The Variety of Life: A Survey and a Celebration of all the Creatures that Have Ever Lived by Colin Tudge

From the Publisher

Here, between the covers of one capacious book, is an illustrated summary of all the creatures that have ever lived, a vast compendium of earth's current and former inhabitants in all their dazzling and infinite diversity.

Colin Tudge argues that we are entering a new phase of biology in which, for the first time, biologists are achieving profound insight into life's true diversity and developing the tools to keep track of it. The Variety of Life heralds this new phase. The first part of the book describes why biologists now feel that there could have been as many as 4,000 billion species on Earth since life began. It then discusses the need for classification, beginning with the most basic principles—the strictly practical classification of fishmongers and foresters, who speak of "shellfish" and differentiate "hardwood" from "softwood"—and moves on to explore the intriguing deliberations of the modern "transformed cladists" and the novel contributions of molecular genetics. Part II describes the creatures themselves. It is divided into 24 sections, each describing a different group, illustrated by nearly 50 double-page spreads which present genealogical "trees" that summarize the evolutionary relationships between the creatures in each group. Some sections describe large, comprehensive groups such as the kingdoms of the Animals or the Plants. Others treat similar sub-groups in more detail, such as the Mammals, a class, or the Hominids, a family. In lively and accessible prose, all the significant groups of creatures—both alive and extinct—are described and their relationships clarified.

For general readers and serious biologists alike, The Variety ofLife offers an unprecedented storehouse of knowledge of life on earth.

Library Journal

Science writer Tudge (The Time Before History) has taken an enormous subject--the inventory of all living things past and present--and created a very readable work on the science of classification and the classifications of life. He draws from the work of dozens of scientists from around the world as he endeavors to bring the theories into a workable whole. Tudge imbues his work with a contagious passion for an area of biology that has dropped in profile in recent decades. The first part of the book serves as a well-developed introduction to the history, philosophy, and potential future of classification suitable for the interested lay reader, the undergraduate biology student, or the biologist specializing in any area other than taxonomy. The latter part of the book contains the actual survey of all living things. One of the highlights of this work is Tudge's writing style. He diligently explains every concept using a wide variety of clear examples and down-to-earth analogies. Highly recommended for science collections in both public and academic libraries.--Marianne Stowell Bracke, Univ. of Houston Libs., TX Copyright 2000 Cahners Business Information.\

Independent Weekly

A beautifully illustrated introduction to everything that's ever lived. It bears the hallmarks of genuine enthusiasm.

What People Are Saying

Edward O. Wilson
Exceptionally clear, user-friendly, and up-to-date.... An easily accessible reference work to the entire spread of biodiversity.

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Table of Contents

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Chapter One

`SO MANY GOODLY
CREATURES'

When I was at school and university in the 1950s and early 60s teachers and pupils took it to be self-evident that biology was about living creatures. It was also, of course, about processes—physiology, ecology, and above all evolution—but at the core of all our enquiries were the organisms themselves. We never stopped asking, `What are they? What's out there?'.

    So we worked steadily through all the known organisms group by group: the annelids (earthworms, leeches, and their like); the arthropods (crustaceans, insects, spiders, trilobites, and so on); echinoderms (starfish, sea urchins, and the rest); vertebrates (backboned creatures, such as fish, dinosaurs, and ourselves); the confusing host of creatures from amoebae to diatoms that in those days were lumped together as `protists'; seaweeds; fungi; slime moulds; plants, which then, as now, were assumed to include some seaweeds but not others, plus mosses, ferns, conifers, and flowering plants; and the creatures loosely called bacteria, which in those days nobody seemed to link satisfactorily to the rest and were sometimes (grotesquely) thrust in among the plants. The general art, craft, and science of classification was and is called taxonomy; and modern taxonomy based on evolutionary principles is commonly and properly called systematics.

    I loved this natural-historical foray among our fellow creatures. This to me was what biology was for; to admire and get to know what was out there. True systematics, too, required that creatures should beclassified not by arbitrary criteria but according to their actual phylogenetic relationships, in so far as these could be judged—where phylogeny implies `evolutionary history'. Classification, in short, was based on the phylogenetic tree, which, at least in general form and shape, resembles a genealogical tree of a human dynasty. Annelids thus emerged as metaphorical cousins of the arthropods, and echinoderms as metaphorical cousins of the vertebrates, although the annelid/arthropod lineage was only very distantly related to the echinoderm/vertebrate lineage. The phylogenetic tree was thus a summary, in graphic form, of evolutionary history. Once you grasped this principle you could see, in your mind's eye, how an earthworm and a bee—two unsurpassingly homely creatures—must have shared a common ancestor who lived, undoubtedly in the sea, at least 600 million years ago. In short, an eye for classification is a constant reminder that nothing on this planet is as homely as it seems—there are several thousand million years of evolutionary drama behind everything that moves and breathes.

    Taxonomy, though, used to have a down side. In some courses (although not, I am pleased to say, in the ones that I was lucky enough to take), students who had been drawn to biology by the romance of it, the desire to associate with their fellow creatures—or even, perhaps, to find a cure for cancer or to feed the world—found themselves counting the bristles on the legs of shrimps, to see if shrimp A was more like shrimp B or shrimp C. Taxonomy could sometimes seem less than riveting. Meanwhile, as the 1960s, 70s, and 80s unfolded, wonderful things were happening in other branches of biology: in ecology, animal behaviour, and evolutionary theory. Most spectacular of all was the rise of molecular biology, which—together with classical genetics and the rapidly unfolding ideas of evolution—has now provided biology with a core of ideas that is beginning to feel as robust and satisfying as the classical and quantum theory that lies at the heart of modern physics.

    There was also another kind of threat to the traditional craft of classification. Molecular biology is concerned primarily with the workings of DNA—the stuff of which genes are made: and genes shape the bodies of all living things. Although there are large and significant differences in the operation of bacterial DNA on the one hand, and that of `eukaryotes' on the other (creatures like us, and fungi, and protozoans and oak trees, whose body cells contain distinct nuclei), DNA on the whole operates in remarkably consistent and similar ways across the whole spectrum of living things. In other words, molecular biology has increasingly emphasized the underlying unity of life. Many molecular biologists, therefore, simply do not care what kinds of cells they work on; any one, they have tended to feel, can serve as a `model' for all the rest. Indeed a breed of molecular biologists has grown up who actually cannot tell the difference between a frog and a toad—or, indeed, when you boil it down, between a toad and a toadstool—because, quite simply, the difference does not seem to matter to them. DNA is DNA is DNA.

    So taxonomy—systematics—has in many circles been marginalized these past few decades. My two daughters both studied biology at school and university and neither was taken on the thorough conducted tour of living things that I enjoyed (although my younger daughter, Amy, did receive excellent instruction on the classification of birds at her university course in Manchester). To many biologists classification seemed to suffer from a series of cumulatively fatal flaws. First, it could be dull. Second, the various major branches of biology were all acquiring an increasingly firm core of theory whereas the traditional craft of classification began to seem almost amateurish: a series of manoeuvres without convincing, rational foundation. It was natural history of the most nerdish kind, an exercise in train-spotting—or so it seemed to those who were not versed in it. Formal courses in biology are of limited duration and other subjects (ecology, behaviour, evolution, molecular biology) just seemed to be so much more interesting and `relevant'. Besides, if it was truly the case, as molecular biology seemed to imply, that the perceived variety of living creatures is merely superficial—that we are all just variations on a theme of DNA—then why bother to distinguish one creature from another? Why differentiate between a frog and a fungus if, at the most fundamental level, they were both the same?

    That is how many biologists came to feel about taxonomy during the last decades of the twentieth century So why write a book on such a lame-duck subject?

THE UGLY DUCKLING

In truth, the art and science of taxonomy should never have allowed itself to slump into dullness. As I hope will become apparent in these introductory chapters, all who have sought to classify living creatures from Aristotle onwards have been immersed perforce in some of the most profound issues of biology—and indeed of philosophy and also, in earlier times, theology. But taxonomy is also a practical craft: practitioners of tropical medicine, for example, really do need to distinguish one mosquito from another; and minute variations in leaf sheen and stem bristle really can mark the difference between a plant that can save lives and one that is merely pretty. It is sad, therefore, that in some university courses the practicalities have dominated, and the intellectual currents have been lost to view.

    Yet it was during the decades when taxonomy seemed to lose its pole position in biology that the science of systematics truly came of age. Biologists must, of course, classify creatures according to their observable characteristics (as discussed in more detail in Chapter 2); and to infer relationships they look both at living species and at the fossils of extinct kinds. The nature and the amount of data available to them have leapt ahead in the past few decades. In particular, palaeontologists seem constantly to unearth the most wondrous caches of fossils—Aladdin's caves of ancient creatures whose existence we had no right to anticipate. Even as recently as the 1960s, biologists still doubted whether they would ever find significant fossils from the Precambrian period—from the geological time, more than 545 million years ago, when no creature had evolved hard skeletons, so that fossilization seemed impossible. Now rich beds of Precambrian fossils are known on several continents. From the later Cambrian period—notably from the Burgess Shale in Canada, around 530 million years old, and studied in particular by Simon Conway Morris of the University of Cambridge—have come series of arthropod-like animals quite unlike those of today.

    It is not unusual for creatures to diversify in many different directions when they first evolve, with most of the variants later dying out: we see the same pattern when fish first evolved, or mammals or birds, or what you will. But to have glimpsed the first wild fling of the arthropods is luxury indeed. Western Australia is yielding fossil fishes, 400 million years old, of astonishing detail. Palaeontologists now have a wonderful series of fossil birds—fragile creatures that do not fossilize easily—greatly enriching the picture of bird evolution that used to be provided only by Archaeopteryx. In 1998 came reports of dinosaurs with feathers—dinosaurs that are clearly related to birds, but certainly would not traditionally be classed as birds. Human fossils, always so elusive and confusing in the past, now show a satisfying, if more diversified sequence back to the plains of Africa around 4.5 million years ago; only one of several different trails led to Homo sapiens. In short, the fossil discoveries of recent years have been wonderful: and all these new, extraordinary creatures are grist to the taxonomists' mill. If we do not attempt to classify them, we simply will have no idea what they are.

    Vast new caches of information, too, have come from a quite different source: study of the body's chemistry and, in particular, of DNA. For the stuff that reveals the underlying unity of Earthly life also reveals the true depths of diversity, and is providing new insights into systematic relationships. Traditional studies of anatomical features can deceive for all kinds of reasons, not least because different creatures that are unrelated often adapt to similar circumstances in similar ways, and so they come to resemble each other. Molecular studies can uncover such deceptions and, indeed, can be revelatory. They have confirmed, for example, how recently human beings shared a common ancestor with chimpanzees (probably around 5 million years ago), and how closely whales are related to cows. Yet all is not so simple, as the following chapters reveal: if and when molecular studies conflict with anatomy, it is often far from obvious which set of data should be believed.

    Yet the recent influx of fossil and molecular data is only half the story. The greatest advance in modern taxonomy has been made in the underlying theory and the method: in particular by the method of cladistics developed from the 1950s onwards by a German entomologist, Willi Hennig. Without cladistics to keep taxonomists on the rails, the mass of new information could simply be an embarrassment. With cladistics it seems possible at last to unravel the true history of living creatures and hence to reveal their evolutionary relationships—and so to reflect those relationships in classifications.

    One last input—a bonus, but a necessary one—is the computer. When cladistical methods are applied to masses of data and, in particular, to the potentially infinite mass of molecular data, the resulting calculations are of horrendous complexity. Without computers to help us out—and, more to the point, the kind of software developed specifically for the purpose by, for example, David Swofford at the Smithsonian Institution in Washington DC—the library of data would largely be beyond analysis, at least in ways that were liable to yield worthwhile answers.

    This, then, is the first reason for taking systematics seriously. It is quite simply the core of modern biology. Its ambition is vaunting—to reveal and to present in summarized form the evolutionary history of all creatures on Earth. Its methods and its philosophy tax the deepest thinkers and deploy the most subtle of techniques. Those who mocked the dull feathers of taxonomy while other, flashier game was taking flight, misconstrued the nature of the beast: the ugly duckling has grown up. Yet even if this were not so, the fact is that all of humanity needs taxonomy. `O wonder!', cries Miranda in The Tempest, `How many goodly creatures are there here!' Indeed, there are—far more than Miranda could have dreamed of. For our own sake and theirs, we need to keep track of them.

KEEPING TRACK

The inventory of known living species stands at around 1.7 million—but there is no central inventory, so nobody knows for sure. There is a curious insouciance in this: we keep better tabs on the stars, which, for all they do for us, are simply points of light. It seems certain, though, that this figure of less than 2 million falls short of reality by at least an order of magnitude (at least tenfold, that is) and probably by several orders of magnitude (hundreds or thousands of times). Thus in the 1970s Terry Erwin of the Smithsonian Institution anaesthetized and then counted all the species of beetle in just one tree in Panama, perceived that the number of unknown species far outweighed the ones that had previously been identified, and through a sequence of reasoning that may seem a trifle tortuous but is widely agreed to be reasonable, calculated that the true number of all species on Earth is probably nearer to 30 million. If the proportion of the different kinds of creatures among the unknowns was about the same as in the ones we do know about, then most of that 30 million would be animals; most of those animals would be insects; and most of those insects would be beetles. God, as the great British biologist J. B. S. Haldane (1892-1964) once commented, seems `inordinately fond of beetles'.

    Some biologists feel that Terry Erwin got a little carried away, and that the true number of living species is probably nearer 8 million. But others feel he may not have been bold enough; that the true number is perhaps nearer 100 million. Many, too, feel that God is perhaps not quite so enamoured of beetles as Haldane supposed; it's just that they are attractive and much studied and so we know a lot about them—compared with what we know about the rest, that is. It has been suggested, after all, that every species of creature on Earth of reasonable size has at least one specialist nematode worm attendant upon it (nematodes are commonly called `roundworms' and sometimes `eelworms') living either as a parasite or at least a commensal (just a resident). Mites, too, diminutive relatives of spiders, are ubiquitous and much understudied, except for the few that do obvious harm—for example, to horticultural crops. There must be many more thousands of them as well. So the real total of biodiversity, on this planet at this moment, seems to be somewhere between 8 and 100 million, with 30 million standing as a reasonable guess.

    Even 30 million, however, now seems a far too modest estimate. In the seventeenth century, a Dutch linen-draper and pioneer microscopist called Anton van Leeuwenhoek (1632-1723) showed that the world contains creatures too small to see with the unaided eye, which he called `little animalcules'. Such creatures are nowadays collectively called `microbes'—a useful term—and they are known to include creatures from three quite different categories: bacteria; the newly discovered, bacteria-like archaes; and creatures of the kind colloquially lumped together as `protozoans' or, more broadly, `protists'. In France, in the nineteenth century, Louis Pasteur (1822-95) showed how very important these microbes are, in brewing and pickling, and in causing disease. Huge industries—brewing, baking, and pharmaceuticals—have been based on the cultivation of microbes. Today these industries in modern guise are subsumed within `biotechnology', and are spreading beyond their traditional bounds into all of industrial chemistry. Microbial diseases continue to dominate world health, because antibiotics and vaccines have removed only the top layers of infection, and only in some countries. Because they are so obviously important, microbes have been well studied, and the inventory of known bacterial and archaeal species now stands at around 40 000.

    The figure of 40 000, however, is now known to be a desperately feeble estimate. Traditionally bacteria could be identified and indeed discovered only by cultivating them: taking a fragment of soil, say, putting it into culture, and seeing what grows. The present catalogue, then, contains only those bacteria that can be cultured. But modern biologists such as Norman Pace at the University of California at Berkeley are now able to pick out bacteria in the soil, or in any other medium, just by looking for their DNA; and so they are finding new types without culturing them first, and indeed without ever setting eyes on the intact organism. The DNA alone is the shibboleth; by their DNA shall they be known, at least for the time being. By such means microbiologists are now suggesting that the real number of bacteria and archaes in the world may exceed the number that are known—the compliant types that can be grown—by 10 000 to one. So the real number of bacterial and archaeal species may not be 40 000 but 400 000 000. Add this to Terry Erwin's estimate of macroscopic creatures and we begin to see that our grasp of `biodiversity', and what it really implies, is tenuous indeed.

    Is this the limit? Not quite. In fact, not by a long way; not at least if we take account of time. Thus it has often casually been suggested that the species now on Earth represent only about 1 per cent of all the organisms that have ever lived on our planet. It is easy to see how this might be so—but easy to see, too, that this again must be a serious underestimate. Thus the world now contains only two species of elephant—sole representatives of the mammalian order Proboscidea. But about 150 proboscidean species are known from the past 50 million years, including a huge array of `true' elephants (members of the family Elephantidae) plus mastodonts, gomphotheres, deinotheres, and others. There are only five living species of rhinoceros, three in Asia and two in Africa, but the fossil inventory now runs to 200. The rhinoceros superfamily probably arose in Eurasia, which has yielded a huge array of ancient species. Many more lived in North America, which they could in principle have reached across the landbridges that at times in the past linked Siberia and Alaska, or Scandinavia/Greenland/Newfoundland. All the American and European rhinos are long gone and the modern African types, the black and the white, are johnnies-come-lately. Similarly, there are only four living species of hyaena—brown, striped, spotted, and the peculiar termite-eating aardwolf—but about 70 species are known from the time that hyaenas first appeared around 20 million years ago.

    Thus, even from such a rapid recce of big, conspicuous, creatures, we can easily see how the number of extinct types could exceed the living species by a hundredfold. But then we might consider that all these conspicuous types are modern by the standards of the world. The specific lineages of elephants and rhinos extend back only 50 million years or so, and the hyaenas much less than that. But life is known to have appeared on Earth at least 3500 million years ago, and perhaps nearer 4000 million; `only' a few hundred million years after the Earth itself was formed, about 4500 million years ago. Thus there has been life of some kind on this planet for at least 70 times longer than there has been any creature resembling an elephant. Elephants are slow-breeding and their generation times average around 30 years but, even so, there have been 70 times or so as many species in the past 50 million years as there are now. So how many more species of all kinds of creature might there have been over the past 3500 million years, given that most are small and some have a generation time measurable in hours? It would be surprising if the total number of species in the past did not exceed the present inventory by at least 10 000 times.

    In short, the number of species that have lived on Earth since life first began could easily be about 400 million times 10 000, which is 4 million million, or 4000 (American) billions—roughly a thousand species for every year that life has existed on Earth. Of course, these estimates may be out by an order of magnitude, or even by several orders of magnitude. But even if they were exaggerated a millionfold the total would still be vast; and far too great for any human mind to grasp.

    What is more interesting in this world than our fellow human beings and other living creatures? Why do we know so few of what must be out there? What kind of philistines are we? Yet we can make a more practical point than this. We need to interact with other species whether we want to or not. They are our food and our environment: homes, scenery, soil, even the oxygen in the air is provided by courtesy of plants and photosynthesizing bacteria. We need actively to exploit our fellow creatures to survive. This is not an option: we have to exploit them unless we prefer to die. Therefore purely for selfish reasons (as well as for reasons that we may hope are less selfish) we also need to conserve them. Besides, even if we learnt to do without our fellow creatures—perhaps found some inexhaustible supply of food on some distant planet—they would not necessarily ignore us. We are flesh, too, for all our conceit, and many are more than happy to feed upon us. To contain, exploit, or conserve our fellow creatures we need to keep tabs on them.

    But how can we, when there are so many? If we simply made a list of the known species now on Earth, we would need 1.7 million names. Because each living species is conventionally given a two-part name, according to the binomial system devised by the great Swedish biologist Carl von Linné (or Carolus Linnaeus in the Latinized form) in the eighteenth century, we would need 3.4 million words. An average-sized novel contains about 100 000 words; a fat, one-volume encyclopaedia has around 500 000. So we would need around seven fat volumes just to make a list of present-day known species. Clearly, if we seriously set out to find and list all the creatures now on Earth, the seven volumes would probably extend to 70 or more; and if we could find out all the creatures that have lived in the past we would need a substantial library—just to make the barest list, without footnotes and explanation. How can we possibly cope?

    Classify, is the answer. Put the different creatures into groups; and then nest those groups within larger groups; and so on and so on. Once classify—at least if you do it well—and any list, no matter how prodigious it is or is liable to become, becomes tractable. It is miraculous when you think about it, but it is the case. So here is a second reason for taking taxonomy seriously. We need to do it because we need to share this planet with a vast if as yet sadly unquantified host of other creatures, and we need at least to keep them under surveillance.

    Yet there is one more reason to consider systematics, and I think it the best. The prime motive of science is not to control the Universe but to appreciate it more fully. It is a huge privilege to live on Earth and to share it with so many goodly and fantastical creatures—albeit a privilege of which we are grotesquely careless. In truth, if we did not need to exploit other species we might simply and unimprovably spend our lives in admiration of them; they are so extraordinary. But in order to get close to them we do have to give them names, and keep them in some kind of order in our minds. Those who have been brought up to believe that naming is an unpoetical pursuit—`Today we have naming of parts'—might care to consider that many of the greatest nature poets have also been keen naturalists, with a detailed knowledge of what is out there: Shakespeare, Wordsworth, John Clare, D. H. Lawrence—the list goes on and on. Indeed it seems we can never appreciate anything fully unless we first put a name to it and have some feel for what it is and where it comes from. A parable will make the point.

A VIRUS OF FINE ART

Imagine that in some pleasantly out-of-the-way art gallery, let's say the incomparable Dulwich in South London, there arises an information virus, analogous to the kinds that infect computers. This hypothetical virus does no harm to the paintings. It merely obliterates the signatures. But then it creeps out of the frames and into the catalogues and reference books, and into the minds of the visitors and critics and everyone who has any vestige of knowledge. It spreads from the Dulwich to the Tate and then to the National, and so to the smaller collections, the Courtauld's and the Wallace, then out to Britain's other cities and grand houses, and across the channel to the Rijksmuseum and the Louvre, the Prado and the Uffizi, and east to the Hermitage and into Asia, and across the Atlantic to the Metropolitan Museum of New York, the Guggenheim and the Frick, and all the great galleries of the United States, and out beyond there into all the world, all the time working its way through every work of reference, and all the memories in the minds of every art enthusiast. The paintings themselves remain unscathed. But soon, nobody in the world knows who painted what, or where, or when.

    What would we lose through such a virus? According to one school of criticism, very little; for some have argued that works of art should be self-contained and need no extraneous information to be appreciated: no biography, no history, no referents of any kind. But most would be appalled by the loss. The paintings would still be there, right enough, as stunning as ever. But without any knowledge of who, when, and where—let alone of why—the paintings would have no meaning. We would lose all sense of development—of ideas, subject, style, and technique. There would still be clues to history and provenance: a preponderance of paintings in Italy of religious and mythological motif, bold in colour and with a distinct tendency to glow, would surely help future scholars to perceive that Italian paintings have a particular `feel', and to infer that comparable pictures even in the furthest corners of the world probably came from Italy in the first place. Soon, with luck and fingers crossed, they would be grouping Dutch landscapes with Dutch landscapes, English portraits with English portraits, and so on and so on; although Dutchmen who chose to paint like Italians, like Cuyp, would give them pause.

    With time, though, they would be able to ascribe particular groups of paintings to particular artists: Rubens, Rembrandt, Vermeer, Constable, Turner, Poussin, El Greco, Tiepolo would surely be among those who might reasonably be distinguished from the rest. But these authors would remain hypothetical, and would have to be given arbitrary names because the originals would be lost. Around such relative certainties, others might be grouped as `school of'. But it would be a bold scholar indeed (and undoubtedly forever controversial) who suggested that all the known works of Picasso or Cezanne could each be ascribed to one hand; in fact no one might ever realize that that was the case.

    As more decades passed, scholars would begin to see connections between groups that they had previously been at pains to separate: for example, that the human figures of Cezanne have much in common, in the way they are perceived and constructed, with those of Rubens. But without knowledge of dates the scholars would still be obliged to ask, `Did Rubens influence Cezanne, or did Cezanne influence Rubens? Or were they perhaps contemporaries, drawing their ideas from a common source?'. These questions seem ludicrous from our present vantage point but in the post-virus age the matter would have to be addressed afresh; and, without historical knowledge, the answer is far from obvious. Is it more reasonable to infer that Cezanne's sketchier style is an allusion to Rubens, or that Rubens represents a more finished version of Cezanne's prototypes?

    In short, such a virus would take away much of our insight; and without that insight, we would also lose much of the pleasure that painting holds. The critics who have argued that everything we need to know is contained within the frame would surely realize the error of their ways. The loss of historical knowledge would be as catastrophic as, say, the loss of an entire gallery. No one would doubt, either—would they?—that the first task of scholars in a post-virus age must be to recapture the lost biographies and histories. `Who?' and `when?' are the essential questions, and to a lesser extent `where?'. When those questions are addressed, and not until, we can ask begin to ask `why?' with some hope, of a satisfying answer.

    But animals and plants, fungi and bacteria, do not come with signatures and historical asides. They just are. Where they came from, and why they take their present forms, we have to work out for ourselves. As naturalists we begin like the art-scholars in a post-virus age. Whatever the practicalities may be, we need for aesthetic reasons to find out what is what. The reasons for studying systematics are hence threefold: intellectual, practical, and aesthetic. In this volume, I want to explore all three, but particularly the first and last. Why, though, write this particular book, in this particular way? Various critics in a spirit of helpfulness have suggested several other ways of going about it. Why haven't I always taken their advice?

WHY THIS BOOK IS LIKE IT IS, AND HOW TO MAKE BEST USE OF IT

As recorded in the Acknowledgements and in the Sources and further reading, a huge number of scientists and friends worldwide have helped me with this book. All have been extraordinarily kind but some have raised a few doubts. What's this book for?, they ask. More to the point, who is it for? Does it not fall between two stools—too detailed for the `layperson' and too general for the professional? The phylogenetic trees are shown without detailed notes to explain precisely why they are arranged as they are—so are they really of use to undergraduates, or university teachers, seeking to set courses in the taxonomy of crustaceans, or echinoderms, or whatever? Then, again, systematics is developing rapidly, as established classifications are subjected to sharper and sharper cladistic and molecular analyses, and more and more fossils come on line, and bacteria and protists turn out to be ever more weird. New phylogenetic trees and classifications are published daily on the internet. What use, in such a field as this, is a book published between hard covers, with all the passage of time that this entails?

    Cogent criticism—but I never actually doubted, not for a second, that this book should be done. For one thing, I have been writing about science for more than three decades and have never been convinced that the gap between the professional scientist and the non-scientist is anything like so vast or unbridgeable as it is commonly held to be. At least: a zoologist may know a thousand times more about animals than a non-zoologist (or about the particular animals they happen to study), but I have often been shocked to find how little zoologists tend to know about plants, or how little botanists commonly know about animals. In fact, although a few scientists are remarkably broad and put most of us to shame, most are extreme specialists, with chasteningly little knowledge of anything outside their own specialty. So a general account of plants that would satisfy a weekend naturalist—and indeed can be appreciated by a bright nine-year-old—should also be of use to a professional biologist who does not happen to be a plant taxonomist. There is no shame in this. It is hard to stay ahead in any specialty, and life is short. Botanists have very little time to think deeply about animals and generally have little reason to; and most zoologists have little time or reason to think seriously about plants.

    But botanists do sometimes need to think about animals—or may simply be curious, like any naturalist; and zoologists do tend to like plants, even if they don't have much time for them. So there is a place for books on plants for zoologists, and books on animals for botanists; and a book that fills such a role will be the same as one intended for amateurs, who look at living things for pleasure. Is this really so? Many argue, after all, that although professional biologists must specialize, they still share many ideas and have a common vocabulary They all know what DNA is, for example, and have a rough idea how it works. They don't need to be told the basic things that a non-biologist might not be aware of, and so the gap between the professional and the amateur remains.

    There are several points here. First, most of the technical ideas and vocabulary in this book apply specifically to the modern science of systematics; and, I have found, most biologists who are not themselves systematists do not know these specialist terms. If you know any practising biologists (who might include doctors, for example) ask them what they understand by `symplesiomorphy'. It is a pleasant word when you grow used to it—roll it around the tongue—and fundamental to the modern method of cladistics. But I would bet that at least nine out of 10 biologists have no idea what it means.

    Even so, some will argue that nobody who is not a professional taxonomist need understand a word like symplesiomorphy. It is, the detractors will suggest, merely a piece of `jargon' that should be confined to the specialist journals, and perhaps expurgated even from them. But again I will contend the precise opposite. First, we should distinguish between `jargon' on the one hand and `technical terms' on the other. The word `jargon' derives from the French for the twittering of birds and implies incomprehensibility—and, of course, all insider groups from generals to cat-burglars develop their own semiotic pidgins that sound like twittering birds to the outsiders and indeed are largely intended to keep outsiders at bay. Technical terms, on the other hand, refer to things and phenomena that exist only in recondite contexts and therefore need to be described in their own specialist language. Symplesiomorphy can be explained in non-technical terms—it refers to characteristics that different creatures hold in common, but which are simply `primitive' features that are also found in other creatures. All will become clear as this book unfolds, including the special meaning of the word `primitive'. The concept of symplesiomorphy is crucial; and the word, despite its abundance of syllables, alludes to that crucial concept precisely and unambiguously.

    Thus if you feel that systematics is worthwhile at all, then `symplesiomorphy' is one of a short list of technical terms that is worth getting to grips with. This is meant to be an introductory text—an introduction to the true implications of biodiversity and of the means by which biologists try to come to terms with it; and the whole point of such a book is, I suggest, to guide readers from a position of essentially zero knowledge to the point where they can, if they choose, begin to read the specialist literature for themselves. Needless to say, the specialist literature is technical, so an introduction will fail miserably in its task if it does not explain the technicalities. So the technicalities are explained and used in this book; and the explanations, as I see it, are of equal value to the graduate biologist as they are to the weekend naturalist—unless the biologist happens already to be a specialist systematist. In fact, introductory books that avoid technical terms, as many seem to do as a matter of policy, sell the reader short. They do not actually introduce the subject at all: they merely provide a pastiche, and a filleted one at that.

    What of the critics who suggest that this book is lacking in detail, so that it would be hard to base a university course on it? Well, this is not intended to be a textbook, but textbooks are not the only kinds of books that are useful. I hope that teachers might feel they can base general courses on biodiversity on this book; but if they want to offer a specialist course on a particular group, crustaceans or ferns or whatever, then if they are serious teachers they should, of course, go back to the specialist literature. On the other hand, I hope that both they and the students will still find this book useful for background and context.

    I have deliberately not filled the phylogenetic trees with specialist notes because I am assuming that if anybody does want to base a course on any particular group, then they will indeed go to the specialist literature. But because I have not appended such notes, I am referring to the phylogenetic trees throughout this book simply as `trees', rather than as `cladograms'. A cladogram, as we will see, is a diagrammatic representation of a specific hypothesis, about the way in which particular creatures are thought to be related; and it must carry notes, to show the reasoning behind the hypothesis. A tree is a more general statement that may or may not be based on a cladogram, or on a series of cladograms. The trees in this book are all based on cladograms, but the reasoning behind the cladograms is not spelled out in detail.

    So I have written this book at the level that I feel is right (and it is, in the end, a matter of `feel'). I did not want to write a textbook to squeeze particular groups of students through particular exams. That's for other people to do. In the end, the only sensible course for authors is to write the kinds of books that they would like to read themselves—and hope that there are other people out there who have the same kinds of tastes. Unless the author comes from Mars, that ought to be true at least some of the time.

    But still, some will ask, what use is a book that may take a decade to write (as this one has) and a year to publish, on a subject that changes so fast? New schemes of classification are now published on the internet, and the details change by the week. Well, the details may indeed seem labile but the deep ideas are far more constant. Carl Woese's notion from the 1970s that living things should be classified in three `domains'—bacteria, archaes, and eukaryotes—will surely be with us for many decades to come, and perhaps effectively forever. Mitch Sogin's suggestion that the eukaryotes include at least a dozen lineages so distinct that they each deserve the rank of `kingdom' seems to be founded just as firmly. Even if the details change we can be sure that biologists will never again accept the peremptory, eighteenth-century division of all living things into `animals' and `plants'. Cladistics seems to have an inexorable logic, and surely will stay with us; and molecular biology will continue to unfold for the next 1000 years. These are the kinds of ideas that the browser on the internet needs to understand if the deluge of new ideas is to make any sense at all.

    A book like this has two functions, then: first, to provide the background to all the new data; and, second, to provide, a database with which to compare the new findings. After all, if you don't know what people thought previously, you will not appreciate the innovations. Besides—and very importantly—not every newly published cladogram is actually true. Because different studies begin with different data, and the data in any one study can be interpreted in many different ways, that is obviously the case. The information in this book cannot be called the orthodoxy, because there is no generally agreed, all-embracing canon. Neither can its ideas be guaranteed to be correct; all ideas in science are hypotheses, awaiting improvement. But the ideas represented here are the best opinions of current world authorities both as published and as told to me. That is not a perfect representation of the whole truth: but in the real world, in real time, I don't know how to get closer to it.

    Overall, then, I have had two broad aims in writing this book, which some may feel are hugely pretentious (though I am prepared to live with that charge). My specific aim is to help to restore the art, craft, and, indeed, the modern science of taxonomy, or systematics, back to the centre of biological teaching and thinking, which is where it ought to be. Systematics is the discipline that introduces us to the creatures themselves. Biology without quantified ecology and animal behaviour, and without the unifying grandeur of evolutionary theory and molecular biology; is just natural history But biological theory without actual, living, breathing creatures, is just philosophy. I like natural history and I like philosophy, but it is only when we put the two together that we have biology, the subject that truly merits a lifetime's absorption.

    My second broad aim is simply to point out that nature is wonderful, and that much of the wonder lies in its variety. Without classification, the variety is simply bewildering, and bewilderment gets in the way of thought. The act of classifying focuses our thoughts and the more we think, the greater the wonder becomes; for this, as Hamlet said in a somewhat different context, is the appetite that grows from what it feeds on. Classification, in short, is not a dull pursuit for obsessives. It is the essential aid to understanding, the means by which to come to grips with life's variety. It puts us in touch. This book has become a hobby. It is good to see it finished, but in many ways I am sorry it is over.