Free The Tree by Colin Tudge

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