For us today, the connection between individual development and evolution is something that can be taken for granted. Back in the eighteenth century, however, there would have been no reason to expect or imagine that the embryonic development of an individual should have anything to do with the overall plan of nature. We have nature-philosophy to thank for making this connection explicit, for it sought correspondences between the microcosm of individual development and the macrocosm of nature.
The science of comparative embryology that sprang directly from nature-philosophy in the mid-nineteenth century shed the mystical baggage of its progenitor, but still held strongly to the notion that the events in the development of the individual reflected its station in nature. Without knowing they had done so, the nature-philosophers had managed to explain, in one coherent scheme, two essential properties of the genome: first, the course of individual development, and second, how changes in development are recorded in evolution, allowing the careful experimenter to use embryology to unlock the evolutionary history of a species.
Nature-philosophy would never have been able to explain another property of the genome – the origin of diversity. Nature-philosophers still saw the world as a static statement of Creation in which each species represented an instance of some divine plan, or archetype. If nature was to be anatomized Man, then the reason lay in the divine plan, and that was that. The abundance of one species or another was not something that could be explained without recourse to the divine. Had God intended there to be a million species of beetle but only one coelacanth, then that choice rested solely with God, and it would be as pointless to ask why matters should not be otherwise as it would to speculate on the metaphysics of beings that might exist on other worlds.
There was another reason why the conception of a static nature militated against any investigation of diversity. An understanding of diversity rests on an appreciation of variation – the small differences between individual animals or plants. Nature-philosophy, though, stressed similarity. In their constant striving to find patterns to unify disparate nature, Goethe and Geoffroy would have regarded natural variation as a distraction. The discounting of variation was a consequence of the adherence of nature-philosophers to the classical ideal of the archetype, of which each earthly instance was but a poor copy. The fact that mundane cats may be black, white or tortoiseshell was therefore never of interest in itself, just a consequence of earthly imperfection – it would never be possible to achieve an exact representation of the divine conception of ‘cat’ in an imperfect world. So matters stood until Darwin’s insight that variation was not, in fact, the noise that cluttered up the signal. Variation was the signal.
The young Charles Darwin (1809–82)1 was the unpromising scion of a wealthy doctor’s family in the English Midlands, related to the famous Wedgwood ceramics dynasty. Charles’s grandfather, Erasmus Darwin (1731–1802), was a noted scholar in what might be called an English strand of nature-philosophy. Erasmus was fortunate in that he lived in an age of cultural enlightenment and religious tolerance. Expansive in figure and dress as well as prose and poetry, he was able to entertain ideas of the transmutation of species, and was thought only mildly eccentric for doing so.
The Darwins and the Wedgwoods were nonconformists – that is, they were Christians, but unattached to the established Church of England. Erasmus’s grandson, though, grew up during a period of increasing political and religious conformity in England, a reaction to the revolutions that had struck Europe in the late eighteenth and early nineteenth centuries – the same revolutions that had shaped the ideas and careers of Geoffroy and Cuvier. Not that such elevated themes preoccupied the young Darwin. He preferred robust country pursuits to scholarship – he was a hearty, outdoorsy type much given to hunting and shooting, with few cares about how he might make his way in the world. In 1825, aged just sixteen, his respectable, physician father sent him to Edinburgh to study medicine, but Darwin found the anatomy demonstrations stomach-churningly grisly. While he was there, however, he fell under the influence of lecturers on the radical fringe of academic society who were prepared to entertain the idea that species were not fixed by divine law, but could change according to the circumstances in which they found themselves. This idea of the transmutation of species was, by the 1820s, considered as seditious as anything in revolutionary Europe – something to be scorned by establishment scientists such as Cuvier – whereas in Erasmus’s day they were thought merely extravagant and exotic.
After a year of medicine at Edinburgh, Darwin drifted into natural history before dropping out completely. The standard destination for people in his position in those days was the established Church of England, and so the squeamish Darwin was dispatched by his despairing parent to Cambridge, where in due course the young man took a degree in divinity. At the time, the old universities – Oxford and Cambridge – were seats of Anglican conformity. College fellows, irrespective of what they taught, were first required to be ministers. Darwin received his instruction in botany from one such divine, the Reverend John Stevens Henslow (1796–1861), who became a firm friend. In an alternate universe Darwin would have become a minister, taken up a quiet country living, and spent the rest of his life botanizing and sermonizing. But his life was about to take a most unexpected course.
The early nineteenth century was the great era of maritime exploration, particularly in Britain, then at the zenith of its naval might and political influence. Ships were dispatched to remote places to make observations for accurate maps and to record anything else of interest – including the indigenous flora, fauna and geology. One such survey ship, HMS Beagle, was due to set sail on a trip around the world, concentrating on a survey of the Atlantic coast of southern South America.
The long isolation of such voyages posed a problem for the Beagle’s captain, Robert Fitzroy (1805–65). Social niceties prevented him from fraternizing with his crew – yet the effective solitary confinement this imposed would strain his mental health. Fitzroy was well aware of the insanity that ran in his family. A relative, Robert Stewart, Viscount Castlereagh, a politician of consummate skill who had served as Foreign Secretary throughout the Napoleonic Wars, had committed suicide. What Fitzroy needed was a companion who could sustain five years of dinner-table conversation at a level sufficiently elevated to save him from his inherited demons.2 News of this opening came to Henslow, who put Darwin’s name forward. Darwin was affable and sociable, with the bearing and background of a gentleman, and so might have been expected to be capable of cheering up the morose captain. Henslow, of course, recognized Darwin’s acumen as an observer of nature and thought he would profit from exposure to the biological diversity of the tropics. Darwin’s father, no doubt, hoped that this expedition would be the making of his feckless offspring.
On 27 December 1831, Darwin set sail on the Beagle as unpaid companion to the captain. In this task he was largely unsuccessful. Although they were social equals, they were poles apart politically and religiously. Fitzroy was High Church and High Tory; Darwin was a Whig (that is, a Liberal), and beneath a thin Anglican veneer there lurked the nonconformist tendencies of his Darwin ancestors and Wedgwood cousins. Darwin spent less time than planned with the captain, and more on natural history, to the chagrin of the official ship’s naturalist, who abandoned the expedition when the Beagle reached South America. Apart from some seasickness, Darwin was in his element – collecting flora, fauna and fossils with enthusiasm and sending crates of specimens back to Henslow and other scientists, all the time making acute notes on everything he could find. After a long period surveying the coasts of Brazil and Argentina, during which time Darwin spent extended periods exploring ashore, the Beagle rounded Cape Horn, and in the autumn of 1835 spent a month among the bleak, rocky Galapagos Islands in the Eastern Pacific. The voyage continued across the Pacific to Australia and then, by degrees, home. Darwin made landfall in England on 2 October 1836.
The voyage of the Beagle had indeed made a man of Darwin. The cornucopia of specimens he had sent home over five years, not to mention his popular travel book, The Voyage of the Beagle (1839), had turned the college drop-out into a respected scientist and minor celebrity. Yet he found his fame uncomfortable. He hid from the public eye in a quiet Kent village, choosing the sedate life of the leisured gentleman, marrying his cousin Emma Wedgwood in 1839. He fathered ten children, performed a multitude of experiments in his home laboratory and greenhouse, and, painstakingly, wrote up his research.
There has been much speculation about why it took Darwin more than twenty years to publish the idea of evolution by natural selection. The answer is complicated. A busy family life would have been one factor: from 1839 until 1856, almost the entire period in which Darwin was collecting his thoughts on evolution, there was at least one school-aged child at home. During this period, Darwin was frequently laid low by a series of debilitating illnesses whose cause – or causes – remain mysterious. The pervading atmosphere of political conservatism and religious conformity, antithetical to radical ideas such as the transmutation of species, would have been another reason for his holding back. Darwin might have felt he owed too great a debt to his father, to Fitzroy, to his various patrons, such as Henslow, and most of all to his devout wife, to be able to publish such an idea with impunity. And there is another, more mundane reason: the idea took time to coalesce into an easily expressible form, especially as the idea of natural selection broke genuinely new ground in the history of thought. Darwin, a naturally cautious man, painstakingly exhausted every avenue of investigation before going public.
Conventional mythology has it that Darwin’s visit to the Galapagos Islands was pivotal in the genesis of the idea of natural selection, but it was not as simple as that. This remote group of dry, volcanic islands is home to a range of peculiar creatures not seen anywhere else in the world. Many of the islands have (or had, until recently) their own species of giant tortoise. There are iguanas that – unusually for lizards – swim in the sea, and a species of flightless cormorant. There are also several species of finch, each unremarkable in appearance but adapted to a particular dietary niche. Some have thin, probing beaks for extracting insects from crevices within bark. Others have thick, robust beaks for cracking nuts and seeds. Darwin looked at these finches as a group and speculated that they were all descended from a single ancestor, or a few ancestors, that arrived on the islands from mainland South America, presumably by accident. Once there, each island’s finch population became adapted to the local conditions. After millions of years, the process of adaptation to these different environments – natural selection – moulded the ancestors of the birds in each habitat until, generations later, the result was the variety of finches we see today. It would not be true to say, however, that Darwin observed the finches, made a few notes and had a ‘eureka’ moment. The visit to the Galapagos Islands made relatively little impression on Darwin at the time. Besides, there is no reason to suspect that Darwin’s own thinking had advanced enough for him to know that the fauna of this out-of-the-way place was significant, apart from its evident yet parochial oddity.
It is a common mistake, when looking back at the history of figures that posterity has made into heroes, to attribute to those people an insight they did not have. Darwin was not a Darwinist – he was a conventional Christian, more or less, although one who, through his family connections and university experiences, had come up against exotic ideas such as the possibility that species might not be immutable, but could in some circumstances be transmuted into new forms. The Darwin who boarded the Beagle would have been sympathetic at least to this idea of transmutation, in the same general way that his family had historically entertained liberal views on politics and religion. Darwin was no out-and-out transmutationist, and even if he were, he would have had no greater insight into the possible mechanisms of transmutation than would any other well-informed observer of his day.
The exposure of this naturally observant young man to the wealth of tropical diversity left him intrigued by the possibilities of variation and transmutation, but it was not clear to him how the two might be related. It is certain that Darwin – before, during and after his voyage – was well-read. He knew and admired the work of European scholars such as Von Baer and Goethe, derived from or influenced by nature-philosophy. These scholars saw a pattern to nature, but not a pattern understood to have been generated by the transmutation of species. Nevertheless, nature-philosophy might have given Darwin a clue. Nature-philosophers stressed that the largest patterns in nature were evident in the smallest things, that the microcosm mapped the macrocosm. The early embryologists, inspired by nature-philosophy, applied this philosophy to the origin of form, showing that every embryo replays its own evolutionary heritage over the course of its development. Darwin, however, took nature-philosophy in a new direction by showing how individual variations on the most trivial scales could, through transmutation, be harbingers of the greatest evolutionary changes.
A problem for anyone sympathetic to the idea of transmutation of species was the great length of time that the process would require – far greater than the few thousand years allowed for in the Bible, then the supreme authority on the history of the Earth. If any changes had happened on the planet since its creation, they would have to have been sudden and dramatic. When fossils started to be collected in a systematic way, in the eighteenth and early nineteenth centuries, and it became evident that the creatures to whom these bones belonged were extinct, it was suggested that they perished in a sudden catastrophe. The biblical flood of Noah was said to stand for several cataclysms in which organic life perished, each time to be created anew in an improved form. Cuvier was a prominent advocate of this ‘catastrophist’ view.
A new strain of thought emerged among geologists who found that evidence for convenient disasters was rather thin on the ground. They suspected that catastrophes were something that characterized ineffably remote ages, and happened rarely, if at all, in the modern world – in the same way that the Deity seemed to have spent a lot of time intervening directly in the lives of the Patriarchs, but had lately got out of the habit of manifesting himself as a burning bush or turning people into pillars of salt. Examining the world around them, what geologists saw was a landscape shaped by the kind of gradual processes we see all around us today: the slow accumulation of change in which, for example, raindrops take millions of years to turn a mountain, grain by grain, into a valley, rather than the violent irruptions required by the catastrophist view, which conveniently allowed for great change to be accommodated within the chronological confines of scripture. This new tendency of geological thought, set up in opposition to catastrophism, was the uniformitarian school, and its leading exponent was Charles Lyell (1797–1875).
The first volume of Lyell’s book Principles of Geology had been published just before Darwin embarked on the Beagle, and it was Darwin’s favourite reading during the voyage. The tale of the slow, gradual processes that have shaped the Earth, over immensities of time not accounted for in the Bible, must have spoken to the nonconformist, liberal side of Darwin’s nature. If the Earth was immensely – perhaps immeasurably – old, there would have been time enough for species to transmute, and Lyell even suggested a form of transmutation in which species would become adapted to their environments. Lyell’s book provided the backdrop against which Darwin would eventually stage his drama of evolution by natural selection.
When Darwin arrived back in England, he had other matters besides transmutation to occupy his mind. There was the curation of the specimens he’d sent back from the Beagle voyage; there was marriage, children, writing up his adventures and his continuing and prolific researches into everything from the taxonomy of barnacles to the growth of orchids. Amid all this activity he began to arrange his thoughts about transmutation. He set out to synthesize his own observations, from the Beagle and from his experiments in the greenhouses and gardens at his home, Down House, with the Lyellian view of geology, with the picture of idealized nature as promoted by the nature-philosophers, and with the more revolutionary ideas of transmutation as suggested by his grandfather and his teachers at Edinburgh.
Some years elapsed before Darwin started to bridge a gap that nobody else had dreamed even existed. The great theme of transmutation over the unthinkably vast intervals of time suggested by Lyell might be rooted in the commonplace variation between individual creatures that we can observe in the here and now – a new reading of the nature-philosophic connection between the microcosm and the macrocosm. Time and chance acted on finches, on giant tortoises, on breeds of dog, to produce a distinct variety, each suited to its own circumstances as Lyell had suggested. But how would each variety become suited to its environment, rather than vary in some other way, or not at all? Darwin had an answer in a mechanism he called natural selection. This term is shorthand for the action of environmental circumstances on a population of creatures that produces more offspring than there are resources to support them. As a result, only those offspring best adapted to the prevailing circumstances will survive for long enough to reproduce themselves. Over many generations, the complexion of the population will come to reflect those better-adapted offspring and their descendants. For this scheme to work, the population has to be varied in constitution, and this variation has to be inheritable.
Crucially, natural selection contains no element of destiny or direction. It was only later that scientists such as Ernst Haeckel (1834–1919) misappropriated natural selection as a motor for a kind of evolution based on destiny – the kind of quest for perfection promoted by nature-philosophy. However, Darwin himself was not immune from such ideas – and why should he have been? In his notes on transmutation, and reading widely from the then-current nature-philosophy, he tried out a number of concepts to see how they suited, including the evolution of complex life from spontaneously generated particles, or monads; the idea that species, like individual creatures, have cycles of life and death (a notion inspired by Lyell’s own ideas about cycles in geological time); and the idea that transmutation might have some directional, progressive character. But spontaneous generation, monads and ideas of progression were gradually stripped away as Darwin came to understand that only three things are necessary for natural selection to work – time, the fact that creatures generally produce more offspring than the environment can support, and the inherited variation in these offspring.
Lyell’s Principles of Geology provided Darwin with the ingredient of time. The element of superabundance came later, after Darwin had begun to construct his theory of evolution as something directed and progressive – which was ultimately antithetical to his final theory of natural selection. Such ideas were abandoned some time after 1838, when Darwin read Essay on the Principle of Population by the economist Thomas Malthus (1766–1834), first published as a pamphlet in 1798 but expanded in successive editions up to 1826. Malthus’s essay started with the premise that whereas populations grow in geometric progression, the resources necessary to support them will grow at a slower, arithmetic rate. Thus populations will outstrip their resources until checked by famine or war, if not by self-restraint. This idea struck a chord with Darwin, who found within it the ingredient he needed – a mechanism that would make evolution work.
Malthus was working during the Industrial Revolution, perhaps the greatest social upheaval in England since the Black Death of the fourteenth century. During the Industrial Revolution, the social, economic, political and even the physical landscape changed beyond recognition in the space of a few decades. People left the land and the populations of cities surged. Some people, such as Darwin’s Wedgwood cousins, grew rich from industry. Many others remained poor – the new urban, working class. Crowded into slums, working people experienced poor health, epidemics of diseases such as cholera, and high infant mortality. Until the twentieth century – and then only in those societies equipped with adequate sanitation and pension provision – people relied on their children to look after them in their old age. But because infant mortality was high, people had as many children as possible in the knowledge that many had to be born to ensure that a few would survive long enough to become breadwinners themselves. Such problems were particularly acute in the overcrowded slums of the growing industrial cities, and the resulting poverty, and its alleviation, was what concerned Malthus. However, families on all social levels were stricken by the deaths of infants and children. Even the Darwins, comfortably wealthy as they were, lost two of their ten children in infancy. A third, their daughter Anne Elizabeth, died from scarlet fever at the age of ten, a blow from which Charles Darwin never fully recovered.
Darwin read Malthus and made a dramatic leap of intuition: what applied to the urban poor could be true of the entire world of nature. Darwin had long observed that animals and plants produce vastly more offspring than can ever be supported either by the parents or the resources available, simply so that a few would survive to reproduce themselves. You do not have to travel to the Galapagos Islands to see this principle in action. Every oak tree in the park produces thousands of acorns each year, each one the germ of a new tree. But if every acorn became a tree, we would soon be unable to move for oak forests. In the real world, most acorns rot or are eaten by animals long before they have a chance to take root.3
Darwin built upon Malthus’s insight to observe that, of all the offspring produced by animals and plants, only those most suited to the environment would survive to reproduce – only those acorns most adept at avoiding being eaten would become oaks. This is why acorns are tough and poisonous rather than tender and delicious. Only the toughest and most poisonous of them will last long enough to grow into trees. (If fruits are tender and delicious, this is a ruse to attract birds or other animals who will eat the fruits and disperse the hard and resistant seeds within.) In the ‘struggle for existence’ as painted by Malthus, only a few offspring would be tough enough, or ‘fittest’, to survive. But because the world is a harsh place, a superfluity of offspring must be produced, just to be sure that a few will be capable of survival and reproduction themselves.
Lyell gave Darwin the time, and Malthus gave him the abundance. The final ingredient was variation. Variation is the keystone of Darwin’s theory, for within it lies the possibility of change, and it was Darwin’s connection of variation with change that gave him the idea of natural selection. If all offspring – whether of oaks or people – were exactly the same, it would hardly matter which among all the millions survived: it would be a lottery. But if offspring differ from one another, even slightly, the possibility is raised that some will just happen to carry traits favoured by the prevailing environment, and these individuals will have a greater chance of surviving to perpetuate the species, at the expense of their fellows. Over generations, the pool of traits carried by the species as a whole would be enriched in those favoured traits, and depleted in those less favoured. Given the large intervals of time offered by Lyell, the species itself would have the potential to transmute into others, as time and circumstances allowed.
After his return to England, Darwin started and maintained a voluminous correspondence with people of every kind, with the aim of understanding the extent of variation and how it could be moulded and influenced. Among his correspondents were stock breeders, nurserymen and pet fanciers – people who set store by the promotion of desirable traits in animals and plants, and the elimination of undesirable ones. The Origin of Species contains much of interest on this score, especially concerning pigeons. From common species of wild pigeon, fanciers have created a range of breeds not found in the wild, such as pouters, tumblers and fantails, by a long process of selection. A breeder will select those birds with most signs of the favoured trait (a more fan-like tail, for example) and allow only those birds to breed. The process is repeated, so that over many generations this trait will come to predominate. Such is the ‘artificial’ selection which Darwin used as an analogy for the grander process in which nature takes the place of the pigeon fancier, selecting which of the many offspring of an animal or plant, in any generation, will survive long enough to reproduce.
Pigeon fanciers, and breeders in general, can produce dramatic changes in the appearance and behaviour of animals and plants in much less than the span of a human lifetime. Given the immensities of time granted by Lyell and his colleagues, nature would have had ample opportunity to create the whole diversity of life from some primordial creature floating around in the deeps of time. And as pigeon fanciers can create several distinct varieties from a single stock of pigeons, so could nature produce different species of, say, Galapagos finch – from a single pair of progenitors. This leads to the insight that because natural selection, in any generation, must work with what it has to hand, and cannot start again from the beginning each time, the form of a species will be moulded as much by its heritage as by its current circumstances, a realization that immediately reconciles the seemingly antithetical views of Geoffroy and Cuvier. It follows from this species-on-species accumulation of heritage that the forms of embryos necessarily track the changing forms of the adults into which they will grow. The nature-philosophers saw the connection between individual development and large-scale patterns of form in nature, but Darwin was the first to explain why this connection existed.
Time, abundance and variation. Darwin needed time to understand all three factors before he was able to piece his theory together in its final form. But even when his theory was complete, he prevaricated endlessly rather than go into print – collating his notes, piling example on example, and building a compendium of evidence in support of his idea with the slowness of a Lyellian geological process. Darwin understood that his theory of evolution (he called it ‘descent with modification’) by means of natural selection was more than just another stab at transmutation, but a qualitative advance in thought, and one which people in general might find hard to accommodate. Events, however, overtook him. Like much unwelcome news, it arrived in the mail.
In 1855, Darwin came across a paper by Alfred Russel Wallace (1823–1913), then an unknown land surveyor turned professional collector of natural history specimens, who had worked in South America, but who was lately in the East Indies. Wallace reported that new species tend to appear in regions close to the home ranges of the species they most closely resemble. Wallace was close to producing a theory of evolution similar to Darwin’s, but he had not got there yet, and Darwin calculated that he still had time to collect his own thoughts before publication. Three years later, in June 1858, Darwin saw another paper by Wallace in which the younger man had hit the nail on the head, describing natural selection in a few pages, quite independently of Darwin. What was worse, Wallace had sent it to Darwin in person, to see what the famous naturalist and Beagle veteran thought of it.
Apart from discussing his emerging theory with a few scientific friends (including Lyell), Darwin had kept his thoughts on natural selection to himself. Now here was Wallace, arriving out of nowhere and about to put his name to an idea which Darwin felt was his own property. Darwin was thrown into agonies of moral indecision. By rights, the first person to publish should claim the credit, and that was likely to be Wallace. On the other hand, Darwin had been thinking about natural selection for many years – presumably, for longer than Wallace had been – and was working up a long and exhaustive treatise on the subject. The problem was compounded by the fact that Wallace, in his naïvety, had written to Darwin, completely unaware that he was an interested party.
Darwin did what many would have done in such a situation – he asked a close friend and mentor for guidance. The friend was Lyell, who arranged an honourable tie. On 1 July 1858, Lyell and another of Darwin’s friends, the botanist Joseph Hooker, read Wallace’s paper and a few short items from Darwin at a meeting of the Linnean Society, London’s senior institution for biology. Neither of the main protagonists was there. Wallace was in the East Indies, and would not return to England until 1862. Darwin was at home, distracted by a spate of family illnesses, and took little interest in the proceedings in London; neither, as it turned out, did many other people. Summarizing the business of the year, the Secretary of the Linnean Society reported that 1858 had been unremarkable for scientific advances.
The fireworks began when Darwin, spurred on by his friends, composed an ‘abstract’ of his ongoing work as a stopgap, to show that although Wallace had threatened to beat him to publication, he had been thinking along these lines for years and had amassed a large amount of evidence to support his case. That short book was, to give it its full title, On the Origin of Species by Means of Natural Selection, Or the Preservation of Favoured Races in the Struggle for Life, and it was published on 22 November 1859. This time the world sat up and took notice. The Origin was an immediate sell-out: orders for 1,500 copies outstripped the first printing of 1,250, and a second edition was hurriedly prepared. As for the reaction of the public, the historian of evolution David Hull put it thus: ‘One can safely say that all hell broke loose.’4
Darwin’s theory of evolution by natural selection has endured because it explains a great deal about how and why the natural world came to be the way it is. It shows how the interaction between inherited variation and external circumstances can mould, or adapt, species to their environments – and, if necessary, reshape species to meet changing circumstances. Natural selection provides a unified explanation for phenomena which might otherwise seem unconnected, from the nesting habits of certain moths that make their homes only inside the fruits of yucca plants, to the sexual preferences of peahens; from the profusion of species of cichlid fishes in the East African Great Lakes to, perhaps, why gentlemen prefer blondes. It also explains how several species can emerge from a single ancestor; why the pattern of life is hierarchical, like the branches of a tree; and, finally, how all the species of the Earth might have descended from some blob of protoplasm in what Darwin himself called a ‘warm little pond’.5
One of the most attractive features of natural selection is the disarming elegance with which it explains so much by postulating so little. The mark of a sound theory is that it explains evidence with minimal recourse to extraneous assumptions. There had been other schemes to explain transmutation – and before that, generation – but they could be made to work only by positing the existence of elaborate microscopic structures, mysterious forces or essences, or divine intervention. Before Darwin, perhaps the most influential theory of transmutation had been that of Lamarck, who had proposed that animals evolved through an inherent improving force, or besoin (‘need’). There is nothing inherently wrong with this as an explanation. However, Darwin’s natural selection is better because it explains the natural world perfectly well without having to assume the presence of extra internal mechanisms. Natural selection required only three things, and all of them could be observed and measured – time, abundance and variation.
Natural selection was the first theory in which all the important properties of the genome – its direction of individual development, the relationship of this development to evolution, and the generation of diversity – were united in a single coherent scheme. Nature-philosophy had linked the first two, but its conception of a static nature blinded it to the third. In his insight that variation was important, and not merely a distraction, Darwin found a way to explain the diversity of nature in terms of the overall pattern of life.
There remained something that natural selection did not explain. Whereas Darwin understood the importance of variation, he could not account for how this variation was generated and maintained. Natural selection does exactly what its name suggests – it selects variant forms from a larger range of variation presented to it. But without some mechanism to regenerate that variation, selection would soon wear it away into bland homogeneity, and the transmutation of species would stop. Darwin could offer no credible mechanism for the generation and maintenance of variation. As Harvey and the preformationists had been with the question of the origin of embryonic form, Darwin was ultimately confronted with what looked like an insoluble problem: where did variation come from?
Darwin was acutely aware of this shortcoming, and it worried him greatly. He realized that variation must be inheritable, but at the time there was no clear idea about how this variation was organized or stored as information, nor how traits from the parents were apportioned in the offspring. It seemed clear that offspring manifested traits of both parents, a circumstance that had bothered preformationists who theorized that traits would be passed down through either the male or the female line, but not both. In general, the view of inheritance in Darwin’s day was that the characters of both parents became indissolubly blended in the offspring. If such blending inheritance were the rule, then any variation present in a population would sooner or later be eroded, homogeneity would result, natural selection would have nothing to select, and evolution would stop.
A cursory glance at the variety and diversity of organisms around us instantly shows that this model of blending inheritance cannot be sustained. Variation must be encapsulated and passed down in some other way – but how? The logical solution, Darwin reasoned, was a system in which inheritance is carried by inviolate, atomistic particles that can be passed down from one generation to another without destruction or alteration, and whose combination and assortment in the offspring might determine the expression of this trait or that. Such atomistic theories of generation have a surprisingly long history. As long ago as 1520, the alchemist Paracelsus – the same who published a recipe for homunculi – wrote:
Both the man and the woman each have half a seed and the two together make a whole seed. And note how they come together. There is in the matrix an attractive force (like amber or a magnet) which draws the seeds unto itself . . . Once the will has determined, the matrix draws unto itself the seed of the woman and the man from the humours of the heart, the liver, the spleen, the bone, the marrow, blood vessels, muscles, blood, and flesh, and all that is in the body. For every part of the body has its own particular seed. But when all these seeds come together, they are only one seed.6
Alchemy aside, the leading atomistic theory of generation came from the fertile mind of the naturalist and encyclopaedist Georges-Louis Leclerc, Comte de Buffon (1707–88). In Buffon’s hypothesis, particles from every organ of the body unite and crystallize in the sperm or egg, interlocking to produce the ‘essence’ of a whole organism that would provide the germ for the next generation. Birth defects would result from damaged or missing particles, or from an imperfect melding of particles. The physicist Pierre-Louis Moreau de Maupertuis (1698–1759) had a similar idea that involved the attraction between oppositely charged particles, drawn from a large pool of such particles, each of which would represent a different part of the organism. Elaborate speculations on the hypothetical behaviour of invisible particles were easily ridiculed by hard-nosed, experimentally minded preformationists such as Spallanzani, who famously remarked that ‘we descend from the observations of Leeuwen-hoek to those of Buffon’.7 Neither have Buffon’s views been in any danger of contemporary rehabilitation: in Early Theories of Sexual Generation (1930), F. J. Cole wrote with characteristic asperity that:
The commanding position which Buffon occupied in the biological world in the middle of the eighteenth century, which he owed rather to an eloquent and forceful personality than to the possession of great scientific merit, was nevertheless inadequate to ensure the acceptance of his elaborate system of pangenesis, which was universally applauded but quietly shelved. 8
Faced with no credible alternative, Darwin came up with an atomistic theory of generation which owed something to Buffon – and which met the same ignominious fate. In 1868 Darwin described an idea he called pangenesis in which each part of the body produces a microscopic representative, or ‘gemmule’. The gemmules would travel to the sperm or egg cells, and the characteristics inherited by the offspring would then depend on the number and nature of the gemmules that happened to be in the sex cells at the time. Darwin’s cousin Francis Galton (1822–1911) suggested that pangenesis might be tested by looking for alterations in the distributions of traits in the offspring of animals that had received blood transfusions from animals with different traits. If gemmules were carried in the blood, the offspring might resemble the blood donor as well as the parent. These experiments were carried out on rabbits and revealed no such effect. Darwin’s response was uncharacteristically evasive: he suggested that gemmules must be carried by some means not involving the blood.
This, then, is the agonizing situation in which Darwin found himself: for his theory of natural selection to work, it was necessary to posit a complicated scheme of particulate inheritance for which no concrete evidence existed whatsoever. This did not, initially, affect the wide prominence enjoyed by natural selection after 1859, even if it was not universally accepted. To begin with, critics were less concerned with its failure to explain the sources and nature of variation on which the theory rested, than with the challenge that evolution posed to established religious and social views. People were quick to grasp the idea that change is the usual state of nature, shaped by external circumstances, and that organisms were not tied to divinely ordained stations with respect to their fellows. It is not surprising, therefore, that ‘Darwinism’ was adopted by social reformers and radical politicians, and fought with vigour by representatives of the establishment. This, of course, was just the kind of reaction that Darwin had been afraid of, and one reason why he kept quiet about it for a long time. The scientific reaction was slightly different. Unlike the social reformers who saw in Darwinism the unlimited possibilities of change offered by freedom from the divine plan, scientists saw natural selection as a progressive force for inexorable evolutionary improvement.
Blame for this perversion of evolution as an instrument of destiny may be laid, in part, at the door of Ernst Haeckel – prolific biologist, embryologist and populist of the theory of evolution. That Haeckel was predisposed to see evolution as progressive is perhaps no surprise, schooled as he was in the traditions of German embryology and anatomy that had, in turn, received their impetus from nature-philosophy. Haeckel revered Goethe as much as he idolized Darwin: he fused Darwinism with the nature-philosophic picture of the world as a series of stages striving towards the human ideal – with natural selection miscast as the motor. Haeckel fused Darwin with Goethe and created a monster. It is no accident that this bastardized Darwinism was misappropriated by the Nazis, and then, all of a sudden, otherwise quite ordinary citizens were committing unspeakable crimes in the name of the evolution of a more perfect human species. Nazism may have been defeated, but Haeckel’s dismal legacy lives on. In the public mind, ‘evolution’ means progression and improvement. Everyone has seen images featuring a line of figures from apes to Man – sometimes culminating with the latest consumer product, accompanied by a slogan such as ‘Move Up to the Next Stage in Evolution’.9
Natural selection is neither progressive nor a force; it has neither memory nor foresight, and works only in the here and now. Neither is it a spirit or essence, separate from the materials on which it acts. Yet it remains all to easy to adopt a view of evolution driven by an inherent bias towards improvement, and with it a kind of teleology in which structures were seen to have evolved to fulfil some pre-existing purpose. An example that has been exercising scientists recently is that of the feathers of birds, structures that seem perfectly adapted for flight. And so they are: but it is one thing to say that feathers are adapted for flight now, and quite another to assert that flight is the purpose for which feathers originally evolved. That would be to conflate the state of feathers in the here and now with the entirely separate process whereby feathers achieved that state over millions of years. Thinking along these lines, some ornithologists have asserted that feathers and flight are features that define birds – but this view has recently been challenged by the discovery of fossils of dinosaurs which, in life, were unlikely ever to have flown yet had structures all but indistinguishable from the feathers of birds. If feathers evolved for any purpose, that purpose was not flight – at least, not initially. To say that if all birds have feathers, then all animals with feathers must be birds is as logical as saying that if all giraffes have four legs, then all four-legged animals must be giraffes.
The trap is to link the evolution of major groups (such as birds) with the supposed adaptation of some evolving structure (such as feathers) to suit some purpose (such as flight). Natural selection is universally proposed as the force that drives this process, as if it were some magical ingredient akin to Wolff’s vis essentialis or Lamarck’s besoin that drove giraffes to grow ever longer necks so that they could pluck leaves from ever higher branches. In this way, natural selection is imbued with a memory and a foresight that it patently lacks, and evolution is reduced to the level of fairy tales. Of course, stories about the evolution of a giraffe’s neck, or the feathers of a bird, are a long way from the nature-philosophic belief in the striving of organisms towards the exalted human state, but beneath all such scenarios lies a general tenor of ‘improvement’.10
Darwin’s failure to account for the origin of variation left his theory vulnerable to adulteration by older ideas of progression and improvement. As a result, the enthusiasm with which Darwinism was greeted among scientists in the 1860s had, by the 1880s, turned to disenchantment. Some scientists reverted to Lamarckism.11 Others turned to experimental biology after the disheartening realization that ad hoc theories about adaptation were very largely valueless as explanations for evolutionary change. Among this latter group was William Bateson (1861-1926), who in 1894 wrote the following excoriating lines:
Any one who has had to do such work must have felt the same thing. In these discussions we are continually stopped by such phrases as ‘if such and such a variation then took place and was favourable’, or, ‘we may easily suppose circumstances in which such and such a variation if it occurred might be beneficial’, and the like . . . ‘If, say we with much circumlocution, ‘the course of Nature followed the lines we have suggested, then, in short, it did.’ That is the sum of our argument.12
These lines are from Bateson’s book Materials for the Study of Variation, in which the problem of the origin of variation was confronted head on. Six years later, Bateson invented a word for the new science of variation that he and others were pioneering. That word was ‘genetics’.