Darwin had set out from England as an undistinguished young man with an interest in natural history; he returned as a famous scientist. At almost every port on the voyage he had been sending letters and papers home, and they had surprised and impressed some of Britain’s leading scientists. John Steven Henslow, who had taught Darwin botany at Cambridge, collected some of his former student’s observations into a paper and distributed it to his colleagues. Robert Darwin had initially been skeptical about his son’s interest in science but now he was impressed enough to give him an investment income; Charles could now afford to do science full time.
And there was a lot to do. He had collected thousands of specimens of all kinds during the voyage, and most of them hadn’t been properly cataloged. This was a common problem faced by zoologists; there were too many specimens, and not enough people to study and describe them properly. Darwin’s new-found fame gave him an advantage though. Not long after his return he met Charles Lyell, who in turn introduced him to palaeontologist Richard Owen. Owen began enthusiastically cataloging Darwin’s fossil collection, which turned out to be a treasure trove of unknown species. Several bones Darwin had thought were Megatherium turned out to be other giant sloths; he also had the remains of extinct rodents, mastodons and giant armadillos. Meanwhile Henslow took on the plant specimens, and other scientists soon joined in.
Darwin himself was busy. Before leaving England he had arranged with FitzRoy that he would write a book about the voyage, based on the journal he’d kept, and by December 1836 he was working hard on it in a rented flat in Cambridge. He gave presentations at the Geological and Zoological Societies. In March 1837 he moved to London to stay with his brother Ras, which also brought him into a new circle of scientists and thinkers. One of them was Harriet Martineau. Martineau was an early feminist, often described as the first female sociologist, and an enthusiastic advocate of Thomas Malthus. She was also determined to marry Ras, and Darwin soon found that she was at his brother’s home practically every day. At first this amused him, but then he began talking to her about Malthus’s work – and realized it answered many of the objections to evolution.
The Reverend Thomas Robert Malthus was an Anglican cleric with a passion for economics. He first became well known in 1798, when he published an essay on population growth that made some startling predictions. Production of food could only increase arithmetically, Malthus argued, while population could grow geometrically; that made it inevitable that the population would eventually outgrow the capacity of the farms that fed it. Over the next 30 years Malthus released five more editions of the essay, each one expanded and refined; the last was published in 1826, eight years before his death. Now Harriet Martineau explained it to Charles Darwin. Malthus had concluded that when the population grew too large for the food supply it would be cut down again by famine, disease or war. When it came to humans and agricultural output Malthus was wrong – food production has comfortably kept pace with population growth – but Darwin saw how the basic message could be the key to evolution.
Previous theories of evolution, like those of Lamarck and Darwin’s grandfather, had assumed that animals somehow changed to suit their environment and then passed those changes on to their descendants. What they couldn’t explain was how those changes happened and were passed on. Darwin had never accepted the old theories but what he’d seen on the Beagle voyage had made him doubt the idea that all living things had been created as they now were. He was willing to at least tentatively accept evolution if he could see a plausible way for it to happen, which up to that point he couldn’t. Now, by combining evolution with Malthus’s work then throwing away large parts of both, he could see the skeleton of a new theory – a plausible one. He started to scribble notes about “transmutation of species” in his notebook. It seems to have been around July 1837 that he realized the importance of his insights because that’s when he started a new, secret, notebook dedicated solely to evolution.
All living things reproduce and most of the ones Darwin had studied did so sexually, in male and female pairs. To maintain a stable population each pair had to have two offspring which, in turn, lived long enough to breed. But every animal he’d studied didn’t have two offspring; they had more – often many more. A female mouse lives for about two years and can have a litter of twelve young every two months – around 140 in her life. If every generation of mice multiplied by a factor of 70 then within four or five generations the entire planet would be literally knee-deep in mice. Obviously, if the mouse population wasn’t going to outrun the resources it depended on, it had to stay fairly constant. That meant that 138 of those 140 baby mice had to die before they reproduced themselves. But which 138?
Darwin had collected many specimens by this time and he knew that no two individuals of the same species were exactly the same. Now he started to wonder if these differences affected which animals were likely to live long enough to reproduce. First he tested Malthus’s theories. He marked out a square yard of earth in the garden and checked it every day; whenever a new plant sprouted he marked the spot with a length of wire. At the end of the summer there were a few living plants, but dozens of markers – most of the plants had died. The deaths might have been random, but Darwin wondered if the survivors had been slightly different in a way that had given them a better chance.
The basic idea was simple; if an animal had characteristics that gave it a better chance of survival than its siblings – slightly better eyesight, for example, or the ability to run faster – it was more likely to survive long enough to reproduce. The difference to the odds of surviving might be small, but Darwin realized that even a small difference could add up over generations. If an animal had a difference that made it only 1% more likely to survive, but it could pass that difference on to its children, then its descendants would slowly become more common until eventually they dominated the population. With enough time, and large enough changes, a new species could appear.
The next question as whether animals really could pass characteristics on to their offspring. That, Darwin decided, needed to be tested. He already suspected that changes in nature would take too long to observe in one lifetime but he thought he knew where the evidence could be found. Humans had been breeding animals selectively for centuries, picking the offspring with the characteristics they were looking for then breeding those with each other, again and again and again. In just a few years it was possible to make a big difference to the animal. But how big? Darwin wanted to find out; he joined several pigeon and dog breeding clubs and started his own collection of fancy pigeons. What he found confirmed, beyond doubt, that selecting animals for their characteristics could produce change. Of course the pigeons and dogs were being changed by human intervention but, in the growing theory that filled the B notebook, nature could do the intervening – by killing those less suited to their environment.
By the early 1840s Darwin was spending much of his spare time working on evolution, and he began to talk about it to his friends. In a letter to Joseph Hooker he said his new idea – that species had evolved, rather than been created – was “like confessing a murder”. Hooker was skeptical but interested; he believed there had been a number of acts of creation, but encouraged Darwin to continue looking for an explanation of species change. Darwin was also making a name for himself as a leading expert – perhaps the world’s greatest – on barnacles; he had collected many on the voyage and now spent hours studying, dissecting and classifying them. He started to find interesting things. He already knew that some barnacle species have males and females, while most are hermaphrodites. Now he found that some species thought to be hermaphrodites actually had tiny parasitic males embedded in them. This suggested that major differences – between hermaphrodites and sexual reproduction, for example – could be bridged by a series of intermediate stages. In 1853 his work on barnacles won him a medal from the Royal Society. The next year he collected all his notes together and finally, 18 years after HMS Beagle had returned to England, set to work in earnest on making sense of it all.
Between his observations on the Galapagos, his experiments with breeding pigeons, the study of barnacles and his knowledge of geology and Malthus, Darwin now had all the pieces of the puzzle. Every member of a species was slightly different to its siblings, and those differences could be inherited. A succession of small differences could, given enough time, add up to a major change. There had been plenty of time for changes to occur, because geology showed clearly that the Earth was at least tens of millions of years old and probably much older. Last of all, those plants or animals whose differences made them better equipped to survive were more likely to reproduce, so more likely to pass on their characteristics to the next generation. Evolution did happen, he concluded, and it happened in the same way as men bred greyhounds or fancy pigeons. The difference was that there was no plan and no careful breeder; nature itself selected, blindly and unconsciously, which organisms would reproduce themselves. All the parts had fallen into place. He named his new theory natural selection and, in late 1857, began writing the book that would become On the Origin of Species by Means of Natural Selection.