Three decades after Carolus Linnaeus’s death, Charles Darwin was born in Shropshire, England, the fifth child of a prosperous physician. It was 1809; the intermittently mad George III ruled Britain, in spells; Jean-Baptiste Lamarck had just published the "Zoological Philosophy," James Hutton’s uniformity was slowly enfolding Bishop Ussher’s young earth, and Georges Cuvier was hard at work drafting his alternative theory of catastrophes.

“I was a born naturalist,” Darwin later remarked; his childhood was devoted to collecting, fishing, tracking, and reading natural history. But his father sent him first to the University of Edinburgh to study medicine, and then to Cambridge, hoping to launch him into the church. Neither field interested him (“my time was wasted,” he wrote, “I was . . . sickened with lectures”) and he did more riding than studying, more bird-watching than Greek. “No pursuit at Cambridge,” he recollected, “gave me so much pleasure as collecting beetles.”

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Darwin finished school in 1831 with a decent degree, an encyclopedic knowledge of the natural world, and absolutely no interest in either healing or preaching. But he had impressed several of his Cambridge professors with his extracurricular studies. One of them, the botanist John Henslow, recommended him to another acquaintance, naval officer Robert Fitzroy, as the perfect addition to Fitzroy’s upcoming expedition—a two-year sea voyage that would take a complete geographic survey of the South American coast.

Darwin accepted at once. The planned Christmas departure of Fitzroy’s ship, the HMS Beagle, was delayed when the entire crew got sloshed: “A beautiful day,” Darwin recorded in his diary on December 26, “& an excellent one for sailing—the opportunity has been lost owing to the drunkedness & absence of nearly the whole crew.” Finally, the Beagle set off from Plymouth Sound on December 27, 1831.

The two-year journey extended to five, and the Beagle continued from the South American coast to the GalaÅLpagos Islands, then to Tahiti and Australia, circling the globe before returning home. Darwin kept copious notes on his observations. Again and again, these notes describe his struggle with the problem of species.

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To start with, the whole concept of a species was still poorly defined. “No one definition has satisfied all naturalists,” Darwin wrote, a quarter of a century later, “yet every naturalist knows vaguely what he means when he speaks of a species.” And the fixity and permanence of species (whatever they were) required multiple acts of divine creation. So why were European ground beetles, Alpine cave beetles, and American cave fish all sightless? Had each of these species been created, separately, without sight? Turnips, rutabagas, and various gourds all had enlarged stems; should this be chalked up to “three separated yet closely related acts of creation”? Or perhaps these were not separate species, just varieties? But in that case, the present definitions of species were all drastically inadequate.

Darwin’s questions were only deepened by the vast variations of living creatures that he now saw. Each island of the GalaÅLpagos had its own mockingbird; they did not interbreed, and they differed in vital ways, so each might be considered a different species; yet they were also, essentially, alike. How should they be classified? What accounted for their differences, and (even more) their similarities?

“When I was on board the Beagle,” Charles Darwin later wrote, “I believed in the permanence of species, but, as far as I can remember, vague doubts occasionally flitted across my mind. On my return home in the autumn of 1836 I immediately began to prepare my journal for publication” (the account would be published in 1839 as "Journal and Remarks," although it is usually now known as "The Voyage of the Beagle") “and then saw how many facts indicated the common descent of species, so that in July, 1837, I opened a note-book to record any facts which might bear on the question; but I did not become convinced that species were mutable until, I think, two or three years had elapsed.”

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That notebook was only the first of a series; and all of them were filled with problems. In the notebook that Darwin created between July 1837 and February 1838, he wrote, in part,

Species are constant over whole country?

Every animal has tendency to change.—This difficult to prove. . . .

No answer because time short & no great change has happened.

Unknown causes of change. . . .

Each species changes. Does it progress?

Changes not result of will of animal, but law of adaptation.

There is nothing stranger in death of species than individuals.

Difficult for man to be unprejudiced about self.

While he was struggling with the species problem, Darwin was also reading the works of fellow natural philosophers: borrowing some of their principles, rejecting others. Charles Lyell had published the "Principles of Geology" just as Darwin was setting out on the HMS Beagle; “I had brought with me the first volume . . . which I studied attentively,” Darwin notes, “and the book was of the highest service to me in many ways.” He found Lyell’s long-and-slow philosophy of change entirely convincing and adopted it for his own (“Natura non facit saltum,” he wrote—Nature does not make sudden jumps) but disagreed with Lyell’s insistence that changes have no particular progression forward. Darwin read Lamarck’s "Zoological Philosophy" and appreciated Lamarck’s vision of adaptation leading toward more complex, more “perfect” forms—although he made vigorous marginal notes criticizing the theory of use and disuse. “It is absurd this way,” he scribbled, “he assumes the want of habit causes animals annihilation of organ and vice versa.”

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In the fall of 1838, he picked up the most recent edition of Thomas Malthus’s best-selling "Essay on the Principle of Population." Malthus, a professor of history and political economy at the East India Company’s training college for its administrators, had first published the essay in 1798, and had been refining it ever since. The future of the human race, Malthus argued, was shaped by two factors: First, That food is necessary to the existence of man. Secondly, That the passion between the sexes is necessary and will remain nearly in its present state.

In other words, humanity has an innate drive to reproduce, which means that the population constantly increases. But because the food supply does not increase as rapidly as the population, a large percentage of those born will always die of starvation: the “difficulty of subsistence” provides a “strong and constantly operating check on population.”

This, for Malthus, meant that there would never be such a thing as a perfect society, in which all members live “in ease, happiness, and comparative leisure”; some part of the human race will always be suffering from poverty and hunger. But Darwin was at once gripped by another thought. “It at once struck me,” he later wrote, “that under these circumstances favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be the formation of new species.”

Darwin had found, he believed, the key to the species problem. He mulled it over for some time and in June of 1842 began to work on setting it down in writing. By 1844 he had completed a first draft of the essay that would become "On the Origin of Species"; a few years later he added to this draft the idea that these variations come about as living creatures adapt to the “economy of nature”—the environment around them.

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But he was not yet ready to publish his argument; and he was still perfecting it when, in 1858, he received a letter from the British explorer Alfred Russel Wallace. Wallace, fourteen years younger than Darwin, had greatly admired Darwin’s "Journal and Remarks." He had followed Darwin’s example and taken a field trip abroad—in his case, first to the Amazon rain forest, and then to the East Indies. He had collected his own observations on tens of thousands of different species and had come to a novel conclusion: species change, become different, evolve, because of environmental pressures.

Wallace, then in Indonesia, had been forced by a recurrent fever to spend hours every day lying down. “I had nothing to do but think,” he later wrote, and one day

something brought to my recollection Malthus’s “Principles of Population,” which I had read about twelve years before. I thought of his clear exposition of “the positive checks to increase”—disease, accidents, war, and famine—which keep down the population of savage races to so much lower an average than that of more civilized peoples. It then occurred to me that these causes or their equivalents are continually acting in the case of animals also. . . . Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, Why do some die and some live? And the answer was clearly, that on the whole the best fitted live. From the effects of disease the most healthy escaped; from enemies, the strongest, the swiftest, or the most cunning; from famine, the best hunters or those with the best digestion; and so on. Then it suddenly flashed upon me that this self-acting process would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior would remain—that is, the fittest would survive.

Wallace jotted these insights down into a quick essay, “On the Tendency of Varieties to Depart Indefinitely from the Original Type,” and enclosed it in his letter to Darwin, asking him to pass it on to Charles Lyell, or any other natural philosophers who might find it interesting.

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Darwin was gobsmacked: “This essay,” he exclaimed, “contained exactly the same theory as mine.” As requested, he sent it along to Lyell (“I never saw a more striking coincidence,” he wrote, in his cover letter, “. . . all my originality, whatever it may amount to, will be smashed”), along with a short abstract of his own work in progress.

Lyell and his colleague Joseph Hooker, director of the Royal Botanical Gardens and a personal friend of Darwin’s, presented both works to the members of the Linnean Society of London, a century-old club for the discussion of natural history; in August of 1858, Wallace’s and Darwin’s theories were published side by side in the Linnean Society’s printed proceedings.

This was the first articulation of the theory of evolution by natural selection. It was a watershed moment in natural history, but apparently no one noticed. The president of the Linnean Society famously remarked, in his annual report for 1858, “The year . . . has not, indeed, been marked by any of those striking discoveries which at once revolutionize . . . science.”

The following year, Darwin, energized by Wallace’s codiscovery of the principle of natural selection, finally published his entire argument. This first edition—"On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life," laid out a series of arguments, all supporting Darwin’s main conclusion: that life, no less than the earth itself, is changing constantly, and that natural causes alone account for that change. He had solved the species problem to his own satisfaction: Species were not permanent, fixed, or bridgeless. They appeared when previous species developed variations and those variations proved helpful in the fight for survival.

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"On the Origin of Species" immediately sold out. The book was widely discussed, widely criticized, widely praised and condemned: “The reviews were very numerous,” Darwin remarked later; “for a time I collected all that appeared . . . but after a time I gave up the attempt in despair.” In 1864 the well-known biologist and philosopher Herbert Spencer used the phrase “survival of the fittest” to describe Darwin’s theory; the phrase soon became inextricably entwined with Darwin’s work.

Over the next two decades Darwin revised the "Origin of Species" five times. Even in his final revision, he did not take the theory to its logical end; but he had already privately concluded that his principles of natural selection applied to the human race as well. “As soon as I had become . . . convinced that species were mutable productions,” he wrote in his later "Autobiography," “I could not avoid the belief that man must come under the same law.” In 1871 he finally published "The Descent of Man," an extension of his evolutionary principles to the human race.

The "Descent" brought the full implications of the "Origin of Species" into plain sight.

Charles Darwin had put biology on a collision course with the human race’s most cherished idea about itself: its uniqueness. “The question raised by Mr. Darwin as to the origin of the species,” one reviewer wrote, “marks the precise point at which the theological and scientific modes of thought come into contact. . . . We are brought face to face in this book with the difficult problems which previously had only revealed themselves more or less indistinctly on the dim horizon.”

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Those difficult problems were now in plain sight—and would remain there.

Excerpted from "The Story of Science: From the Writings of Aristotle to the Big Bang Theory" by Susan Wise Bauer. Published by W.W. Norton and Co. Copyright © 2015 by Susan Wise Bauer. Reprinted with permission of the publisher. All rights reserved.