The land on which species lived, after all, was not eternally unchanging. Darwin’s finches now lived on islands that had at some point breached out of the ocean. Once the Galápagos had emerged, an original species of finch might have colonized them from South America, and over time its descendants on each island had changed into the distinct species with their distinct bodies that were now adapted to their way of life. The descendants of the original settlers branched apart into distinct lineages. The same branching might have happened among the mammals of Patagonia. The animals that left behind the gigantic fossils that Darwin had discovered might have given rise to today’s mammals, with their smaller bodies.
In his notebook Darwin sketched out a tree, with old species branching into new ones.
Darwin found this idea terrifying. He began to suffer from heart palpitations and stomach-aches, to wake up out of strange dreams in the middle of the night. He knew that whatever laws governed finches or anteaters must also govern human beings. He began to think of humans as merely one more species of animal, albeit with some peculiar mental gifts. “It is absurd to talk of one animal being higher than another,” he wrote in his notebooks. “People often talk of the wonderful event of intellectual Man appearing—the appearance of insects with other senses is more wonderful…. Who with the face of the earth covered with the most beautiful savannas & forests dare say that intellectuality is the only aim in the world?”
Perhaps humans were the result of evolution, just like the finches. Darwin visited the zoo to look at a newly captured orangutan named Jenny and saw in her face the expressions he could also see in babies. “Man from monkeys?” he wrote.
Although his ideas were still embryonic, Darwin knew that they were dangerous. A public announcement that humans had evolved might alienate him from Lyell and the other naturalists whom he respected, and on whom his career depended. Nonetheless Darwin kept scribbling in his notebooks, working out his theory and gathering facts to support it.
Darwin searched for signs of how traits were handed down from one generation to the next and how they changed in the process. He interrogated gardeners and zookeepers and pigeon fanciers. He interviewed his hairdresser about breeding dogs. Although he could see signs that species were not eternal, he still didn’t know of any way a species could take on a new form. Lamarck had claimed that an animal could change over its lifetime and pass on its acquired characteristics to its offspring, but there was little evidence that this actually happened. Darwin looked for a different explanation for how evolution could unfold.
He found it in a gloomy book about humanity’s inevitable sufferings. In 1798 Thomas Malthus, a country parson, had written An Essay on the Principle of Population. In it he pointed out that a country’s population, if it is unchecked by starvation or sickness, can explode in a matter of years. If every couple raised four children, the population could easily double in 25 years, and from then on, it would keep doubling. It would rise not arithmetically—by a factor of 3, 4, 5, and so on—but geometrically—by a factor of 4, 8, and 16.
If a country’s population did explode this way, Malthus warned that there was no hope that its food supply could keep up. Clearing new land for farming or improving the yields of crops might produce a bigger harvest, but it could only increase arithmetically, not geometrically. Unchecked population growth inevitably brought famine and misery. The only reason that humanity wasn’t already in perpetual famine was because its growth was continually checked by forces such as plagues, infanticide, and simply putting off marriage till middle age.
Malthus pointed out that the same forces of fertility and starvation that shaped the human race were also at work on animals and plants. If flies went unchecked in their maggot making, the world would soon be knee-deep in them. Most flies (and most members of every species) must die without having any offspring.
In Malthus’s grim essay, Darwin found the engine that could push evolution forward. The fortunate few who got to reproduce themselves wouldn’t be determined purely by luck. Some individuals would have traits that would make them better able to survive under certain conditions. They might grow to be big, they might have a particularly slender beak, they might grow thicker coats of fur. Whichever individuals were born with these traits would be more likely to have offspring than weaker members of their species. And because offspring tend to be like their parents, they would pass on those winning traits to their young.
This imbalance would probably be too small to see from one generation to the next. But Darwin was already comfortable with imperceptible geological changes producing mountains. Here was mountain making of a biological sort. If a population of birds ended up on a Galápagos island, the individual birds that were best suited to life on the island would produce the next generation. And with enough time, these changes could produce a new species of bird.
Darwin found a good analogy for this process in the way farmers tend their crops. They breed their plants by comparing how well each stalk or tree turns out. They then use the seeds only from the best ones to plant the next generation. With enough breeding the crops become distinct from other varieties. But in nature there is no farmer. There are only individual animals and plants competing with one another to survive, for light or water or food. They undergo a selection as well, a selection that takes place without a selector. And as a result, Darwin recognized, life’s design could come about naturally, with no need for a string of individual acts of creation.
Darwin took a little time away from scribbling heresies to find a wife. Before his voyage he had fallen in love with a woman named Fanny Owen, but shortly after he set sail she married another man. When he got home he wondered if he should get married at all. Ever the methodical scientist, Charles drafted a pro-and-con balance sheet. He wrote “Marry” on the left side, and “Not Marry” on the right, and “This is the question” in the middle. This nuptial Hamlet reasoned that as a single man he would have more time for science and for conversations in men’s clubs. He wouldn’t have to earn enough to support children. On the other hand, a wife would offer “female chitchat” and constant companionship in old age. He added up the columns and made his conclusion: “Marry—Marry—Marry. QED.”
Darwin chose his cousin Emma Wedgwood. He had no interest in the sophisticated women he encountered in London. Instead, he looked back to his mother’s niece, who had grown up as he did, in the country. Emma had already become interested in Darwin during his occasional visits to the Wedgwood house. She was happy to be courted by him, although he wooed her awkwardly with a series of oblique comments and half-made gestures. She was completely unprepared when he nervously blurted out to her one day that he wanted to marry her. She said yes, but she was so stunned that she promptly went off to teach her Sunday school class.
Soon, though, Emma grew happy with the thought of marrying a man she considered “perfectly sweet-tempered.” Darwin, meanwhile, worried that his time on the Beagle had made him too unsociable for marriage, but he found hope in the prospect of marrying Emma. “I think you will humanize me,” he wrote to her, “and soon teach me there is greater happiness than building theories and accumulating facts in silence and solitude.”
Emma’s only worry came when Charles talked about nature and the laws that might govern it. Emma, a devout Anglican, could tell that Charles had his doubts about the Bible. “Will you do me a favor?” she wrote to him. She asked him to read part of the Gospel according to John: “A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.” If he began with love, Darwin might become a proper Christian.