The Tree

by Colin Tudge

  Naming, however, is only the first step. Classification requires another order of endeavor.

  GETTING SORTED

  Classification at its most basic is an exercise in convenience; and if convenience is all we are interested in, then any of us is free to carve up the world as we choose. So it is that fishmongers and chefs recognize the category of “shellfish,” which includes anything that lives in water and is crunchy on the outside and soft on the inside—in practice, an astonishingly mixed bag of crustaceans (such as shrimps and crabs) and mollusks (such as whelks and oysters). Timber merchants label all conifers “softwoods” and all broad-leaved trees “hardwoods.” They do this even though some conifers are a lot harder than many hardwoods, and the softest woods of all are in truth “hardwoods.”

  But there seems to be an innate order in nature; and at least some—perhaps most—of the terms by which most peoples classify living things do seem to reflect that underlying order, something more than mere convenience. Thus in English as surely in most languages we recognize the category of “insect” and on the whole take “insect” to be different from “spider.” “Birds,” “horses,” “dogs,” and, indeed, “conifers” and “flowers” (meaning flowering plants) are vernacular categories but, again (unlike “shellfish”), they do seem to reflect some real quality of nature: a true orderliness. In short, deep in the human psyche (and deep in the psyche of animals, too, as can be shown in laboratory trials) is the belief that nature is orderly, at least to some extent. Behind the terms “robin,” “duck,” “eagle,” and “canary” lies, very clearly, the broad general concept of “bird.”

  This simple musing raises a series of deep questions—deep enough to have kept philosophers and biologists occupied for thousands of years. First, is the order that we think we perceive “real”? Intuitively it seems obvious that shrimps and oysters are very different, even if they are lumped together as “shellfish,” while ducks and robins really are variations on a single theme that we might as well label “bird.” But can we trust our intuition? Might it not be that all creatures are entirely independent of one another, and that ducks in reality are no more similar to robins than shrimps are to oysters? Intuition tells us that there is indeed order; but we know that our intuitions can be wrong.

  Second, if the order is “real,” not just a trick of our minds, how do we pin it down? Insects, for instance, are immensely various, so why do we call them all “insects”? By what criteria do we place butterflies and beetles and grasshoppers in the same grand category, which we take to be different from the grand category of spiders? Are those criteria valid?

  Then, third, there’s the issue that has especially exercised theologians (and a great many biologists) over the past two hundred years: why should nature be orderly? Where does the order come from? Is it orderly simply because it is the creation of God (as Genesis tells us), and God has a tidy mind? Or are there other feasible or indeed necessary explanations?

  If the order in nature is “real,” if it reflects some deeper underlying intent or force, then it would (would it not?) be very good to reflect this in the classification. A classification based purely on convenience (shellfish, softwood, hardwood) is just a temporary device, a throwaway thing, that meets the needs of particular trades at particular times. A classification that reflects the true order of nature—a “natural” classification—provides true insight: insight, so many philosophers have opined, into the mind of God; or insight, so others have insisted, into forces that have brought order into being, whether inspired by God or not. Thus, the idea of providing a truly “natural” classification has engaged philosophers since, well, at least since the beginnings of philosophy.

  Plato and his pupil Aristotle are commonly taken as the twin founders of modern Western philosophy, and they had different ideas about where “order” comes from—and both ideas have been reflected in the attempts of later biologists to provide a natural classification. Thus Plato thought that everything on earth is merely a copy, and a flawed one at that, of some “ideal” counterpart that exists in what might be called “heaven.” These ideals were in fact more “real” than the things we see around us. Plato’s ideas were absorbed into Christianity, and Christianity has been a driving force in Western science, so biologists until well into the twentieth century were wont to think, Platonically, that all earthly things and creatures are ideas of God. Thus in the late nineteenth century Louis Agassiz, then an extremely influential professor of biology at Harvard, declared that each separate species is a “thought of God.”

  Aristotle was on the whole more down-to-earth and rejected Plato’s “ideals.” Instead he spoke of “essence”: there is no ideal insect, of which beetles and butterflies are reflections; what we see is what there is. Nonetheless, all insects share some “essence” of insecthood. Aristotle, unlike Plato, was a naturalist; he liked to look at nature. And he was the first philosopher that we know about who tried to devise a “natural” classification that truly reflected the essences of different forms. In doing so, he set out the most basic rules of taxonomy—and identified some of the key problems. Thus, he said, if we really want to see who belongs with whom, then we have to see what features they have in common. More specifically, the taxonomist must pick out particular “characters” (the biologist’s term for “characteristics”) of each of the creatures in question, and then see which and how many of those characters they share with other creatures.

  This is fine as far as it goes. Feathers are a very clear character of birds, and all living creatures with feathers may reasonably be classed as birds. But what about, say, number of legs? That is a clear character, too. But birds have two legs, and so do humans. Do birds and people belong together? Everything else about humans seems to suggest that we are closer to dogs, monkeys, and other mammals: like them we have hair rather than feathers, and we produce live babies rather than eggs, and women suckle their babies as other female mammals do and birds do not. So what do we make of our two-leggedness? Well, the broad generalization is that in seeking the true order of nature, some characters are more informative, or less deceptive, than others. Feathers are a good guide. Number of legs is a less good guide. Or at least this is true in this instance. When it comes to telling insects from spiders, the number of legs is a very good guide indeed.

  From the time of Aristotle, and with many a diversion, the art and craft of taxonomy shuffled along, as naturalists and apothecaries, and anyone else with an interest in nature, tried to classify the creatures they dealt with, and to some extent at least tried to create systems of classification that were “natural” and reflected the true order of nature. The medieval herbalists made great progress, describing an impressive variety of plants, with Latin (or Latinesque) descriptions to match. In the Middle Ages emerged the idea that different species of similar plants could be grouped together into genera (singular: “genus”), and this thinking is reflected in the names they gave to their plants. They did not have enough data; communications were not good, and they tended to work semi-independently; and they had few robust principles to guide their thinking. But they did a lot of vital groundwork nonetheless.

  The seventeenth century saw the birth of recognizably modern science, both in method and philosophy. The method included close, repeatable, quantified observation and orderly experiment. The philosophy included the final acknowledgment of the idea that the universe is indeed orderly. It was run, so Galileo and Newton and other great seventeenth-century physicists averred, according to natural “laws,” an idea that is still with us, at the heart of science. Naturalists quickly got in on the act. Living creatures are far more various in form and in their behavior than are the planets, or the mechanical devices that the physicists and engineers played around with. But even so, the naturalists felt, biology should have its “laws” too. This general feeling reinforced the idea that the apparent orderliness of nature, which is reflected in general terms like “bird” and “insect,” did indeed have deep origi
ns.

  John Ray was outstanding among the seventeenth-century naturalists who sought to broaden the scope of classification, to include many more creatures than the herbalists had, and to devise ground rules for finding the true order of nature that lies behind appearances. Notably, in our present context, he distinguished two great categories of flowering plants—a distinction that still persists. Some flowering plants, he pointed out, have long, narrow leaves, like lilies and grasses; others have broad leaves. More than a century later the French taxonomist Antoine-Laurent de Jussieu pinned down the deep difference that lies behind this distinction. The embryos of all flowering plants, still within their seeds, have leaves, known as “cotyledons.” The embryos of narrow-leaved flowering plants, such as lilies, grasses, and palm trees, have only one cotyledon. The embryos of broad-leaved plants, like oak trees and daisies, have two cotyledons. Hence the two great groups of flowering plants: monocots and dicots (much more of this in Chapter 6). Jussieu’s discovery illustrates another great principle, in line with Aristotle’s musing over the number of legs: that the characters that really count, and really show who is related to whom, are often ones that are not particularly obvious; indeed, they are “cryptic.”

  Jussieu was a child of the Enlightenment, in which thinkers of all kinds sought to integrate all the wisdom of the world into one grand “rational” framework. The Enlightenment was centered in France, and Jussieu was only one of a host of late-eighteenth-century French biologists who made an enormous and lasting impact. Best known of them all was Jean-Baptiste Lamarck, who was a fine botanist and devised keys to aid identification. But the Enlightenment touched all of Europe, and perhaps the most influential Enlightenment biologist of all was a Swede, Carolus Linnaeus or Linneus (whose name is sometimes Germanized for no good reason to Carl von Linné). Linnaeus was primarily a botanist and led several expeditions deep into Europe, much of which in his day was still very wild and woolly, discovering many hundreds of new species. He was also a marvelous extrovert and led botanical expeditions from his native Uppsala with the local band out in front and everyone dressed in a uniform of his own design. This demonstrates once more how much easier it is to be a botanist than a zoologist. Animals faced with such a mob would surely have packed themselves off to Russia.

  More to the point, between the 1730s and the 1750s Linnaeus built upon the ideas of his contemporaries and predecessors to create the system of classification that is with us still, and is called “Linnean.” In truth, since Linnean classification has been significantly modified over the years, it should surely these days be called “neo-Linnean.” But so far as I know, I am the only person to use the term “neo-Linnean” (and will continue to do so until it catches on).

  At the root of Linnaeus’s classification is the “binomial” system of naming living creatures. Each creature has two names, as in Quercus robur or Homo sapiens. The first name is “generic,” denoting the name of the genus, and the second is the species. In truth, Linneus did not invent the binomial system from scratch—it is evident in the work of the medieval herbalists—but he made it formal. It remains one of the few items of language that is universally acknowledged worldwide. Absolutely unbreakable convention rules that these scientific names are always written in italic; that the generic name always begins with a capital letter; and that the specific name is always written lowercase, even when it is based on the name of a country (as in indica or africana) or a person (as in williamsii or cunninghamii). (Newspapers almost invariably get the convention wrong.) The names are properly called “scientific” but are often known as “Latin” even though they commonly include just as much Greek, and also may incorporate the names of people and places or bits of Swahili or Inuit or what you will.

  Linnaeus also proposed that similar genera should be contained within larger groups known as orders, and that similar orders should be grouped into classes, and classes into kingdoms. He regarded kingdoms as the biggest grouping of all and recognized only two: Plantae and Animalia. He was not a good microscopist (even though microscopes were very popular in the eighteenth century) and had little to say about the creatures that cannot be seen without them (such as protozoa and bacteria). Somewhat perversely (he should have known better) he rammed the fungi in with the plants.

  Early in the nineteenth century the English anatomist Richard Owen provided one more crucial conceptual advance—one that answered Aristotle’s problem of how to distinguish important characters from less important. The important characters, said Owen, are “homologous”—they are features that may have different functions in different creatures but nonetheless clearly have a common origin. Thus the wings of birds, the front legs of horses, and the arms of people serve very different functions, yet they all originate as forelimbs. This can be determined from common observation—and can be seen unequivocally when you look at the embryos. The wings of flies serve the same general purpose as the wings of birds but are clearly very different. They arise as projections from the back, quite independently of the limbs. Bird wings and fly wings are merely “analogous.” The creatures with homologous, shared features should be grouped together (and birds, horses, and people are all classed as “vertebrates,” with flies in the separate category of “insects”).

  Cases like this are obvious. But when biologists are looking at unfamiliar structures in unfamiliar plants—and especially at fossils that have been reduced to fragments—the crucial distinction between homologies and analogies can be very hard to make. Even in what may seem like clear-cut cases, the distinction may not be easy. Charles Darwin wondered whether flowers were homologous with the cones of conifers. They have a roughly similar structure (at least when compared with primitive flowers, such as magnolias), and they do the same job. The general consensus today is that they are not homologous. Conifers and flowering plants invented their sexual organs separately.

  The trek from Aristotle to Linnaeus, with the additional insight of Owen, takes us halfway to modern taxonomy. But even by the time of Linnaeus, a sea change was in the offing.

  THE FINAL ROAD TO MODERNITY

  Until well into the nineteenth century, most European and American biologists took it for granted that life began on earth in the way described in Genesis. God created everything. He made each creature separately: the enormous diversity reflects the fertility of his mind. He placed each one in the environment to which it was best suited—shaggy bears in the north, smooth-haired bears in the tropics (Malaysia, South America), and so on. Each creature is “adapted” to its environment—for if it were not, it could not live there, and the general phenomenon of adaptation was explained by God’s beneficence. He molded creatures to thrive in whatever conditions he placed them in. Of course he did. He is benign.

  But Genesis also implied that the world was created quickly—on the first day, on the second day, and so on. Furthermore, in the seventeenth century a zealous Irish bishop called James Ussher added up the reported ages of all the patriarchs listed in the early books of the Old Testament and concluded that the earth must have been created in 4004 B.C., which made it less than six thousand years old. Genesis also describes the Flood, in which Noah rescued a male and female of all the creatures. Present-day creatures are all descended from the couples that Noah took on to his ark. Clearly the creatures that lived before the Flood were the same as the ones that live now.

  The general rationalism of the eighteenth century, the huge exercises in civil engineering that ate deep into the bedrock, and the new, growing, formal science of geology nibbled away at the details offered in Genesis. It was clear by the end of the eighteenth century that the earth was much older than six thousand years (even though the geologists who discovered this, such as Scotland’s James Hutton, typically remained as devout as ever). In the early nineteenth century formal collections of fossils, most spectacularly of dinosaurs and other ancient reptiles, showed beyond reasonable doubt that a huge range of creatures existed before the Flood, yet did not survive it—and also su
ggested that many of the creatures that surround us now, like elephants and oak trees, did not exist at the time of the dinosaurs. Clearly there had not been a once-for-all creation of plants and beasts that had remained unchanged ever since. Clearly the ones that were created first were long gone, replaced by others. Either there had been a series of separate creations (not recorded in Genesis) or the initial creatures had changed over time, to give rise to those of the present day. The idea that creatures might change over time was, and is, the idea of evolution.

  Many people floated general notions of evolution in the eighteenth century. Even Linnaeus, it seems, though on the whole content with conventional theology, was veering toward it at the end of his life. Several formal descriptions and explanations were published in the late eighteenth and early nineteenth centuries, of which the best known is that of Lamarck. What was lacking, though, was a plausible mechanism: a way of explaining how and why there are so many different creatures on earth and how each one is adapted to its surroundings; and also how there could be change over time even though all creatures in general give rise to offspring who resemble themselves (“like begets like”).

  The biologists who finally provided the convincing account, and the plausible mechanism, were two Englishmen: Alfred Russel Wallace and Charles Darwin. Independently, they came up with the idea that Darwin called “natural selection.” Creatures do give birth to offspring that are like themselves—but the offspring (if sexually produced) are not identical with their parents. There is variation. Some variants, inevitably, will be better adapted to the prevailing conditions than others. And not all can survive, because all creatures are able to produce more offspring than the environment can support. The survivors, therefore, are the ones that are best adapted to the conditions. To the Victorians, the word “fit” meant “apt.” So the ones that were best adapted were the “fittest.” In the 1860s Herbert Spencer, a philosopher who at that time was extremely famous, summarized the idea of natural selection as “survival of the fittest,” a phrase that Darwin later adopted.