The Tree (7 page)

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 suggested 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.

In 1858 Darwin and Wallace presented their ideas in a joint paper, which was read on their behalf to the Linnean Society of London. The Linnean is an august society of biologists, still with its headquarters in Piccadilly, that was founded to commemorate Linnaeus. Darwin and Wallace’s paper was surely the most momentous ever presented to them—indeed, it was one of the most momentous ever presented anywhere. But the Linnean’s president, in his annual report for 1858, dourly reported that nothing much of interest had happened that year. In 1859 Darwin (who had been thinking about the ideas longer than Wallace had and had a much broader scientific background) expounded the ideas more fully in
On the Origin of Species by Means of Natural Selection,
generally referred to as
The Origin of Species.
The
Origin
changed the course of modern biology, and also changed all philosophy and theology. In it, Darwin speaks of “descent with modification.” Most other biologists preferred, and prefer, the term “evolution.”

In truth, Darwin made four outstanding contributions that are central to our theme. First, he established once and for all that evolution is a fact. Second, he provided the plausible mechanism: natural selection. Third (a separate issue), he argued that species are not as the Platonists still conceived them to be—once-for-all creations that could not be changed. Over evolutionary time, he said, species could change into other species, and the lineages could branch, so that any one species could give rise to many different types that would all then evolve along separate lines.

Finally, he proposed that all the creatures that have ever lived on earth are descended from the same common ancestor that lived millions of years in the past (although Darwin did not know how many millions). We share a common ancestor with robins and mushrooms and oak trees. This at a stroke answers the deepest problem:
why
there is order in nature. To be sure, we can say that God designed butterflies and bees along similar lines simply because he has a tidy mind. But we can also argue that butterflies and bees are similar because, in the deep past, they shared a common ancestor: the first ever insect. Deeper back in time, the first ever insect shared a common ancestor with the first ever shrimp—and insects and shrimps clearly have a lot in common. Even before that, the common ancestor of insects and shrimps shared a common ancestor with spiders. So although insects are clearly very different from spiders, they too still have quite a bit in common.

Since all creatures are literally related, they can all be represented on one great “family tree” (although a family tree drawn on such a scale is more properly called “phylogenetic,” from the term “phylogeny,” which refers to the evolutionary relationship between different groups of creatures; it comes from the Greek
phylos,
meaning “tribe”). This idea chimes beautifully with Linnaeus’s classification. Linnaeus’s kingdoms represent the great boughs of this all-embracing phylogenetic tree. The classes and orders are the thinner branches. The individual species are the twigs.

Some people were offended by Darwin’s grand view of phylogeny. Some continue to argue that it is blasphemous, because it seems to contradict Genesis, which states that God created human beings separately from all other creatures, in his own image. Others are affronted by Darwin’s particular suggestion that human beings are most closely related to apes. The creationist movement is still strong worldwide—not just in the United States. Some professional biologists are creationist fundamentalists. In absolute contrast, many modern biologists and philosophers argue that since evolution by means of natural selection seems to offer a plausible alternative to the account in Genesis, this means that religion in general is obsolete and God is dead.

In truth, neither of these extreme positions is valid. It makes no sense to reject evolutionary ideas; and it makes no sense to try to use those ideas to justify atheism. Leading churchmen of the late nineteenth century knew this (and Darwin is buried in Westminster Abbey). Many modern biologists who are steeped in evolutionary theory remain devout. Many take the wondrousness and subtlety of evolution as further proof that God is indeed marvelous, and demands reverence. Many, indeed, continue to argue in the spirit of the seventeenth century that the true purpose of science is to enhance appreciation of God’s works. For my part, I feel that Darwin’s is a glorious vision. I love the notion that we are literally related to all other creatures: that apes are our sisters, and mushrooms are our cousins, and oak trees and monkey puzzles are our distant uncles and aunts. Conservation, on such a view, becomes a family affair.

Conceptually, too, with Darwin’s great insight the task of taxonomy became easier. All the taxonomist has to do is identify creatures that share common ancestors. The way to do that is to identify shared characters that are homologous. In fact, Richard Owen remained wedded to the conventional theology of his day and never fully accepted the idea of evolution—and yet, ironically, his idea of homology provides a principal clue to evolutionary relationships. But in practice it can be very difficult to decide which of the characters that different creatures share are truly homologous—and even if the difficulties are overcome there is still one theoretical snag.