The meeting had little obvious impact. In May 1859 the president of the Linnean Society gave a report of the happenings of the previous twelve months. His verdict was that the year “has not, indeed, been marked by any of those striking discoveries which at once revolutionize, so to speak, the department of science on which they bear.” Looking back, this seems like one of the great misstatements of all time – but that’s because we know what happened next. Darwin, at last, had been galvanized to act. During the next weeks and months he wrote and wrote, drawing together materials, observations, and results amassed over more than twenty years; and late in November 1859, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life was published for the first time.
I have one of these first editions in front of me. The binding is green leather; the lettering on the spine is in gold. The volume is thick – just over five hundred pages. When I open it, it smells musty. Which makes me think for a moment of a chilly, misty November day in London – a jostling, Victorian, imperial London, a London of women in crinolines, men in top hats, and children sweeping manure from the streets. Turkey still had the Ottomans; Russia, the Tsars; convicts were still being transported to Australia. Slaves were picking cotton in the American South. Back then Germany did not exist as a country; nor did Italy, though it soon would. The composer Richard Wagner was writing Tristan und Isolde, his great opera of doomed love; Charles Dickens had recently published A Tale of Two Cities. Telephones, cars, airplanes – none existed, except in imagination.
And from that earlier world, comes the Origin.
The Origin is simultaneously an argument and a massive compilation of evidence, and it is this evidence that, more than anything else, is so persuasive and gave the book such impact. This is what sets the Origin apart from what was presented to the Linnean Society or from the writings of Patrick Matthew.
Darwin drew from every branch of biology then known – from fossils, animal embryos, human efforts at plant and animal breeding, the way animals and plants are distributed around the world, patterns of extinction, the presence of vestigial organs like the rudimentary eyes of cavefish, and so on – as well as from observations and numerous experiments, including many he conducted himself.
To see how his thinking unfolds, consider his treatment of islands. Darwin points out that the inhabitants of oceanic islands tend to be similar to – yet clearly different from – the inhabitants of the nearest continent. As an example, he gives the Galápagos Islands. These lie on the equator more than five hundred miles from the coast of South America, yet to a great extent the plants and land birds resemble those of the continent. So much so, says Darwin, that a naturalist feels he is “standing on American land.” To explain the resemblances, he proposes that the animals and plants on islands are not specially created for island life; rather, they arrive on the islands from the nearest mainland and begin to evolve in new directions. But if this is right, they need a way to travel across the ocean. Darwin points out that birds can do this easily – they can fly – but most land mammals cannot. Which explains why oceanic islands tend to have many unique species of bird, but (until humans sailed in with rats, pigs, goats, and other members of their entourage) no mammals except bats.
But what about plants? How do they travel? At the time, it was “known” that seeds could not survive being soaked in saltwater. But Darwin tested this. He set up jars of saltwater in his study and put seeds into them for various lengths of time; he then removed them, planted them, and investigated whether or not they sprouted. Sure enough, he discovered that many seeds can survive immersion, some for as long as 137 days. He then opened an atlas and, from the rate of the ocean currents, calculated that more than 10 percent of plants have seeds that could float more than nine hundred miles and still grow, should they happen to land in a favorable spot. And that’s not the only way plants can get around. They can also be carried by birds. Darwin observes that when birds eat fruit, the seeds often pass through the digestive system intact. A bird blown hundreds of miles off course in a gale could thus carry seeds to far-flung places. More astonishing is that birds that have eaten seeds may in turn be eaten by hawks or owls – and these twice-eaten seeds can still germinate. Similarly, if a fish eats a seed and is then eaten by a bird, the bird may become the agent of seed dispersal. This is not mere conjecture. Darwin forced seeds into the bellies of dead fish; fed the dead fish to fishing eagles, storks, and pelicans; and found that some hours later the seeds were either regurgitated or excreted, and that some of these seeds could still grow.
Again and again, Darwin takes an observation – in this case, the patterns of island life – and tests as many implications as he can think of. In doing so, he anticipates and defuses criticism after criticism.
He also makes a number of important and strikingly modern insights. For example, he realized that much of the time the greatest challenges in an organism’s environment come from other life forms – predators, parasites, competitors, potential mates, and so on – rather than aspects of the physical environment such as climate. It is, after all, the living environment that generates much of the intricacy in nature – the elaborate displays that have evolved to attract mates, or (in the case of plants) pollinators, the astonishing camouflage with which some animals blend into their surroundings, the various mechanisms by which organisms of all kinds fight off disease. Through evolving interactions, beings such as birds, moths, and flowers can shape each other in complex and beautiful ways.
Moreover, as Darwin recognized, these intricate relationships have broader implications. One is that nature is a web of relationships, and changes that affect life forms in one part of the web can have strong effects elsewhere. In one of his most famous passages he observes that red clover is pollinated only by bumblebees. But the number of bumblebees depends on the number of field mice, because field mice destroy bumblebee nests. The number of field mice is in turn affected by the number of cats, which means that sometimes cats will have a strong effect on the frequency of particular flowers.
A second implication is that evolution is local. So animals and plants living in Australia, or New Zealand, or Mauritius, or wherever, develop an evolving web of relationships among themselves. If an interloper that has evolved in a different web of relationships should happen to arrive, it will sometimes be able to flourish at the expense of the residents. For example, since oceanic islands typically lack ground-dwelling mammals, life forms there repeatedly evolve characteristics that they tend not to evolve if ground-dwelling mammals are present, such as birds losing the power of flight and evolving to nest on the ground. If rats, pigs, or humans should then arrive, these animals find themselves highly vulnerable to predation, or even extinction.
Darwin also realized that although behaviors and mental capacities seem somehow different from traits such as the color of a feather or the scent of a flower, they are not. This means that behaviors can evolve in the same way as any other trait. So if individuals vary in a behavior, and if that variation has some genetic basis, then the behavior can evolve through natural selection.
Over time, then, simple behaviors can potentially evolve into far more elaborate forms. To show how this could happen, Darwin takes the hexagonal combs built by honeybees and asks how the impulse to build such complex structures could have evolved through the slow accumulation of small but useful variations. This passage is one of the finest in the book; he sets up the full difficulty of the problem to be solved and then solves it with great elegance. The problem? Honeybees have evolved to build a complex comb that holds the maximum amount of honey while using the least amount of wax. To solve it, Darwin marshals evidence from other bees that build less complex combs, he writes to a mathematician to investigate the geometry of honeycombs, and he performs experiments to elucidate how honeybees actually build their combs. He concludes that the steps to making complex combs can be reduced to a few simple rules of thumb – and that the ability to follow these rules of thumb can readily