Darwin saw twining plants as ancestral to other climbing plants, and he thought that tendril-bearing plants had evolved from these, and leaf-climbers, in turn, from tendril-bearers, each development opening up more and more possible niches—roles for the organism in its environment. Thus climbing plants had evolved over time—they had not all been created in an instant, by divine fiat. But how did twining itself start? Darwin had observed twisting movements in the stems, leaves, and roots of every plant he had examined, and such twisting movements (which he called circumnutation) could also be observed in all “lower” plants: cycads, ferns, seaweeds, too. When plants grow toward the light, they do not just thrust upward, they twist, they corkscrew, toward the light. Circumnutation, Darwin came to think, was a universal disposition of plants, and the antecedent of all other twisting movements in plants.
These thoughts, along with dozens of beautiful experiments, were set out in his last botanical book, The Power of Movement in Plants, published in 1880. Among the many charming and ingenious experiments he recounted was one in which he planted oat seedlings, shone light on them from different directions, and found that they always bent or twisted toward the light, even when it was too dim to be seen by human eyes. Was there (as he imagined of the tips of tendrils) a photosensitive region, a sort of “eye” at the tips of the seedling leaves? He devised little caps, darkened with India ink, to cover these, and found that they no longer responded to light. It was clear, he concluded, that when light fell on the leaf tip, it stimulated the tip to release some sort of messenger which, reaching the “motor” parts of the seedling, caused it to twist toward the light. Similarly, the primary roots (or radicles) of seedlings, which have to negotiate all sorts of obstacles, Darwin found to be extremely sensitive to contact, gravity, pressure, moisture, chemical gradients, etc. He wrote:
There is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle…. It is hardly an exaggeration to say that the tip of the radicle…acts like the brain of one of the lower animals…receiving impressions from the sense-organs, and directing the several movements.
But as Janet Browne remarks, The Power of Movement in Plants was “an unexpectedly controversial book.” Darwin’s idea of circumnutation was roundly criticized. He had always acknowledged it as a speculative leap, but a more cutting criticism came from the German botanist Julius Sachs, who, in Browne’s words,
sneered at Darwin’s suggestion that the tip of the root might be compared to the brain of a simple organism and declared that Darwin’s home-based experimental techniques were laughably defective.
However homely Darwin’s techniques, though, his observations were precise and correct. His ideas of a chemical messenger being transmitted downward from the sensitive tip of the seedling to its “motor” tissue were to lead the way, fifty years later, to the discovery of plant hormones like auxins, which, in plants, play the roles that nervous systems do in animals.
Darwin had been an invalid for forty years, with an enigmatic illness that had assailed him since his return from the Galápagos. He would sometimes spend entire days vomiting, or confined to his sofa, and as he grew older, he developed heart problems, too. But his intellectual energy and creativity never wavered. He wrote a total of ten books after the Origin, many of which themselves went through major revisions—to say nothing of dozens of articles and innumerable letters. He continued to pursue his varied interests throughout his life. In 1877, for instance, he published a second, greatly enlarged and revised, edition of his orchid book (originally published fifteen years earlier). My friend Eric Korn, an antiquarian and Darwin specialist, tells me that he recently had a copy of this in which there was slipped the counterfoil of an 1882 postal order for two shillings and ninepence, signed by Darwin himself, in payment for a new orchid specimen. Darwin was to die in April of that year, but he was still in love with orchids and collecting them for study within weeks of his death.
His final book, The Formation of Vegetable Mould Through the Action of Worms, with Observations on Their Habits, published the year before his death, returned to a favorite subject—earthworms—which he had first written about more than forty years earlier. Starting from homely experiments in his own garden and extrapolating from these, Darwin brought out that these seemingly insignificant creatures—hitherto thought of, if at all, as pests—had been instrumental in altering the geography and geology of the Earth, digesting organic matter and turning it into soil. He calculated that in England alone, worms could transform more than a hundred billion tons of earth within a thousand years. Darwin’s book on worms was, as Robbin Moran points out,
about how slow, gradual processes working over a long time could have a great cumulative effect. Given enough time, worms can bury a house. The parallel to natural selection is obvious.
Natural beauty, for Darwin, was not just aesthetic; it always reflected function and adaptation at work. Orchids were not just ornamental, to be displayed in a garden or a bouquet; they were wonderful contrivances, examples of nature’s imagination, natural selection, at work. Flowers required no Creator, but were wholly intelligible as products of accident and selection, of tiny incremental changes extending over hundreds of millions of years. This, for Darwin, was the meaning of flowers, the meaning of all adaptations, plant and animal, the meaning of natural selection.
It is often felt that Darwin, more than anyone, banished “meaning” from the world—in the sense of any overall divine meaning or purpose. There is indeed no design, no plan, no blueprint in Darwin’s world; natural selection has no direction or aim, nor any goal to which it strives. Darwinism, it is often said, spelled the end of teleological thinking. And yet, his son Francis writes:
One of the greatest services rendered by my father to the study of Natural History is the revival of Teleology. The evolutionist studies the purpose or meaning of organs with the zeal of the older Teleologist, but with far wider and more coherent purpose. He has the invigorating knowledge that he is gaining not isolated conceptions of the economy of the present, but a coherent view of both past and present. And even where he fails to discover the use of any part, he may, by a knowledge of its structure, unravel the history of the past vicissitudes in the life of the species. In this way a vigour and unity is given to the study of the forms of organised beings, which before it lacked.
And this, Francis suggests, was “effected almost as much by Darwin’s special botanical work as by the Origin of Species.”
By asking why, by seeking meaning (not in any final sense, but in the immediate sense of use or purpose), Darwin found in his botanical work the strongest evidence for evolution and natural selection. And in doing so, he transformed botany itself from a purely descriptive discipline into an evolutionary science. Botany, indeed, was the first evolutionary science, and Darwin’s botanical work was to lead the way to all the other evolutionary sciences—and to the insight, as Theodosius Dobzhansky put it, that “nothing in biology makes sense except in the light of evolution.”
Darwin spoke of the Origin as “one long argument.” His botanical books, by contrast, were more personal and lyrical, less systematic in form, and they secured their effects by demonstration, not argument. According to Francis Darwin, Asa Gray observed that if the orchid book “had appeared before the Origin, the author would have been canonized rather than anathematized by the natural theologians.”
Linus Pauling records in an autobiographical essay that he read the Origin when he was ten. I was not that precocious, and could not have followed its “one long argument” at that age. But I had an intimation of Darwin’s vision of the world in our own garden—a garden which, on summer days, was full of flowers and bees buzzing from one flower to another. It was my mother, botanically inclined, who explained to me what the bees were doing, their legs yellow with pollen, and how they and the flowers depended on each other.
While most of the flowers in the garden had rich scents and colors, we also had two magnolia trees, with huge but pale and scentless flowers. The magnolia flowers, when ripe, would be crawling with tiny insects, little beetles. Magnolias, my mother explained, were among the most ancient of flowering plants and had appeared nearly a hundred million years ago, at a time when “modern” insects like bees had not yet evolved, so they had to rely on a more ancient insect, a beetle, for pollination. Bees and butterflies, flowers with colors and scents, were not preordained, waiting in the wings—and they might never have appeared. They would develop together, in infinitesimal stages, over millions of years. The idea of a world without bees or butterflies, without scent or color, affected me with a sense of awe.
The notion of such vast eons of time, and the power of tiny, undirected changes which by their accumulation could generate new worlds—worlds of enormous richness and variety—was intoxicating. Evolutionary theory provided, for many of us, a sense of deep meaning and satisfaction that belief in a Divine Plan had never achieved. The world that presented itself to us became a transparent surface, through which one could see the whole history of life. The idea that it could have worked out differently, that dinosaurs might still be roaming the earth or that human beings might never have evolved, was a dizzying one. It made life seem all the more precious, and a wonderful, ongoing adventure (“a glorious accident,” as Stephen Jay Gould called it)—not fixed or predetermined, but always susceptible to change and new experience.
Life on our planet is several billion years old, and we literally embody this deep history in our structures, our behaviors, our instincts, our genes. We humans retain, for example, the remnants of gill arches, much modified, from our fishy ancestors—and even the neural systems which once controlled gill movement. As Darwin wrote in The Descent of Man, “Man still bears in his bodily frame the indelible stamp of his lowly origin.”
In 1837, in the first of many notebooks he was to keep on “the species problem,” Darwin sketched a tree of life. Its brachiating shape, so archetypal and potent, reflected the balance of evolution and extinction. Darwin always stressed the continuity of life, how all living things are descended from a common ancestor, and how we are in this sense all related to each other. So humans are related not only to apes and other animals, but to plants too. (Plants and animals, we know now, share 70 percent of their DNA.) And yet, because of that great engine of natural selection—variation—every species is unique and each individual is unique, too.
The tree of life shows at a glance the antiquity and the kinship of all living organisms, and how there is (as Darwin originally called evolution) “descent with modification” at every juncture. It shows too that evolution never stops, never repeats itself, never goes backward. It shows the irrevocability of extinction—if a branch is cut off, a particular evolutionary path is lost forever.
I rejoice in the knowledge of my biological uniqueness and my biological antiquity and my biological kinship with all other forms of life. This knowledge roots me, allows me to feel at home in the natural world, to feel that I have my own sense of biological meaning, whatever my role in the cultural, human world. And although animal life is far more complex than vegetable life, and human life far more complex than the life of other animals, I trace back this sense of biological meaning to Darwin’s epiphany on the meaning of flowers, and to my own intimations of this in a London garden, nearly a lifetime ago.
I am especially indebted to David Kohn, Eric Korn, and, at the New York Botanical Garden, Robbin Moran, Dennis Stevenson, and Jan Stevenson for their stimulus and helpful criticisms of this essay.