If one word could name the great cause of Edward O. Wilson's later life, it would be biodiversity — a term he did much to popularize and even more to invest with meaning. The diversity of life, the sheer multitude of species with which we share the planet, was for Wilson the most wonderful phenomenon in nature and the most gravely threatened, and the effort to comprehend it, to measure it, and to defend it occupied the last decades of his career. This chapter concerns that diversity itself: what it is, how much of it exists, how it came to be over the long history of life, why it matters, and how it is now being lost. It is, in a sense, the heart of Wilson's mature vision, for the diversity of life was the thing he loved most and understood best — the endless variety of living forms that his science had spent a lifetime cataloguing, and that his conscience spent its last years struggling to save. The conservation response to the crisis this chapter describes is treated in Chapter 12; here the subject is the diversity itself, in its splendor and its peril.
To grasp what Wilson meant by the diversity of life, one must begin with a fact he found at once exhilarating and humbling: that we do not know, even to the nearest order of magnitude, how many species share the Earth with us. Confronting what he called the scale of our ignorance, Wilson observed that science has named and described only a fraction of the living world — roughly 1.4 million species, at the time he wrote — while the true number very likely runs to ten million or more, and by some estimates far higher still. The great bulk of life on Earth, in other words, remains unknown to science, undescribed, unnamed, its existence unsuspected in any catalogue. This astonishing ignorance was, for Wilson, both a scandal and an invitation: a scandal that so little of life had been documented after centuries of biology, and an invitation to the great work of exploration and discovery that he believed should be among science's highest priorities. We live, he insisted, on a little-known planet, most of whose inhabitants we have never met.
This ignorance is not for want of effort but a measure of the sheer magnitude of the task. Since Linnaeus established the modern system of naming in the eighteenth century, taxonomists have labored to describe and classify the living world, and the roughly one and a half million species they have catalogued represent an immense scientific achievement. Yet set against the estimated total, that achievement reveals how much remains: the described species are the visible tip of a largely submerged diversity, and the estimates of the true number span a wide and uncertain range, from ten million upward, precisely because so much of life lies beyond the reach of current knowledge that even guessing at its extent is fraught. Wilson had spent his own career at this work, naming new species of ants by the score, and he knew intimately both the labor each description demands and the vastness of what remains undone. The naming of life, he came to believe, was among the most fundamental and most neglected of scientific enterprises, the necessary first step toward understanding and protecting a diversity most of which science had not yet even registered as existing.
Insects, and the arthropods generally, are where the great reservoir of unknown diversity chiefly lies, and Wilson — himself an entomologist — never tired of pressing the point. The insects dominate known diversity, with some three-quarters of a million species already named, and yet the number still awaiting discovery may dwarf even this, for every expedition into an unexplored tropical forest or an uninvestigated corner of the world turns up creatures new to science. Wilson's own late expeditions, into the forests of Mozambique and the remote reaches of the Pacific, continued to yield insects likely unknown to any catalogue, a reminder that discovery was not a finished chapter of biology's past but an ongoing and urgent enterprise. The small and the numerous, the six-legged multitude that Wilson had championed all his life, held the bulk of terrestrial biodiversity, and it was among them, not among the conspicuous vertebrates, that the real dimensions of life's variety were to be found — and lost.
The unknown is not distributed evenly, and Wilson took particular delight in pointing out where the great mass of undiscovered life lies hidden. It is not, for the most part, among the birds and mammals, which are relatively well catalogued, but among the small and the inconspicuous — the insects above all, of which some three-quarters of a million species were named in his day yet countless more await discovery, and beyond them the mites, the nematodes, the fungi, and the almost wholly unexplored universe of microorganisms. Whole realms of the biosphere remain scarcely investigated: the depths of the ocean, the canopies of tropical rain forests, the teeming world within the soil. Wilson surveyed these hidden realms with the eagerness of an explorer describing unmapped continents, for that is what he took them to be — vast tracts of living diversity awaiting their discoverers, a world of wonders still unknown lying all around us and beneath our feet, most of life still waiting, as he put it, to be found and named.
Nowhere does the diversity of life reach a higher pitch than in the tropical rain forest, and Wilson wrote of these forests with a rapture born of long personal acquaintance. He had camped beneath their storms and walked their floors, and he conveyed the staggering biological richness of the Amazon and its like as the very summit of terrestrial diversity — places where a single tree might hold more species of ant than all the British Isles, where the profusion of life outruns any possibility of complete census. The rain forests, occupying a small fraction of the Earth's land surface, harbor a wildly disproportionate share of its species, concentrating the planet's diversity in green bands about the equator. To Wilson these forests were the great treasuries of life, the richest expression of what evolution had achieved, and their destruction was therefore the gravest of losses — for to clear a tract of tropical forest is to extinguish more diversity than the clearing of any other habitat on Earth. The coral reefs, he noted, play a similar role in the sea, marine hotspots of extraordinary richness rivaling the forests in the profusion of life they sustain.
Wilson opened his central book on the subject, The Diversity of Life, not with abstractions but with a memory: himself camped in the Amazon rain forest through a night of tropical storm, the violence of the elements giving way, as the storm passed, to the teeming life that the darkness concealed and the lightning briefly revealed. It was a characteristic gesture, grounding the vast theme of biodiversity in a single vivid experience of a particular place, letting the richness of one patch of forest stand for the richness of the whole living world. From that concrete beginning he drew the reader outward to the general truth it embodied: that the rain forest is the planetary peak of diversity, the place where life's variety reaches its greatest concentration, and that to stand within it is to be surrounded by more kinds of living things than anywhere else on Earth. Wilson's genius as a writer on biodiversity lay in just this movement, from the felt particular to the comprehended whole, making the abstract fact of diversity palpable through the remembered texture of a real forest on a real night.
The unevenness of life's distribution across the globe was itself, for Wilson, a fact of the first importance. Diversity is not spread uniformly but concentrated, rising steeply from the poles toward the equator and reaching its maximum in the tropical forests and coral reefs, so that a small fraction of the Earth's surface holds a large fraction of its species. This concentration has a double edge: it means that the diversity of life, though vast, is also spatially focused and therefore vulnerable, since the destruction of relatively small but supremely rich areas can extinguish a wildly disproportionate share of the whole. The geography of diversity, in other words, is also the geography of risk, and Wilson's attention to where life is richest was inseparable from his alarm at where it is most endangered. The tropics, which took the longest to build their diversity and hold the most of it, stood also to lose the most, and fastest, as human pressure bore down upon them.
If the tropical forests are the best-known theaters of diversity, Wilson was quick to remind his readers that vast realms remain barely explored at all, and that the true extent of life's variety may lie largely in these hidden places. The depths of the ocean, which cover most of the planet, harbor communities we have scarcely begun to sample; the canopies of rain forests, a hundred feet above the ground, form an aerial world of their own that biologists only lately learned to reach; the soil beneath every footstep teems with an unstudied multitude of invertebrates and microbes; and the microbial world generally, the bacteria and archaea and their kin, constitutes a diversity so vast and so poorly known that it may dwarf everything visible to the naked eye. Wilson surveyed these hidden realms as the true frontier of biology, the places where the great discoveries of the coming century would be made if the diversity they contain could be reached before it was lost. To him they represented both the measure of our ignorance and the promise of wonders yet to come — an unexplored biosphere lying all around us, most of life still waiting in the dark places of the world to be found, named, and understood, if only there were time.
How did all this diversity come to be? Wilson's answer drew on the whole of evolutionary biology, for the diversity of life is the accumulated product of the process by which one species becomes two. The engine of diversification is speciation — the splitting of a single ancestral species into distinct descendant species, most often when populations become geographically isolated and diverge until they can no longer interbreed. Each act of speciation adds to the standing diversity of life, and over the immensity of evolutionary time these additions have accumulated into the millions of species alive today. Wilson tied the mechanics of speciation directly to the growth of biological diversity, for the two are one process seen at different scales: what looks, close up, like the divergence of two populations is, from a distance, the branching growth of the tree of life itself. The rate at which new species form varies enormously among groups and circumstances — some lineages, like the orchids exploiting innumerable microhabitats in tropical mountains, speciate with remarkable speed — but the cumulative result of all this branching, across hundreds of millions of years, is the diversity we behold (the mechanisms of speciation are treated more fully in Chapter 7, Natural Selection and Evolutionary Theory).
Speciation often proceeds not one species at a time but in great creative bursts, and Wilson gave special attention to the phenomenon of adaptive radiation, by which a single ancestral lineage diversifies rapidly into a whole array of specialized descendant forms. When a lineage gains access to new and unexploited ecological opportunity — a new island, a new continent, a new way of life opened by some key innovation — it may radiate explosively, its descendants fanning out to fill the available niches with a spray of new species adapted to different modes of living. Wilson pointed to the classic cases, the finches of the Galápagos and the honeycreepers of Hawaii, as vivid illustrations of how a few colonizing ancestors can give rise, in geological short order, to many species. Adaptive radiation, he argued, is one of the central means by which life fills the world with diverse forms, the process that turns ecological opportunity into biological diversity. It is diversification at its most exuberant, the tree of life throwing out whole new branching systems where fortune opens the way.
Diversity is shaped, too, by the great geographic rearrangements of the Earth, and Wilson relished recounting one of the most dramatic: the Great American Interchange, which followed the rise of the Isthmus of Panama some three million years ago. For most of their history the two American continents had been separate, each evolving its own distinct fauna in isolation; when the land bridge formed and joined them, the two long-separated biotas mixed, animals streaming north and south across the new connection, with consequences both creative and destructive as formerly isolated species met, competed, and in some cases displaced one another. Such episodes of biotic interchange, when previously separated faunas come into contact, are natural experiments in the assembly of diversity on a continental scale, revealing how the diversity of a region depends not only on its internal evolution but on its connections to and isolation from other regions. The geography of the planet, shifting over geological time, is thus one of the great authors of biological diversity, joining and sundering the theaters in which evolution plays out.
Wilson was fascinated, too, by the question of how many species a given place can hold — by the phenomenon he called species packing, the accumulation of coexisting species within a habitat, the very opposite of extinction. How closely can species be packed into an ecosystem before competition sets a limit? How is it that a single tropical tree can support so many species of insect, a single reef so many kinds of fish and coral? These questions linked the study of diversity to the ecology of coexistence, for the number of species a place can hold depends on how finely they divide its resources among themselves (a theme pursued in Chapter 10, Ecology and Ecosystems). The diversity of a rich habitat, in this light, is a measure of how many ways of life its species have contrived to pursue side by side, how densely the ecological space has been filled with specialized forms. Species packing was diversity viewed from the standpoint of the community, the local accumulation of coexisting life that, summed across all the habitats of the Earth, constitutes the global diversity Wilson sought to comprehend and defend.
The standing diversity of life at any moment, Wilson understood, is not a fixed quantity but the running balance of two opposed processes: origination, which adds species through speciation, and extinction, which subtracts them. Diversity rises when speciation outpaces extinction and falls when extinction gains the upper hand, and over the long history of life the balance has generally tilted, haltingly and with great reversals, toward increase, so that more species are alive today than at any previous moment in the history of the Earth. Yet this abundance coexists with a starker fact that Wilson never let his readers forget: that virtually all species that have ever lived are now extinct, that the species alive today are a tiny remnant of the vastly greater number that evolution has produced and death has claimed across the eons. Life is prolific but mortal; its diversity is the momentary balance of ceaseless birth and ceaseless death among species, a standing crop maintained, like the water in a filling basin, by the difference between what flows in and what drains away.
Not all species face extinction equally, and Wilson analyzed with care the factors that make some vulnerable and others resilient. Body size is one: larger-bodied species tend to maintain smaller populations, requiring more resources per individual and more space, and smaller populations are more easily extinguished by the accidents of environment and chance, so that the great beasts are, as a rule, more extinction-prone than the small. Geographic range is another: a species confined to a single island or a narrow patch of habitat can be wiped out by a local catastrophe that a widespread species would survive, so that endemics — the unique species of restricted regions — are especially at risk. Population size, ecological specialization, position in the food web all bear on a species' vulnerability or persistence. Wilson drew these factors together into an understanding of why some lineages endure while others perish, an understanding that had direct bearing on conservation, for it identified which species stand in greatest danger and why. The largest, rarest, and most narrowly specialized creatures — often the very ones humans most cherish — are frequently those most likely to be lost.
The slowness of recovery from catastrophic loss gave Wilson's warnings their particular weight, and he returned to it often. Because the rebuilding of diversity after a mass extinction takes tens of millions of years — a span beyond all human reckoning — the losses now being inflicted are, for any timescale that could matter to human beings or their descendants for hundreds of generations, simply permanent. We are not borrowing against diversity, to be repaid in some foreseeable future; we are destroying, irrevocably, an inheritance that took the whole history of life to accumulate and that will not be restored within any horizon we can imagine. This was the sober arithmetic beneath Wilson's grief: that evolution's long labor of diversification, halted and reversed by human action, cannot be resumed on any human schedule, so that what is lost in this generation is lost not for a while but effectively forever. The permanence of extinction, set against the slowness of nature's recovery, made the present destruction, in his eyes, the most consequential thing our species has ever done.
Wilson balanced this grim arithmetic with a longer view that offered a certain cold consolation, though he was careful not to let it soften the urgency of the present. Across the whole immensity of its history, life has proven ultimately resilient: it has recovered from every one of the great extinctions, rebuilding its diversity over subsequent eons, throwing out new forms to replace those lost, so that the tree of life, however savagely pruned, has always eventually grown back fuller than before. In the very longest perspective, then, life will doubtless survive even the sixth extinction and, given enough geological time, rediversify into new splendors we cannot imagine. But this consolation, Wilson insisted, is no comfort on any scale that matters to us, for the recovery would unfold over tens of millions of years, in a world without us to witness it, and would not restore the particular species and the particular diversity we are now destroying. Life's resilience guarantees a future for life in the abstract; it guarantees nothing for the living world we actually inhabit and love, whose specific richness, once lost, will not return within the entire span of human history to come.
The deep-time perspective also transformed how Wilson understood the present moment, setting it against the vast backdrop of life's history and lending it an almost unbearable significance. Nearly four billion years of evolution had gone into the building of the diversity now alive; the human species had emerged only in the last geological instant, and had risen to planetary dominance in a span briefer still. That this latecomer, in a few centuries, should be undoing an achievement of billions of years struck Wilson as an event of literally epochal magnitude, a turning point in the history not merely of humanity but of life itself. To view biodiversity in deep time was to see the current crisis for what it is: not a local or temporary disturbance but a hinge in the history of the planet, a moment when the trajectory of life on Earth is being bent by a single species' actions. This cosmic framing was not rhetorical inflation but Wilson's honest attempt to convey the true scale of what is at stake, measured against the only timescale adequate to it — the timescale of evolution itself, on which the present generation of human beings has become, all unawares, a geological force.
Extinction, Wilson was careful to insist, is a natural and recurring feature of life's history, not in itself an aberration. The fossil record reveals extinction as a constant background process, punctuating the general rise of diversity, the normal fate of species as environments change and competitors arise and fortune turns against them. Most species persist for a few million years and then vanish, replaced by others; this ordinary background extinction is the counterpart of speciation, the subtraction that balances the addition. To understand the present crisis, Wilson argued, one must first understand this natural rhythm of loss, against which the current acceleration can be measured. For it is not extinction as such that alarmed him — extinction is as old as life — but the rate of extinction, the pace at which species are now vanishing, which he became convinced had risen catastrophically above the natural background to levels seen only rarely in the whole history of the Earth.
Those rare occasions when extinction has vastly outrun its background rate are the mass extinctions, and Wilson surveyed them as the great catastrophes of the history of life. The fossil record reveals five of them — the greatest dying at the end of the Permian, the most famous at the end of the Cretaceous, when the dinosaurs and much else were swept away — episodes in which a large fraction of all species perished in a geologically brief span, through causes ranging from meteorite impacts to vast volcanic outpourings and swings of climate. Wilson emphasized two facts about these catastrophes that bear directly on the present. The first is their severity: each of the great extinctions eliminated a substantial part of the diversity of life, resetting the course of evolution. The second is the slowness of recovery: after each mass extinction, the rebuilding of diversity to its former levels required not thousands or millions but tens of millions of years — five to twenty million years, by his reckoning — a span so vast as to render such losses, on any human timescale, effectively permanent.
Wilson took care to explain how we know these ancient catastrophes at all, for the mass extinctions are legible only in the fossil record, that imperfect but eloquent archive of life's past. The rocks preserve, in their succession of entombed forms, both the ordinary turnover of species and the rare intervals when diversity crashed, allowing paleontologists to reconstruct the tempo of extinction and recovery across hundreds of millions of years. From this record Wilson drew the pattern that framed his argument: the long, halting rise of diversity, punctuated by the five great collapses and their prolonged aftermaths. The causes of these collapses he surveyed with appropriate caution, for they remain in part uncertain and various — the end-Cretaceous event now widely attributed to the impact of a great meteorite, others implicated in vast volcanic outpourings, swings of climate and sea level, or some conjunction of catastrophes. What the causes had in common was their capacity to overwhelm the resilience of life across the whole planet at once, and it was precisely this planet-wide reach that made them so destructive and their aftermath so long.
The balance of origination and extinction, Wilson showed, is what determines the standing diversity of life at any given time, and understanding it as a balance was crucial to grasping the present danger. In normal times, origination through speciation roughly keeps pace with or exceeds background extinction, and diversity holds steady or slowly climbs; a mass extinction is a catastrophic and temporary victory of extinction over origination, after which the slow engine of speciation must labor for tens of millions of years to restore what was lost. The present crisis, in these terms, is an acceleration of extinction so extreme that no conceivable rate of natural speciation could offset it — origination cannot possibly keep pace with a loss unfolding in centuries rather than epochs. This was why Wilson regarded the human-caused extinction as genuinely comparable to the great natural catastrophes: not because its immediate death toll necessarily exceeds theirs, but because it has so violently unbalanced the fundamental equation of diversity, driving extinction to a pitch that origination cannot answer, and thereby threatening to reduce the diversity of life for a span of time that beggars human comprehension.
It was against this deep background that Wilson framed his gravest warning: that the Earth is now entering a sixth mass extinction, this one caused not by asteroid or volcano but by a single species — our own. The human transformation of the planet, above all the destruction of habitat, has accelerated the loss of species to a rate that Wilson and many others judged comparable to the great catastrophes of the geological past, so that we stand at the threshold of a biological cataclysm of our own making. The forces driving this loss he gathered under the framework of HIPPO — habitat destruction foremost, then invasive species, pollution, human overpopulation, and overharvesting — a diagnosis of the human causes of extinction (developed further, with the conservation response, in Chapter 12). What made this sixth extinction uniquely tragic, in Wilson's eyes, was that it is being carried out by the one species capable of understanding what it is doing, and therefore capable, if it chooses, of stopping. The comparison with the five great dyings of the past was not rhetorical excess but Wilson's sober scientific judgment about the magnitude of what is underway.
The scale of the present loss is peculiarly difficult to grasp, Wilson noted, precisely because so much of it goes unwitnessed. Extinctions are rarely observed directly; a species slips away in some remote forest or reef, often before it has even been discovered and named, so that the true toll is systematically underestimated, hidden by our ignorance of how much there was to lose. This meant that the crisis was worse than it appeared, its casualties largely invisible — species vanishing unrecorded, extinguished before science ever knew them, a loss we cannot even fully measure because we never knew its full extent. Wilson documented what could be documented, tracing how the spread of human beings across the globe, and especially the era of Western colonial expansion, had accelerated extinction, leaving figures of vanished birds and mammals and other creatures in its wake. But he insisted that the recorded losses were only the visible portion of a far larger destruction, the measurable fraction of a catastrophe most of which unfolds in silence and darkness, unseen and uncounted.
Why should we care? Wilson knew that the case for biodiversity could not rest on sentiment alone, and he marshaled the full range of reasons that its loss should alarm us. The diversity of life possesses, he argued, value of many kinds at once: practical value, as the source of foods, medicines, and materials, an immense and largely untapped reservoir of biological wealth, for who knows what cures and crops lie undiscovered among the millions of species we have never examined; scientific value, as the record and ongoing experiment of evolution, the library in which the deepest questions of biology are written; aesthetic value, in the beauty that has stirred human beings in every age; and spiritual value, in the sense of meaning and connection that the living world affords. To lose a species is to lose along every one of these dimensions at once — a resource forfeited, a text destroyed, a beauty erased. Wilson refused to reduce the value of life to any single measure, insisting that biodiversity matters in as many ways as human beings are capable of valuing anything at all (the fuller development of these values, and the ethic built upon them, belongs to Chapter 12, Nature Conservation).
Beneath all the enumerable values, Wilson located a deeper one that he found hardest to put into words but held most profound: that each species is a masterpiece of evolution, a unique and unrepeatable product of millions of years of natural selection, encoding in its genes and its form a long history of adaptation that can never be recovered once lost. To extinguish a species is thus to destroy something of immense and irreplaceable value quite apart from any use it might have to us — a distinct solution to the problem of living, a branch of the tree of life grown over eons, a form of order wrested from chaos by the long labor of evolution. Wilson felt the loss of such things as a kind of desecration, the permanent erasure of evolutionary achievements that no human power could recreate, and this sense of extinction as the irretrievable loss of masterpieces gave his writing on diversity its undertone of mourning. The living world, for him, was not raw material but a gallery of wonders, each species a work finished over immense time and now, in case after case, being obliterated before it had even been beheld.
Among the practical values, Wilson gave particular emphasis to the medical and agricultural bounty that biodiversity represents, the storehouse of useful compounds and genes hidden in the living world. Medicines beyond counting, he pointed out, have been discovered in remote and unexpected organisms — in molds and plants and marine creatures no one would have thought to consult — and each extinction forecloses forever the possibilities such an organism contained, closing doors we did not know were open. The wild relatives of crops harbor genetic resources on which the future of agriculture may depend; the untested millions of species constitute a reservoir of solutions to problems we have not yet encountered. To destroy this diversity before we have even catalogued it, Wilson argued, is an act of stupendous imprudence — the burning of a library we have never read, the discarding of a fortune we have never counted. Even on the narrowest calculation of human self-interest, the preservation of biodiversity is sound policy, for we are destroying, unexamined, the very resources on which our future welfare may turn.
Wilson's response to the scale of our ignorance and the urgency of the loss was a characteristically bold proposal: that humanity undertake to discover, describe, and catalogue every species on Earth. He championed the vision of a great biological census — an encyclopedia of life that would gather what is known of every species into a single accessible compendium, and drive the exploration needed to find the millions still unknown before they vanish. This was conservation through knowledge, the conviction that we cannot value or protect what we have not even discovered, and that the documentation of life is therefore an urgent priority, a race against extinction to record the diversity of the planet before it is lost. Wilson also championed the role of citizen science in this enterprise, believing that the great work of biological discovery need not be confined to professionals but could enlist ordinary people in the finding and recording of life. The cataloguing of the living world, he argued, was a task worthy of a civilization, and one that the accelerating crisis made desperately time-sensitive.
There is, running through all of Wilson's writing on biodiversity, a particular quality of feeling that sets it apart from mere scientific reportage: an intense personal love of the living world, communicated with the ardor of a lifelong naturalist. He wrote of the thrill of the search, the exhilaration of finding a creature perhaps unknown to science in some unexplored corner of a forest, the joy that had drawn him to the study of ants as a boy and never left him. This was not incidental to his argument but central to it, for Wilson believed that the case for biodiversity is ultimately made not by statistics but by love — by helping people to see the wonder of the living world, in the conviction that to know it is to love it and to love it is to wish to save it. His books on diversity are therefore works of advocacy through enchantment, attempts to convey the sheer marvel of life's variety so vividly that the reader will come to feel, as Wilson felt, that its loss would be an impoverishment beyond bearing. The diversity of life was, for him, not a resource to be managed but a wonder to be cherished.
To contemplate the diversity of life as Wilson did is to be caught between two overwhelming feelings, and his writing holds them in a difficult balance: wonder at the profusion of living forms that evolution has produced, and grief at the destruction now being visited upon them. The Earth's biodiversity is the grand achievement of nearly four billion years of evolution, a richness so vast we have not begun to catalogue it, so intricate we have not begun to understand it — and it is being dismantled, in the span of a few human generations, by a species that arrived only yesterday and has scarcely paused to learn what it is destroying. This was the tragic knowledge at the center of Wilson's late thought, and it gave his biodiversity writing its peculiar intensity, the union of a scientist's precision with a lover's grief. Yet he refused final despair, holding always that the outcome remains undecided, that much might still be saved if we chose to save it. The diversity of life is our inheritance and our responsibility; whether we will prove worthy of it is, as Wilson knew, the great question of our age (the answer to which his conservation writing sought to shape, as Chapter 12 describes).
In the end, Wilson's lifelong devotion to biodiversity was the natural flowering of everything he was. The boy who had marveled at the ants of an Alabama yard, the young taxonomist who had named new species by the score, the theorist who had explained how diversity accumulates on islands and in the depths of evolutionary time, and the elder who pleaded for the preservation of life — all were expressions of a single, sustained astonishment at the variety of living things. He had spent his career demonstrating that this variety is not a chaos but a comprehensible order, the lawful product of evolution and ecology; and he spent his last years insisting that this order is precious beyond reckoning and imperiled beyond precedent. The diversity of life was the great fact that Wilson's science illuminated and his conscience defended, the wonder that unified the naturalist and the moralist in him. To read him on the subject is to be persuaded, as he was, that the millions of species with which we share the Earth are the most remarkable thing we know of in the universe, and that their preservation is among the highest obligations of the one species able to comprehend, and to choose.