Darwin? Did he not leave all that quaint vitalist theorizing behind? Well, not really . . .

  It’s worth remembering that Darwin spent his formative years in an intellectual milieu that was similar to Lamarck’s and Cuvier’s and that was animated by the central question of nineteenth-century scientific vitalism: how did the well-constructed, coherent, and adaptable organism come to be? The oft-cited metaphor of evolutionism being something “in the air” that wafted over the English Channel from France, a spirit that Darwin just inhaled and brought to fruition, paints Darwin as more of an intellectual blank slate than he actually was. His novel thinking has to be understood in the broader context of nineteenth-century debates over the nature of adaptation.

  We have seen the tumult that was roiling the medical faculties of Europe over this question. On the English side of the Channel, the notion of a marvelously designed living world had long planted its foot in the English school of natural theology, championed by a long line of thinkers from John Ray to William Derham to Thomas Malthus and culminating in the vivid apologetics of the great William Paley (1743–1805).10 Darwin knew this tradition well but came into it sideways, as he did with so much of his thinking. He had come to know and admire Paley’s thought and work when he was ensconced as a young man at Christ’s College, Cambridge.* But he arrived at this point only after a circuitous path.

  Charles Darwin was part of an extended family of free thinkers and liberals, part of the new professional elite that grew up during the Industrial Revolution.11 He was born in Shrewsbury, in Shropshire, along the northern border with Wales. This area had seen considerable industrial development through the eighteenth century, although William Blake’s “dark Satanic Mills” were less in evidence here than they were elsewhere in England.* Instead of belching coal, the clean waters of the River Severn provided most of the power that turned the wheels of the local textile mills. Shropshire was also blessed with extensive deposits of fine clay, which provided the raw materials for the fortune that elevated the Darwins into the upper middle class. This was the fine china produced by the Wedgwood family, whose family tree was intertwined closely with the Darwin’s. But where the Wedgwoods became industrialists, the Darwins became professionals—physicians who could serve the growing numbers of people streaming into the towns from the countryside. Indeed, Charles was, for a time, destined to follow his father and grandfather into the family profession and was duly sent off to study medicine at the University of Edinburgh, where his father and grandfather had studied. There, Charles soon found he could not stomach the dissections and suffering that were part of his medical curriculum, and he left Edinburgh after one year. Exasperated by his son’s failure at medicine, Charles’s father directed him to the other respectable station for diffident sons of the middle class: study at Cambridge to enter the Anglican clergy. It was there that Charles came to know Paley’s Natural Theology (1802), which was de rigueur for aspiring clerics, for it offered irrefutable proof of the existence of a wise, beneficent, and all-knowing deity. Paley was also important reading for students of natural history, toward which Darwin, with his obsession for beetles, shooting, and rat-catching, was drawn as if by gravity.

  The fertile ground of Darwin’s mind had been long prepared more by his family’s profession of medicine rather than by natural history. Charles’s grandfather, Erasmus, had studied medicine at Leiden, which was involved in the tumult over the transformation of vitalism that was roiling the eighteenth-century European schools of medicine. Consequently, there were many close connections between the medical schools at Leiden and Edinburgh. Charles’s father, Robert, had also studied medicine at both universities, and he certainly would have been familiar with the doctrines of scientific vitalism that were emerging there.

  Much of the history of evolutionary thought, and of medical thought for that matter, can be understood as an ongoing argument between two competing visions of nature: Romantic idealism and Enlightenment rationalism. Vitalism, and the insistence on the ineffable nature of the organism, was a reflection of the Romantic, as was the whole notion of natural theology as it applied to natural history. The English tradition of medicine, on the other hand, tended to lean toward the rational: the great physician William Harvey was English, after all, and Cambridge University was the font of Archibald Pitcairne’s “Newtonian medicine,” which sought to organize medical practice as a set of mathematical axioms.12

  The tension between Romantic idealism and Enlightenment rationalism twined itself through the Darwin family tree. Robert Darwin was by all accounts a stolid, responsible, hardworking practitioner of practical medicine, by which he grew wealthy through assiduous attention to the everyday details of sickness and health. Erasmus Darwin, on the other hand, was a noted free thinker and admirer of the more Romantic tendencies of the voluble French. Both tendencies intertwined in Charles’s idyllic youth ranging through the Shropshire countryside. From his grandfather, Charles became familiar with the radical concepts of species transformation streaming in from France: Erasmus had speculated about evolution and natural selection in several of his works, including his epic Zoonomia (1796), as well as in poems such as “The Loves of the Plants” (1789) and “The Temple of Nature” (1791). Erasmus Darwin was also no friend of mechanism, as can be seen in his preface to Zoonomia:

  It happened, perhaps unfortunately for the inquirers into the knowledge of diseases, that other sciences had received improvement previous to their own; whence, instead of comparing the properties belonging to animated nature with each other, they, idly ingenious, busied themselves in attempting to explain the laws of life by those of mechanism and chemistry; … forgetting that animation was [life’s] essential characteristic.13 (emphasis added)

  Physics envy was a problem, it seems, even in the early nineteenth century.

  In the heroic Darwinian story, Charles left all that muddy vitalist nonsense behind as the clear sun of natural selection
began to dawn in his mind. It is certainly true that Darwin had critical things to say about both Lamarck and Cuvier—he disputed Lamarck’s emphasis on progressive evolution and thought Cuvier’s catastrophism was more simply explained as gaps in an incomplete fossil record. Nevertheless, the two core ideas of Darwin’s own theory bear the unmistakable stamp of nineteenth-century vitalist thought. Success in the “struggle for existence” boils down to apt function (physiological adaptation), which draws inspiration, albeit flawed, from Lamarck’s “adaptive force” applied to lineages. Homology, cited by Darwin as strong evidence that lineages could evolve through gradual modification of existing parts, drank deeply from Cuvier’s notion of the correlation of parts and the mutual accommodation of the organism’s “many little lives.”

  Indeed, in the cold light of day, Darwin’s solution to the problem of evolution—natural selection—was not even especially original. Erasmus had crab-walked up to the idea of natural selection several decades before his grandson claimed it as his own. There were also at least two rival claimants to the idea of natural selection who preceded Darwin by many years, perhaps because Erasmus had put the idea “in the air,” as Lamarck and Cuvier had supposedly done for evolution.* Finally, there is that remarkable coincidence of “Darwin’s” idea independently popping up in Alfred Russel Wallace’s fever-addled head.*14 This is not to gainsay Darwin’s own achievement, of course. What made Darwin stand out was his supreme talents as a naturalist, his attention to meticulous detail in observation, and his habit of delving exhaustively into evidence. These habits transferred to paper make for tortured reading, but they allowed him to see the problem of evolution from a perspective that neither Lamarck nor Cuvier nor any of the other armchair claimants to Darwin’s idea could see or even remotely begin to defend.* Where Lamarck saw striving for perfection and unrelenting progress and sought a “system” to explain it, Darwin looked to nature as it was and saw evolution’s sometimes whimsical tricks—and saw that these deserved explanation too.15

  Darwin also recognized the fatal problem lurking in any theory of evolution that did not somehow synthesize apt function with heritable memory. Lamarck had seen the connection but had failed to close the deal—he had said only that they might be combined somehow but had taken it no further. And this is why Darwin undertook to develop his own synthetic theory for heredity, which he trotted out for the world to see in his 1868 book The Variation of Animals and Plants Under Domestication.16

  Darwin’s solution to the heredity problem was a mechanism he called pangenesis. His idea was that adaptation and heredity were melded through the agency of gemmules, tiny particles that all the body’s cells supposedly shed over the course of their lives. Gemmules were like little identification nanochips, containing information about the cell that spawned them: muscle cells shed gemmules that were distinct from those shed by bone cells, and both would differ from gemmules derived from brain cells, and so forth. These myriad nanochips circulated throughout the body during an animal’s life and eventually came to rest in the germinal tissues: the testes and ovaries. There, the gemmules formed a sort of genetic parliament that decided what manner of gamete the sperm or egg would be. If a particular somatic tissue—say, the muscles and bones of the neck of a hypothetical proto-giraffe living in a savanna—was used more, or grew disproportionate to other tissues during the animal’s life, gemmules from the neck tissues would be represented in higher proportion in the germinal tissues of savanna-dwelling proto-giraffes than they would be in, say, forest-dwelling proto-giraffes, where the lush leaves were closer to the ground and access to them was easier. When breeding time came, embryos of savanna-dwelling proto-giraffes would start life with more neck gemmules and would develop more neck—longer necks—than would the offspring of forest-dwelling proto-giraffes. Carry this on for many generations and two distinct lineages would emerge: one in savannas leading to the long-necked giraffe, and the other in forests leading to its shorter-necked relative, the okapi.

  This familiar example of the giraffe just-so story* should look familiar because it can be found in nearly every biology textbook written since. Usually, the giraffe story is filed under Lamarckism, but in all fairness it should be filed under Darwinism, for Darwin’s theory of pangenesis was a Lamarckian scheme for the heritability of acquired characteristics across generations. The principal difference is that Lamarck’s influence des circonstances—the adaptive force that caused disused organs, like cave fish eyes, to disappear over generations—was transmuted by Darwin into the tiny particulate gemmules that accomplished the same thing. Pangenesis thus puts Darwin squarely into the camp of the much-derided Lamarck.17

  It’s worthwhile asking, therefore, why it is Lamarck and not Darwin who is tarred with the brush of the inheritance of acquired characteristics, the wrong-headed theory of inheritance that supposedly discredits Lamarck. Part of it may be due to style: where Lamarck built grandiose unworkable systems, Darwin painstakingly ground his way through evidence, the quirkier the better.

  The famous story of Lord Morton’s mare provides a telling example of Darwin’s approach. The Scot George Douglas was the 16th Earl of Morton (therefore, Lord Morton). He was a well-regarded breeder of horses: among his projects was an attempt to domesticate the quagga, a South African subspecies of the Plains Zebra. Unlike the zebra, whose coat is striped all over, the quagga’s stripes were limited to the neck and some markings along the legs. The quagga also had in common with the zebra the stiff-haired mane, which set it apart from the horse’s mane, which is long and flowing. The quagga, sadly, is now extinct, the last living mare having died in 1870 at the London Zoo. But they were very much present in 1820 when Douglas undertook his effort to domesticate the animal.

  Douglas could acquire only a male quagga for his project, so he decided to breed his acquisition with one of his chestnut mares. The offspring of that mating, as would be expected, showed many characteristics of a hybrid mating: it mostly looked like a horse but had striping along the legs, stiff hairs along the mane, and some subtle zebralike shaping of the face and mouth (Figure 5.4). But what came next was decidedly odd. Douglas then mated his chestnut mare with an Arabian stallion, which you would expect to produce a colt that blended the traits of two horse parents: the chestnut mare and the Arabian stallion. Instead, the colt showed remnants of the striping and stiff-haired mane of its quagga stepfather. To horse breeders, this was a quirky mystery, a bit of lore to ponder as part of the horse breeder’s art. To Darwin, it underscored a deep lesson about heredity. How else could the story of Lord Morton’s mare be explained unless there were some form of heritable particle—quagga gemmules, we might call them—that could impart “quagga-ness” onto the chestnut mare’s future ova?

  Figure 5.4

  The offspring of Lord Morton’s mare. (left) The quagga-horse hybrid. Note the striping along the neck and hind legs and the stiff, short hairs of the mane. (right) The subsequent offspring of Lord Morton’s mare and an Arabian stallion. Note the striping of the hind legs and the stiff hairs of the mane. From Charles Hamilton Smith’s 1841 The Natural History of Horses.

  Add to this quirky little story twenty-six chapters of characteristically dense Darwinian prose, painstakingly bolstered by other examples drawn from hereditary diseases of the eye to the variegated leaves of plants to the effects of heat on the hides of South American cattle, and you come out with Darwin’s pangenesis theory. Darwin came to the only conclusion he reasoned could explain these innumerable examples: it had to be pangenesis, or something like it.