Charles Darwin

Charles Darwin was an English naturalist, geologist, and biologist known for his contributions to the theory of evolutionary biology, specifically through the proposition of the concept that all species descended from a common ancestor and that branching patterns of evolution resulted from a process known as natural selection. He is best known for his work On the Origin of Species, in which he laid out his theories. He is sometimes called the father of evolutionary theory or shares the title with French biologist Jean-Baptiste Lamarck.

A chalk drawing of young Charles Darwin in 1816.

Darwin was born Charles Robert Darwin on February 12, 1809, in Shrewsbury, Shropshire, England. He was the second son of society doctor Robert Waring Darwin and Susannah Wedgwood. Susannah Wedgwood was the daughter of pottery industrialist and Unitarian Josiah Wedgwood, and Robert Waring Darwin was the son of Erasmus Darwin, a freethinking physician and poet fashionable before the French Revolution. Erasmus Darwin was the author of Zoonomia or The Laws of Organic Life (1794-96).

Charles Darwin's mother died when he was eight, and he would be cared for and raised by his three elder sisters. He grew up in awe of his overbearing father, who was known for astute medical observations and taught the young Charles much of what he knew of human physiology.

Charles Darwin was not the best student, disliking the rote study of Classics at the traditional Anglican Shrewbury School, where he studied between 1818 and 1825. During the period, science was considered dehumanizing, and English public schools did not teach science and tried to keep students from learning science. Charles Darwin would earn a condemnation from his headmaster, and reportedly the nickname "Gas" from his schoolmate for dabbling in chemistry as a child.

Drawing of a young Charles Darwin.

By sixteen, Charles Darwin was considered a wastrel by his father and seemed only interested in game shooting. To cure his son of this, Robert Darwin sent Charles to Edinburgh University in 1825 to study medicine. While Charles Darwin later dismissed his time in Edinburgh, it has proven, on reflection and through understanding his notes and diaries, to have been a formative experience for his later thinking. At the time, there was no better science education in a British university, and while there, he was taught the chemistry of cooling rocks on primitive Earth, how to classify plants by the then-modern "natural system," how to stuff birds by John Edmonstone, and how to identify the rock strata and how to identify the flora and fauna in England's colonies.

At the same time, Darwin was exposed to more radical thinkers, such as English Dissenters who were barred from graduating at the Anglican Universities of Oxford and Cambridge. He participated in student societies where he heard freethinkers deny the Divine design of human facial anatomies and argued animals shared the same mental faculties as humans. This, and other materialist thinking, was considered subversive during the conservative decades post-French Revolution, and some of the talks in these student groups would be officially censored. In this way, Darwin witnessed the social penalties of holding deviant views.

At the same time, Darwin was able to engage in his tendencies toward being a naturalist, as he collected sea slugs and sea pens on the shores near the university. In this activity, he was accompanied by Robert Edmond Grant, a proponent of evolutionary theories and disciple of French biologist Jean-Baptiste Lamarck. Grant went on to become a mentor of Darwin, teaching him about the growth and relationships of primitive marine invertebrates. Grant believed these marine invertebrates held the key to unlocking the mysteries that surrounded the origin of complex creatures, and that through the study of invertebrate biology and zoology, could provide an answer to those questions.

Bronze statue of a young Charles Darwin at Christ's College.

Although he was sent to Edinburgh University to study medicine, Darwin was not a fan of the practice. He hated anatomy and surgery, which at the time was pre-chloroform and done without any anesthetics. His father realized medicine was not Charles Darwin's calling and thought the church would be a better career than an aimless naturalist. He was swtiched to Christ's College, Cambridge, in 1828. This presented a complete intellectual reversal, with Christ's College having a conservative environment where Darwin was educated as an Anglican gentlemen, rode his horse, was able to indulge in drinking and shooting, and could go beetle-collecting with other squires' sons.

Christ's College also offered Darwin a look at the conservative side of botany, which young professor Reverend John Stevens Henslow taught him. At the same time, the Reverend Adam Sedgwick, a known follower of the theory of Providential design in the animal world, took Darwin to Wales in 1831 on a geologic field trip. In the same year, Darwin managed a tenth place in the Bachelor of Arts degree. John Henslow was pivotal as well, as he suggested Darwin should take a voyage to Tierra del Fuego aboard a rebuilt brig, the HMS Beagle.

A drawing of the HMS Beagle.

Darwin took Henslow's suggestion and joined the crew of the HMS Beagle not as a surgeon-naturalist but as a self-financed gentleman companion to the twenty-six-year-old captain Robert FitzRoy, an aristocrat who feared the loneliness of his command and the length of the journey. FitzRoy planned to survey the coast of Patagonia to facilitate British trade and return three natives of Tierra del Fuego who had previously been taken to England where they had been Christianized. Darwin armed himself for the voyage with several books, including the first volume of Charles Lyell's Principles of Geology and advice on preserving carcasses from experts of the London Zoo. In December of 1831, the Beagle set sail from England.

The round-the-world voyage of the Beagle from 1831 to 1836.

The Beagle circumnavigated the globe, and the trip would be the making of young Charles Darwin. The trip was five years of physical hardship and mental rigor while imprisoned in the ship's walls. At the same time, as a gentleman naturalist, Darwin was able to leave the ship for extended periods to pursue his own interests. The trip challenged many of Darwin's intellectual notions and, in some cases, confirmed theories he had already encountered, shaping his thinking and his theories to come. For example, on calm days at sea, Darwin saw the multivarious plankton and wondered in his journals why beautiful creatures teemed in the ocean's vastness where humans could not truly appreciate them, as was suggested by some theories of creation at the time.

On the Cape Verde Islands, which the Beagle visited in January of 1832, sailors and Darwin saw oyster shells running through local rocks higher up in the geological formations, suggesting that Charles Lyell's theory of slow geological movements that saw the land rise in some places and fall in others. Other evidence Darwin found of the rising and falling landscape came after the Beagle surveyed the Falkland Islands, the ship sailed the west coast of South America to Valparaiso, Chile, where Darwin climbed 4,000 feet into the Andean foothills and wondered what forces could raise such mountains.

He would meet these forces, as he saw volcanic Mount Osorno erupt on January 15, 1835. And on February 20 of that same year, in Valdivia, Chile, he lay on the forest floor during an earthquake and ensuing tidal wave, which destroyed the great city of Concepcion. In the wake of this, Darwin saw the local mussel beds were all dead and lying above high tide. The land had risen. Darwin had observed the forces that Lyell had argued cause geological formations to rise. It caused Darwin to imagine the eons it would take to raise fossilized trees in sandstone that had once been seashore mud up to 7,000 feet and sent Darwin thinking into longer and longer time scales.

Sketches from fossils in Darwin's collection.

Darwin's periodic trips over two years to the cliffs at Bahia Blanca and farther south at Port St. Julian yielded huge bones of extinct animals, later confirmed as fossilized bones, which Darwin brought back to the Beagle. These included skulls, femurs, and armor plates that he assumed were of rhinoceroses, mastodons, cow-sized armadillos, and giant sloths. This included Darwin unearthing a horse-sized mammal with a face like an anteater's and a 28-inch-long skull. Fossil extraction was another passion for Darwin, pushing his thinking into a primeval world and causing him to consider what would cause these giant beasts to die out.

At Salvador de Bahia (now Salvador), Brazil, Darwin marveled at the rainforest and the tropical splendor, but in the same area, Darwin encountered slavery, which horrified him and made him an abolitionist immediately. In July 1832, on the River Plate (Rio de la Plata), he found Montevideo, Uruguay, in a state of rebellion and followed to watch armed sailors retake a rebel-held fort.

At Bahia Blanca, Argentina, Darwin encountered gauchos working to exterminate the Pampas. He met General Juan Manuel de Rosas, who was in charge of eradicating natives of the area, which led Darwin to believe that beneath the veneer of human civility, the rule of the frontier was genocide. Darwin would again encounter these types of horrors on Tierra del Fuego in December 1832 and led him to conclude, in his journals, that there was a difference between what he called the "civilized" and "savage" man, a difference he considered greater than that between a domesticated and wild animal. He also noted, in Victorian terms, that even those most "savage" or "lowest races" were capable of "improvement," which tantalized Darwin, as it did not seem to fit with contemporary explanations of the origin of humans.

A drawn map of the Galapagos Islands.

In September 1835, Darwin landed on the Galapagos Islands. At the time, these were volcanic prison islands that were home to marine iguanas and giant tortoises. The islands have been, in legend, considered a "eureka" moment for Darwin, but it was not. He spent his time on the islands noting different mockingbirds on each of the four islands and tagged his specimens accordingly. But he failed to label other specimens he brought home with him, such as birds he thought were wrens, "gross-beaks," finches, and oriole relatives by the island they came from. Nor did he collect tortoise specimens, despite prisoners of the islands believing each island had a unique tortoise species.

In October 1836, Darwin's voyage on the HMS Beagle came to an end. Darwin had held on to his aspirations to join the vicarage; but with the end of the voyage, this aspiration had ended. Instead, Darwin was given a 400-pound annual allowance from his father, and he settled in London among the urban gentry as a gentleman geologist. During this period, Darwin befriended Lyell and was reported to have discussed his findings, which supported Lyell's geological theory. They met as Darwin was a new fellow of the Geological Society in January 1837.

He went on to publish his diary as Journal of Researches into the Geology and Natural History of the Various Countries Visited by the H.M.S. Beagle in 1839. He received a 1,000-pound treasury grant, which he obtained through the Cambridge network and used it to employ several experts to examine his specimens, and he published their descriptions in his Zoology of the Voyage of H.M.S. Beagle. These two publications would make him a known figure in London.

Title page of Darwin's Journal of Researches.

During his years aboard the HMS Beagle and later during his years in London, Darwin's thinking began to change, as written in his notes and diaries. At the time, his thinking was considered radical and dissenting as his ideas of antislavery and understanding of human equality began to frame and change his idea of human's place in nature. This was in a backdrop of radical non-comformists who were denouncing the church's monopoly on power, attacking the Anglican status quo and the supposed supernatural creation of life and society that held humans above animals. Darwin's thinking about human equality and human slavery framed his thinking of animals, such that he noted that animals were thought of as lesser beings and were made slaves by humans, and human enslavers tried to do the same with their slaves; by this logic, if all humans were equal, as he accepted, humans and animals should also be considered equal.

Another idea that affected Darwin around this time was the thinking of polymathic Charles Babbage. Some radical thinkers of the period questioned the idea that God "designed" each animal to be unique since all vertebrates shared a similar structural plan. Babbage, in part, answered this question, with the concept of God as a divine programmer, preordaining life by means of natural laws rather than ad hoc miracle. In 1837, Darwin enjoyed the soirees hosted by Babbage and likewise accepted Babbage's ideas that laws dictated the development of creatures. However, the findings of the above experts furthered Darwin's thinking. These experts identified Darwin's fossils, discovering they belonged to long-extinct mega-fauna, including huge armadillos, anteaters, sloths, and capybaras which had long gone extinct.

Sketches of some of the bird specimens Darwin brought back from the Galapagos.

Further, John Gould, an ornithologist of the Zoological Society, looked at the Galapagos birds Darwin brought back and announced that they were not the mixture of wrens, finches, and "gross-beaks" as Darwin had believed, but they were all ground finches with different adaptations to the island they originated. Further, the mockingbirds from the Galapagos were diagnosed by Gould to be three species, unique again to different islands. In March 1837, Darwin examined Captain Robert FitzRoy's collection to discover each island had a representative finch as well, each diverged from a mainland species.

At this time, Darwin lived near his brother Erasmus, known for being a freethinker, in London's West End, and he partook of a dissident dining circle. This included members such as the Unitarian Harriet Martineau and provided a milieu for Darwin's ruminations, which led him to accept the theory of "transmutation" (now known as evolution). This began Darwin's double life, as it has been referred to, as to suggest the possibility of transmutation during the period would be seen as blasphemous and heretical, a potential for corruption of humanity and the spiritual safeguards of the social order, a claim Darwin was sensitive to, if not sympathetic to. This returned to what he witnessed during his time at Edinburgh University, where he had seen censorship and others that espoused "materialist" thinking were publicly disgraced, and he was afraid his new secret would be discovered and he would endure similar attacks.

Example of one of Darwin's journals.

But at the same time, his journals showed that he was continuing to dismantle the scientific bedrock of a religiously-dominated society. Beneath a respectable facade at the Geological Society, Darwin's journals show someone growing more frustrated with what he saw as arrogant and short-sighted ideas. These notebooks showed a doggedness too, as he searched for causes of extinction, accepted life as a "branching tree" rather than other ideas that suggested species existed in a straight line, tackled island isolation, and tackled the problem of whether variation appeared gradually or at a stroke. Further, he dismissed an idea put forth by his predecessor Lamarck, that life strived "upward"—be that to greater complexity or greater intelligence—but his thinking became more relativistic, with each branch of the tree stretching not higher but into niches, and suggesting there was no way to rank humans and bees, no yardstick to suggest one is higher than the other, and thereby dismantling humans' place as the crown of creation.

This so-called radicalization in Darwin's thinking went further, as he began to think that human society was as much a result of evolution as a different beak on a finch. It was an adaptation to help humans survive and thrive. He saw similar social instincts of humans in orangutans at the London Zoo and saw social instincts as an evolutionary development that would lead humans to develop morality. By 1838, his journals detailed his thinking into one of its more radical phases, where Darwin suggested the belief in God was an ingrained tribal strategy, and the love for a deity would be an effect of the organization of the human brain. But even at his most radical, Darwin's journals were marked by a mocking tone, where he derided his thinking, and had a furtive tone, during a period where a gentleman's character had to be above reproach in order for him to be accepted in society. In 1838, Darwin was accepted in the prestigious Athenaeum Club, and by 1839, he was admitted into the Royal Society. He had too much to lose.

It has been suggested (and some of Darwin's existing journals seem to confirm this) that during this time, Darwin time spent with his brother, Erasmus, and Harriet Martineau, who was expected to marry Erasmus, brought him into greater contact with the ideas of Thomas Malthus. Martineau was a proponent of Malthus's ideas, and he sent pamphlets to Darwin about Malthus's ideas while Darwin was on the Beagle.

Darwin's thinking, especially in terms of artificial variations bred into domestic dogs and fancy pigeons (a fashion of the time), saw these changes as bred and suggested the possibility for evolution of species. But Thomas Malthus's September 1838 Essay on the Principle of Population offered a congruity to Darwin in the way nature operated and fanciers produced new breeds. Malthus's essay said there would always be too many mouths to feed, population increases geometrically, but food production rises arithmetically; therefore, according to Malthus, charity is useless. This led to the passing of a Malthusian Poor Law in 1834, which incarcerated paupers in workhouses. Darwin realized that population explosions create struggles for resources, and from these struggles some thrive, others survive, and others die. This would, in his thinking, remove those maladapted to succeeding in those conditions. This was the idea he applied to nature that would eventually be called "natural selection."

His brief argument, around this period, was such that nature was equally uncharitable and overpopulated. This creates a struggle, and from that struggle, all kinds of variations, good and bad, occur. Only the survivors and their adaptations continue through the population, surviving the thousands of trials, and this is then considered the preferred way for improved traits to be passed on. According to Darwin, that is the way species keep pace with the evolution of Earth, as previously laid out in Charles Lyell's thinking.

During this period, Darwin suffered from some health maladies, including heart palpitations and stomach problems, which sent him to the Highlands of Scotland in 1838 for a reprieve. Health problems would plague Darwin for the rest of his life. Some suggested the stress of leading his intellectual double-life resulted in health problems, and others suggested he had picked up some unknown illness from his journeys. The illness eventually extended to chronic exhaustion, eczema, and bouts of nausea, headaches, and persistent heart palpitations that plagued him for the rest of his life. Some have suggested the illness he contracted was a parasitic illness called Chagas disease. Chagas disease eventually results in cardiac damage, which would ultimately be the cause of Darwin's death.

Portrait of Emma Wedgwood.

Beginning in 1838, Darwin was contemplating marriage, but his inclination toward logical and ordered thinking was thus displayed, as he composed a list to consider the pros and cons of marriage. In the pros or "to marry" column, he listed children, a constant companion and friend for old age, and someone to take care of the house. While in the cons or "not to marry" column, he listed freedom, the conversation of clever men at clubs, and the loss of time that came with marriage. However, in 1839, he married Emma Wedgwood, his first cousin. The couple eventually had ten children, seven of whom survived to adulthood.

In part due to his marriage, and in part due to his illness, let alone his concerns about his ideas while moving through society, Darwin moved to Downe in the country in 1842. At about this time, Darwin drafted a thirty-five-page sketch of his theory of natural selection, which was expanded in a longer essay in 1844, but he had no intention of publishing it. Instead, he wrote a letter to Emma in 1844 asking that should he die beforehand, she should pay an editor 400 pounds to publish the work. This letter has suggested to some that Darwin may have wanted to die before his ideas were published so he would not have to suffer the potential criticism, censorship, and reputational damage.

Down House, where Darwin moved to in 1842.

He previously confided his thoughts on evolution to Emma, around 1839, and she had been shocked by his ideas, further confirming to Darwin that he needed to continue to keep his ideas to himself. He later commented in his journals that admitting to believing in evolution was like confessing to murder. This was not far off, as atheists of the time using evolution against what they saw as Anglican oppression had been jailed for blasphemy. During this period, specifically from 1846 to 1854, Darwin added to his credibility as a scientific expert on species through a detailed study of all known barnacles. In this study, he noted his intrigue in their sexual differentiation, as some female barnacles had tiny degenerate males clinging to them, which sparked Darwin's interest in the evolution of divergent male and female forms from what he conceived as an original hermaphrodite creature. His monographs on these topics earned him the Royal Society's Royal medal in 1853, and he could no longer be dismissed as a speculator or dabbler in biological matters.

Early page sketch from Darwin's journals showing species trees.

By the 1850s, Charles Darwin was wealthy. By the late 1840s, he had 80,000 pounds invested, was an absentee landlord of two large Lincolnshire farms, and had invested in railway shares. Further, he had established himself as a serious thinker and biologist, and socially, the 1850s were a quieter and more prosperous period, which began to see a rise in meritocracy through the education establishments across England and saw a changing social composition in which freethinkers were finding a better reception. For example, biologist Thomas Henry Huxley and the philosopher Herbert Spencer were both outsiders opting for a secular approach to nature and deriding the influence of "parsondom," and both received a better reception than they would have in the previous decades.

During this period, Darwin commented on the loss of his last tendrils of belief in Christianity with the death of his oldest daughter, Annie, from typhoid in 1851. While in 1854, he seemed to solve one of the problems of his theory, the forking genera to produce new evolutionary branches. He used a marketplace analogy, suggesting species would act like the division of labor where, in a crowded marketplace, species favored variants that could exploit aspects of a niche. He worked on some practical experiments during this time, such as with seeds in seawater to prove they could survive ocean crossings, and keeping fancy pigeons to see if the chicks would be like the parents or their ancestors. During this period, he perfected his analogy of natural selection with his analogy of fancy pigeons and what he called "artificial selection."

In April 1856, Darwin began writing a book he tentatively called Natural Selection, which was designed to crush any opposition to his theories with an avalanche of facts. His thinking had changed. Where previously, in the 1830s, Darwin thought species remained perfectly adapted until the environment changed, he instead believed by this point that each new variation was imperfect. And rather than perfect adaptation to an environment, perpetual struggle was the rule. By June 1858, Darwin had finished a quarter of a million words. There was a question about when he would be done and whether he would ever publish his massive work, until he received word that Alfred Russel Wallace, an English socialist and specimen collector, was sketching a similar-looking theory.

The title page for On the Origin of Species.

This led Darwin to act, fearing that he would lose priority, and had an extract read at the Linnean Society on July 1, 1858, alongside an extract from Wallace's work. Darwin was sick at the time and missed the first public presentation of his theory. This "absenteeism" would mark the rest of Darwin's life. At the same time, he began an abstract of his sprawling, unfinished Natural Selection, which would grow into the more accessible book, formally titled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, or more popularly On the Origin of Species.

He secreted himself away to a spa on the Yorkshire moors when the book was published, and though he sent copies to experts, they came with self-effacing letters. The book was noted to distress Darwin's Cambridge patrons, but he received sympathy from various groups in London, including from rising biologists and geologists, though few initially adopted Darwin's evolutionary approach. And the newspapers drew the one conclusion from the book Darwin had specifically avoided, that humans had evolved from apes and that Darwin's theory then denied the immortality of humanity.

Diagram showing how Darwin envisaged natural selection creating subspecies.

As noted above, the argument for the development of species and their plurality from evolution was not new, nor was it Darwin's theory. Rather, his argument centered on the argument for evolution by natural selection. Where previous theories, such as Lamarck's theory of evolution—which was a near contemporary theory of Darwin—all the way to the reflections of Empedocles and the Greek atomists circa 495 to 435 BCE had all suggested that the flora and fauna on Earth had come about through evolution, but none were able to offer a process by which evolution did, let alone should, happen. Therefore, the main difference of Darwin's theory was the mechanism of natural selection.

Darwin's theory of evolution followed what was common knowledge, if not always commonly accepted, that species showed a complex network of similarities, forming a tree, composed of groups within groups. To Darwin, these structures of similarities offer evidence of a structure of ancestry, or, that species appear similar to one another because they share common ancestors, with the greater similarity indicating a more recently shared ancestor. This led Darwin to the conclusion that life was originally developed into a few forms from which it would branch. But what would cause this branching? This would be the second, and arguably more important, goal of On the Origin of Species, to argue for the mechanism of natural selection.

Of note, Darwin and his understanding of the concept of species is not as clear as it might seem, especially as the definition of species in Darwin's time was not itself clear and had a variety of complex notions. One such was that species were immutable categories handed down by God. While others have argued whether Darwin believed species categories were merely an epistemic convenience or an objective fact of the natural world.

Darwin's argument for natural selection begins with artificial selection—that breeders of domestic plants and animals are able to use selective breeding to breed traits out of plants or animals and breed traits seen as desirable into those plants and animals. Therefore, Darwin argues, there is a possibility for selection, but then the question remains of what takes the role of the breeder in the natural world. This is where Darwin noted that the competition to survive, with competition referring to efforts for resources in a given environment between species, the competition to survive amongst a species, the competition for mates amongst a species, and the struggle against the environment (such as heat, cold, drought, or natural disaster, for example).

Darwin argued this competition offered all the ingredients to generate variation amongst a species, and variations that were beneficial to survival would, he argued, be continued in a species, whereas a variation—or lack of—that hampered the species' chance of survival would not be maintained. Further, he noted, a variation that was neither beneficial nor harmful, or even legacy variations, could maintain in a species because the variation was not taking a toll on the survival of the species. He used the argument to show how natural selection could lead to increased specialization of creatures, especially as those designed for survival niches would have a better chance of survival than species with a more general approach to survival— especially in the course of feeding themselves. He also suggested that variation could lead to complexity, such as the development of the eye of an eagle; however, unlike other theories of evolution, Darwin noted that natural selection did not have a drive toward complexity and that it would be reasonable for variation to return to simplicity.

The collection of fossils was a relatively new scientific trend, and the recognition of what fossils were, the geological record it offered of flora, fauna, and the history of Earth through geological time, was recent. This allowed Darwin, at the time, to argue that the relative rarity of fossils, the rarity of the conditions for fossilization, and the incomplete knowledge of fossils except for well-explored areas, meant there was not a complete set of the transitional forms that could connect ancestral species to the organisms alive today. The fossils, he also argued, were the record of extinction, which would be the outcome of competition, and the relations between extinct groups would follow, he argued, the same kinds of patterns that could help to predict which species would argue best.

The final part of the book is a "consilience" part, where he continues to discuss how natural selection and evolution would lead to the variety of species that seem to be specifically created for their environments, as they would adapt to the environment. He noted that the flora and fauna of tropical islands are closely affiliated with species living on the nearest major continent, indicating that normal means of dispersal (floating or carried by birds, among others), along with steady evolution, would offer an explanation for these distributional facts. Similarly, he argued for a tree-like structure of larger groups containing smaller groups that relates all species to a common ancestry followed by selective divergence.

This makes his argument unique. It combines a number of different ingredients, such as the analogy of artificial selection, several direct rebuttals of specific counterarguments, and the specific mechanism that explained the variety of phenomena as a result of evolution. Further, although some of his arguments were backed by explicit evidence, others he left as exercises for the reader.

Thomas Henry Huxley, sometimes called Darwin's bulldog.

Darwin continued to hide from society, and the attacks flung his way. Alternatively, Thomas Henry Huxley waded into the public argument, both as the defender of Darwin and as a pugnacious advocate for Darwin's theory. Huxley was noted for relishing in public debate, and he took up the cause with enthusiasm. He wrote reviews of On the Origin of Species, defended the theory of evolution, and more specifically human evolution, at the Oxford meeting of the British Association for the Advancement of Science in 1860, an often mythologized debate, and he would publish his own book on human evolution in 1863 titled Evidence as to Man's Place in Nature. Huxley was Darwin's champion and, more specifically, a champion of Darwin's evolutionary naturalism and his non-miraculous assumptions. These pushed the science of biology into previously taboo areas, and on the back of Huxley and others' advocacy, Darwin won the Royal Society's Copley Medal in 1864.

With the help of Huxley's reaction and his enthusiasm for evolution, which Darwin called "descent," the notion of and word "evolution" would not appear until the last edition of On the Origin of Species was introduced in 1872. Evolution would begin to be acknowledged in various places by 1866. That same year, Darwin met Ernst Haeckel, a German zoologist and admirer of Darwin's theories whose own proselytizing spread the word through the Prussian world and earned Darwin the order Pour le Merite from the King of Prussia in 1868. However, the theory of natural selection—which was called by some the "law of higgledy-piggledy"—received little support.

Darwin, despite almost shunning society at this point, because of his poor health and his apparent fear of social engagement, continued to experiment and revamp On the Origin of Species, guiding it through six editions and continuing to pledge his support to "natural selection," which he continued to believe was the most important part of his theory. He answered critics through his revisions of the work, reemphasizing other causes of change, and bolstered the Lamarckian belief that alterations through excessive use might be passed on.

Sketch of an orchid from Darwin's Various Contrivances.

During this period, Darwin also proved adept at using intellectual flanking maneuvers to get around critics. For example, he took the subject of orchid flowers, then considered intractable, and made the case for them to have evolved through the mechanism of "natural selection." He did this through the 1862 publication of The Various Contrivances by which British and Foreign Orchids are Fertilised by Insects. The publication took many by surprise, as some expected that his next publication following On the Origin of Species would be more directly a book on his theories. Instead, he used orchids to show how his theory debunked the idea that the orchid was a piece of floral whimsy designed by God to please humans but had been honed by natural selection to attract insects for the purpose of cross-pollination. He noted the petals guided bees to the nectaries of the flowers, and pollen sacs were located where they could be removed by the stigma of another flower.

Examples of artificially bred "fancy" pigeons.

Darwin followed his 1862 publication on orchids with further discussions on the variations of plants, the effects of cross and self-fertilization, and how flowers of the same species can change. The first of this series of studies was published in 1868, titled Variations of Animals and Plants under Domestication, which marshaled facts and explored the causes of variation in domestic breeds of plants and animals. The book looked at the impact of artificial selection, as he termed it, or the way in which breeders of plants and animals identified preferred variations in a given species that occurred naturally and then directed the production of the species toward those variations. For example, he went into length on the development of fancy pigeons, which were bred to produce tufts, topknots, and many other variations by breeders known as "fanciers."

Portrait of Darwin's critic George Douglas Campbell, the eighth Duke of Argyll.

This 1868 book was also a response to some critics, most notably George Douglas Campbell, the eighth Duke of Argyll, who criticized what he saw as a blind and accidental process of variation. Darwin's work undermined this providential explanation. Further, Variations aimed to answer the criticism that suggested that any single favorable variation would be swamped by back-breeding within a general population. To answer this, Darwin offered a hypothesis of "pangenesis" to explain the discrete inheritance of traits. At the time, there was no understood mechanism for inheritance, and this theory imagined tissues of organisms threw out "gemmules," which were passed to the sex organs and permitted copies of themselves to be made in the next generation. The theory would eventually be dismissed when they failed to find the supposed "gemmules" in rabbit blood.

1868 photo of Darwin on a family vacation.

These books also raised the question of why cross-pollination may be important. Darwin, in his works, professed his belief that cross-pollinated plants would produce offspring fitter than self-pollinating plants. He conducted thousands of cross-pollinations to prove his hypothesis. The results of these experiments would end up in his 1876 publication The Effects of Cross and Self Fertilization in the Vegetable Kingdom. In 1877, he published another book looking into the result of different male and female forms of flowers and how these different forms facilitated outbreeding and strengthened plants' overall development. This came in his book The Different Forms of Flowers on Plants of the Same Species. In his journals, he agonized over the effects of inbreeding, as he was married to his first cousin and worried about the potential consequences of this on his children. Through these works, he also studied various other plants and their response to gravity and sunlight.

Darwin's second major work borrowed from his unfinished notes for Natural Selection and in the opinion of some was the companion to On the Origin of Species; The Descent of Man, and Selection in Relation to Sex was published in 1871 as a two-volume book. The two parts were discrete, annotated, heavily anecdotal in places, and have been considered authoritative. The first volume focused on the evolution of humans and civilization from the Old World monkeys—which would lead to several caricatures of Darwin. The second volume responded to several critics, such as the above-mentioned Duke of Argyll, who questioned the usefulness and development of colorful feathers for birds—such as iridescent hummingbirds or the plumage of peacocks—under Darwin's evolutionary theory.

The title page of The Descent of Man.

The second book laid out Darwin's theory of sexual selection, which suggested there were variations based on sexual preferences. In the case of birds, Darwin argued that female birds choose mates for their gaudy plumage. Further, he used his network of breeders, naturalists, and travelers worldwide to find evidence for such "sexual selection." He argued that sexual selection happened among humans as well and that this was where the diverse notions of beauty and aesthetic preferences in human societies originated from.

Part of the publication of this book and explanation in The Descent of Man was aimed at Alfred Russel Wallace, who helped push Darwin to formulate and publish his theories, and who had, by the late 1860s and early 1870s, engaged in spiritualism. This spiritualism saw Wallace arguing that an overdeveloped human brain had been provided by "spirit forces" to move humanity toward perfection. Whereas Darwin argued that the development of the human brain was through evolution and natural selection, a larger brain allowed early humans to better adapt to their surroundings and out-compete those without the adaptation.

It should be noted, in the argument from The Descent of Man, that Darwin at no point thought humans merited an exception to Darwin's gradualist, continuous picture of life. Further, he thought there was no drastic difference that separated humans from other animals, even with respect to emotions, communication, intellect, or morality. The Descent is not, therefore, trying to advance an argument for a special feature in human nature. Rather, Darwin had been careful to remove or not include mentions of humans in the Origin and instead thought the argument for humans would need its own treatise to allow him to tackle the opposition he knew would come to this idea. The opposition came from at least two directions: religious and scientific.

A caricature of Darwin as an ape following the publication of Descent.

The religious objection was strong, especially during the period. Any picture of continuity between humans and animals would have to take into account the human soul, at least for theologians of the time. This was considered a supposedly distinctive feature of human beings that would have to be considered in any theory of the existence of humans. Many other authors thought this incorporation was possible. It required significant work, but authors worked to construct an account where the human soul was incorporated into human evolution.

The second major objection, the one that was more problematic from Darwin's perspective, was the scientific opposition. Despite the synchronicity in their theories, perhaps the best representation of this opposition was Alfred Russel Wallace, who argued that the development of human mental capacity exempted humans from natural selection's impact on their autonomy. This special place for humans did not sit with Darwin. He thought natural selection worked for humans and would act no differently in the case of humans.

Image used to explore similarity in structures between humans and the great apes, suggesting a common ancestor.

The Descent works to address these objections and concerns right at the beginning with a demonstration of the similarity between the physical and mental characteristics of humans and other animals. Darwin noted the many physical homologies between humans and animals, with a number of features through the development of an adult human that are seen throughout animal species. He notes the mental capacities of humans and how they differ from even the most intelligent animals. He suggests there is no difference between humans and animals and those mental capabilities not only have analogies in animals, but their difference can be explained through natural selection as a variation that allows for better competition. Darwin would later return to the discussion of emotions and their expression in his work The Expression of Emotions in Man and Animals, which looked at the emotional displays in animals and their similarities in humans.

For Darwin, the evolution of higher intellectual capacities in humans is connected with the evolution of social life and the moral sense. The social instincts identified by Darwin consist of sympathy and reciprocal altruism. He argued these would do tolerably well to knit together a kind of pre-society, although it would be limited to the "group" or "tribe" of the individual. Meanwhile, social instincts, in turn, give rise to a feeling of self-satisfaction or dissatisfaction with one's behavior as it aligns or fails to align with those feelings of sympathy. Communication or language adds to the mix, allowing social consensus to develop along with the ability for a clear expression of public opinion, influences that could be intensified as they become habits, and giving early humans an increasingly intuitive feeling for conformity to emerging social norms.

An image used by Darwin to explore the consistency of sympathetic understanding between species.

From this emerges, according to Darwin's argument, the moral sense. Developed out of a basic kind of instinctive sympathy, humans can move to a habitual, linguistically encoded sense of praise or blame, which in turn develops an instinctive sentiment that an action should have or not have been done, also known as a feeling for right and wrong. Darwin did note that this did not prescribe the content of any morality; rather, it was the development of the moral response, and he argued that often the content would emerge due to the conditions of life, in response to what encouraged or discouraged the survival of the group. In his notebooks, Darwin would speculate the following:

Looking at Man, as a Naturalist would at any other Mammiferous animal, it may be concluded that he has parental, conjugal and social instincts, and perhaps others.... These instincts consist of a feeling of love & sympathy or benevolence to the object in question. Without regarding their origin, we see in other animals they consist in such active sympathy that the individual forgets itself, & aids & defends & acts for others at its own expense.

Darwin's argument here has been criticized by some, as most of Darwin's arguments for natural selection have focused on the individual. But his argument for the usefulness and process of natural selection for this moral imperative comes from group selection, or a process in which those who present more or greater moral characteristics will proliferate through their usefulness to the group. Meanwhile, the individual who is a selfish member of the group and may profit in the short-term will be outperformed in the long run by the selfless member. Those critics who do not like group selection, in this case, suggest that Darwin's explanation may not work. Instead, they have offered a suggestion that "kin selection" or the process by which an organism promotes an "extended" version of its success by helping those related to it would offer an individual-level explanation for an apparent group-level phenomena.

The second part of Darwin's Descent looks at sexual selection and its impact on natural selection. This part is not limited to humans but takes into account all species, from insects, to birds, to other mammals, and takes up a volume-and-a-half of the two-volume work. In this, he appeals to sexual selection to answer questions about how many differences—in human or animal populations—could come about. This goes a long way to answering the question around "standards of beauty" and how organisms have to compete for others with survival, but also when attracting and retaining mates. This would lead, according to Darwin's theory, to the development of changes or characteristics that are specifically focused on mating, producing changes that are non-selective or counter to natural selection.

The tails of peacocks are often considered the perfect example of sexual selection.

The classical example of sexual selection selecting for traits that are potentially counter to survival, and therefore natural selection, is the case of the tail of peacocks. The tail imposes a penalty in terms of the bird's ability to escape from predators, yet the elaborate tail is a way for them to attract mates and therefore have offspring. Therefore, the "selection" performed by peahens will become a vital part of the evolutionary story. Darwin argues this is true for all species, although each species has a different standard of beauty by which it will compete. For humans, he points to a variety of non-selective differences that could be described in terms of socially developed aesthetic preferences. This has been in contention since, as some have suggested that sexual selection should not be considered a process that is distinct from natural selection but should be a part of natural selection, as natural selection is intended to drive toward survival both of the individual and in part of the species, therefore requiring reproduction. But sexual selection as a subset of natural selection would offer an explanation for some cases.

The scientific reception of the work has focused on two objections. The first has been touched on above, in that Darwin never satisfactorily defines what he means by species. The second is Darwin's attempt to treat the generation and distribution of variations as a black box. Darwin is confident, in his writing, that relevant variations could potentially arise in a population, as artificial selection proved variation possible; but he does not have a specific mechanism for what causes variation to arise. However, the larger scientific reception to his work recognized almost immediately that Darwin's arguments for common ancestry and homology were strong. It would not be until the integration of Mendelian genetics with the theory of evolution that the mechanism for natural selection would be found and this specific controversy would be laid to rest.

Another cartoon mocking Darwin's theories laid out in Descent of Man.

The religious response to Darwin's work would be more complicated, shaped by theological disputes contemporary to Darwin, the interactions these theological forces were used to having with science (which in some cases did not have any interaction with science), and questions of Darwin's personal character. Some theological forces were willing to integrate human evolution into their picture of the world, so long as space was made for the special and spiritual creation of humans. Others believed Darwin violated a Baconian image of scientific methodology and rejected his theory. However, overall, the acceptance or rejection of his theory was not necessarily determined by religious affiliation. Many people of the time, including figures in the Church of England, were quite willing to consider and even support Darwin's theories; while some of his harshest critics were in no way religious or friends of religion.

Darwin pictured a few years before his death.

Darwin would finish another major part of his work in 1872, publishing the photographically illustrated The Expressions of the Emotions of Man and Animals, which concluded a long-standing line of work, where Darwin had been studying orangutans at the London Zoo and had been fascinated by expression, especially as he compared the expression of the orangutans to those of his infant children. He had, as a student, been taught that the muscles in the face of humans had been designed by God to allow humans to express their unique thoughts. But in The Expressions of the Emotions of Man and Animals, he expanded the subject of expressions to include asylum inmates and compared them to show the continuity of emotions and expressions between humans and animals.

Toward the end of his life, Darwin was a relative recluse, leaving his house and property only rarely appearing in society. However, those who knew him and those who visited found him to be the kindest and gentlest person, and around him grew a protective, devoted circle of friends. These friends worked to defend Darwin and his theories in public, and Darwin, in turn, helped his friends, such as raising 2,100 pounds to send Huxley on holiday when he was fatigued in 1873 and pestering until Wallace was added to the Civil List in 1881.

Darwin's final resting place in Westminster Abbey.

During this period, Darwin also began to write his autobiography, which he would finish in 1881. It was composed for his grandchildren, and as such, he was particularly candid regarding his dislike of Christian mythology around eternal torment. In 1881, he also finished another of his long-term interests, publishing The Formation of Vegetable Mould, Through the Action of Worms, in which he argued for the ability of these rather humble forms to cause monumental transformations of landscapes. At the same time, Darwin noted that he was looking forward to joining the worms. This came as his chronic illness continued to cause him difficulties, including Darwin suffering from angina. In March 1882, Darwin suffered a seizure, and on April 19, he died of a heart attack. Despite his feelings and Darwin calling himself an agnostic as early as 1870, there were petitions upon his death that he should be buried in Westminster Abbey. It was successful, and on April 26, 1882, Darwin would be laid to rest with the full ecclesiastical pomp attended by the nobility of state and the new nobility of science.

Perhaps the most important part of Charles Darwin's legacy has been the adoption of his theory of evolution. Though the theory has been modified over time, especially as technology and science has allowed people to better understand the mechanisms that led to the process of evolution, it stands as the dominant biological theory explaining the development of the natural world. In this way, Darwin's theories changed the way people view the world and humans' place in the world. For some, Darwin's legacy is not just his theories around evolution but that he was one of the first thinkers who cast the supernatural or a deity out of the equation for evolution. This had been done previously in other scientific fields; but Darwin's removal of these sources of the development of the natural world and his application of those rules to humans was arguably the first such application and made Darwin one of the first scientific thinkers to provide not only a theory but a basis of evidence for the natural, non-spiritual origin of humans.

Social Darwinism is a collection of ideas that emerged in the late 1800s and adapted Darwin's theory of evolution to explain social and economic issues. The theory emerged, in part, during his lifetime, even though Darwin himself rarely commented on the connections between his theories and human society. When he did, he tended to borrow understood concepts, such as "survival of the fittest," which was not his idea, nor does it properly express his theory of natural selection, but was used by Darwin as shorthand to explain his theory. Over time, social Darwinism became more popular and would be used as a justification for imperialism, labor abuses, poverty, racism, eugenics, and social inequality. However, despite the name, it was never an idea Darwin espoused or that he was known to endorse. It was also used to support laissez-faire capitalism and class stratification.

Charles Darwin statue outside of London's Natural History Museum.

As science moved forward and technology was better able to discover and explain the mechanisms of how an organism can develop new traits, and how those traits can be passed on as hereditary traits, the different points of Darwin's theory have been emphasized and understood in their importance. For a period, the thought that organisms evolve and forked on evolutionary trees was considered the most important part of Darwin's theory. However, as developmental biology studied hereditary traits and better understood how those traits could be passed on, the mechanism of natural selection was emphasized in his theory. As the study of genetics has grown and researchers better understand gene expression and how epigenetic changes can become hereditary and persist across generations, the theory has continued to be proven right.

In many cases, Darwin's details may not hold up and be mired in a Victorian-era scientific understanding. However, some details have held up. For example, researchers have discovered the importance that mammal subspecies or a population within a species that differs from having different physical traits and their own breeding ranges play a more important role in evolution than previously thought. These findings held up Darwin's hypothesis, that organisms require relationships between species and subspecies in the future evolution of species.

Example diagram of how a warm pond or tide pool could lead to emergence of life.

Darwin believed, first noted at some point in an 1871 letter to Joseph Hooker, that life likely began in "warm little ponds" rather than in the deep sea. He wrote:

"It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosophoric salts, light, heat, electricity, etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes [...]"

The deep sea theory has long held that life probably began in the deep ocean, especially around deep-sea vents, where many of the known elements for life exist. And it has long been held that Darwin was wrong about his theory. But further scientific understanding, such as the importance of amino acids in the development of proteins and early cells, and the potential in tide pools for there to be very warm water cycling offers a potential for a terrestrial origin of life. Research into this potential has found several mechanisms, including the chemical reactions and physical properties of hydrothermal and tide pools, a gel phase as a candidate progenote (in which a naturally occurring hydrogel offers the organic solutes and mineral grains necessary to develop functional and inert polymers that can be used to develop small-cellular, microbial, organisms), and the resulting protocell gel progenote has shown an adaptive radiation, which could give rise to simple life, from which could evolve multi-cellular life.