The orchid work moved quickly and well, and in 1862 Darwin was able to send his manuscript to the printers. The book had a typically long and explicit Victorian title, The Various Contrivances by Which Orchids Are Fertilised by Insects. His intentions, or hopes, were made clear in its opening pages:
In my volume “On the Origin of Species” I gave only general reasons for the belief that it is an almost universal law of nature that the higher organic beings require an occasional cross with another individual…. I wish here to show that I have not spoken without having gone into details…. This treatise affords me also an opportunity of attempting to show that the study of organic beings may be as interesting to an observer who is fully convinced that the structure of each is due to secondary laws, as to one who views every trifling detail of structure as the result of the direct interposition of the Creator.
Here, in no uncertain terms, Darwin is throwing down the gauntlet, saying, “Explain that better—if you can.”
Darwin interrogated orchids, interrogated flowers, as no one had ever done before, and in his orchid book he provided enormous detail, far more than is to be found in the Origin. This was not because he was pedantic or obsessional, but because he felt that every detail was potentially significant. It is sometimes said that God is in the details, but for Darwin it was not God but natural selection, acting over millions of years, which shone out from the details, details which were unintelligible, senseless, except in the light of history and evolution. His botanical researches, his son Francis wrote,
supplied an argument against those critics who have so freely dogmatised as to the uselessness of particular structures, and as to the consequent impossibility of their having been developed by means of natural selection. His observations on Orchids enabled him to say: “I can show the meaning of some of the apparently meaningless ridges and horns; who will now venture to say that this or that structure is useless?”
In a 1793 book entitled The Secret of Nature Revealed in the Structure and Fertilization of Flowers, the German botanist Christian Konrad Sprengel, a most careful observer, had noted that bees laden with pollen would carry it from one flower to another. Darwin always called this a “wonderful” book. But Sprengel, though he drew close, missed the final secret, because he was still wedded to the Linnaean idea of flowers as self-fertilizing—and thought of flowers of the same species as essentially identical. It was here that Darwin made a radical break and cracked the secret of flowers, by showing that their special features—the various patterns, colors, shapes, nectars, and scents by which they lured insects to flit from one plant to another, and the devices which ensured that the insects would pick up pollen before they left the flower—were all “contrivances,” as he put it; they had all evolved in the service of cross-fertilization.
What had once been a pretty picture of insects buzzing about brightly colored flowers now became an essential drama in life, full of biological depth and meaning. The colors and smells of flowers were adapted to insects’ senses. While bees are attracted to blue and yellow flowers, they ignore red ones, because they are red-blind. On the other hand, their ability to see beyond the violet is exploited by flowers which use ultraviolet markings—the so-called honeyguides which direct bees to their nectaries. Butterflies, with good red vision, fertilize red flowers, but may ignore the blue and violet ones. Flowers pollinated by night-flying moths tend to lack color, but to exude their scents at night. And flowers pollinated by flies, which live on decaying matter, may mimic the (to us) foul smells of putrid flesh.
It was not just the evolution of plants but the coevolution of plants and insects that Darwin illuminated for the first time. Thus natural selection would ensure that the mouth parts of insects matched the structure of their preferred flowers—and Darwin took special delight in making predictions here. Examining one Madagascan orchid with a nectary nearly a foot long, he predicted that a moth would be found with a proboscis long enough to probe its depths; such a moth was finally discovered, decades after his death.
The Origin was a frontal assault (delicately presented though it was) on creationism, and while Darwin had been careful to say little in the book about human evolution, the implications of his theory were perfectly clear. It was especially the idea that man could be regarded as a mere animal—an ape—descended from other animals that had provoked outrage and ridicule. But for most people, plants were a different matter—they neither moved nor felt; they inhabited a kingdom of their own, separated from the animal kingdom by a great gulf. The evolution of plants, Darwin sensed, might seem less relevant, or less threatening, than the evolution of animals, and so more accessible to calm and rational consideration. Indeed, he wrote to his friend Asa Gray, “no one else has perceived that my chief interest in my orchid book, has been that it was a ‘flank movement’ on the enemy.” Darwin was never belligerent, like his “bulldog” Huxley, but he knew that there was a battle to wage, and he was not averse to military metaphors.
It is, however, not militancy or polemic that shines out of the orchid book; it is sheer joy, delight in what he was seeing. This delight and exuberance burst out of his letters:
You cannot conceive how the Orchids have delighted me…. What wonderful structures!… The beauty of the adaptation of parts seems to me unparalleled…. I was almost mad at the wealth of Orchids…. One splendid flower of Catasetum, the most wonderful Orchid I have seen…. Happy man, he [who] has actually seen crowds of bees flying round Catasetum, with the pollinia sticking to their backs!… I never was more interested in any subject in all my life than in this of Orchids.
The fertilization of flowers engaged Darwin to the end of his life, and the orchid book was to be followed, nearly fifteen years later, by a more general book, The Effects of Cross and Self Fertilisation in the Vegetable Kingdom.
But plants have to survive, flourish, and find (or create) niches in the world, if they are ever to reach the point of reproduction. Thus Darwin’s equal interest in the devices and adaptations by which plants survived and their varied and sometimes astonishing lifestyles, which included sense organs and motor powers akin to those of animals.
In 1860, during a summer holiday, Darwin first encountered and became enamored of insect-eating plants—and this started a series of investigations that would culminate, fifteen years later, in the publication of Insectivorous Plants. This volume has an easy, companionable style, and starts, like most of his books, with a personal recollection:
I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew ( Drosera rotundifolia ) on a heath in Sussex…. On one plant all six leaves had caught their prey…. Many plants cause the death of insects…without thereby receiving, as far as we can perceive, any advantage; but it was soon evident that Drosera was excellently adapted for the special purpose of catching insects.
The idea of adaptation was always in Darwin’s mind, and one look at the sundew showed him that these were adaptations of an entirely novel kind, for Drosera‘s leaves not only had a sticky surface but were covered with delicate filaments (Darwin called them “tentacles”) with glands at their tips. What were these for, he wondered? “If a small organic or inorganic object be placed on the glands in the centre of a leaf,” he observed,
they transmit a motor impulse to the marginal tentacles…. The nearer ones are first affected and slowly bend towards the centre, and then those farther off, until at last all become closely inflected over the object.
But if the object was not nourishing, it was speedily released.
Darwin went on to demonstrate this by putting blobs of egg white on some leaves and similar blobs of inorganic matter on others. The inorganic matter was quickly released, but the egg white was retained, and stimulated the formation of a ferment and an acid that soon digested and absorbed it. It was similar with insects, especially live ones. Here, without a mouth, or a gut, or nerves, Drosera efficiently captured its prey and, using special digestive enzymes, absorbed it.
Darwin addressed not only how Drosera functioned but why it had adopted so extraordinary a lifestyle: he observed that the plant grew in bogs, in acidic soil that was relatively barren of organic material and assimilable nitrogen. Few plants could survive in such conditions, but Drosera had found a way to claim this niche by absorbing its nitrogen directly from insects rather than from the soil. Amazed by the animal-like coordination of Drosera ‘s tentacles, which closed on its prey like those of a sea anemone, and by the plant’s animal-like ability to digest, Darwin wrote to Asa Gray:
You are unjust on the merits of my beloved Drosera ; it is a wonderful plant, or rather a most sagacious animal. I will stick up for Drosera to the day of my death.
And he became still more enthusiastic about Drosera when he found that making a small nick in half of a leaf would paralyze just that half, as if a nerve had been cut. The appearance of such a leaf, he wrote, resembled “a man with his backbone broken and lower extremities paralysed.” Darwin later received specimens of the Venus flytrap—a member of the sundew family—which, the moment its trigger-like hairs were touched, would clap its leaves together on an insect and imprison it. The flytrap’s reactions were so fast that Darwin wondered whether electricity could be involved, something analogous to a nerve impulse. He discussed this with his physiologist colleague Burdon Sanderson and was delighted when Sanderson was able to show that electric current was indeed generated by the leaves, and could also stimulate them to close. “When the leaves are irritated,” Darwin recounted in Insectivorous Plants, “the current is disturbed in the same manner as takes place during the contraction of the muscle of an animal.”
Plants are often regarded as insensate and immobile—but the insect-eating plants provided a spectacular rebuttal of this notion, and now, eager to examine other aspects of plant motion, Darwin turned to an exploration of climbing plants. (This would culminate in the publication of On the Movements and Habits of Climbing Plants.) Climbing was an efficient adaptation, allowing plants to disburden themselves of rigid supporting tissue, and to use other plants to support and elevate them. And there was not just one way of climbing, but many. There were twining plants, leaf-climbers, and plants that climbed with the use of tendrils. These especially fascinated Darwin—it was almost, he felt, as if they had “eyes,” and could “survey” their surroundings for suitable supports. “I believe, Sir, the tendrils can see,” he wrote to Asa Gray. How did such complex adaptations arise?