Charles Robert Darwin, the naturalist whose discovery of the theory of evolution by natural selection revolutionized biology, was born in Shrewsbury, Eng., on Feb. 12, 1809. He was the son of Robert Waring Darwin, who had one of the largest medical practices outside of London, and the grandson of the physician Erasmus Darwin, the author of Zoonomia, or the Laws of Organic Life, and the artisan-entrepreneur Josiah Wedgwood. Darwin enjoyed a secure position in the professional upper middle class that provided him with considerable social and professional advantages.

Youth and education.

Darwin's mother died when he was eight years old. Otherwise he enjoyed a golden childhood, cosseted and encouraged by adoring sisters, an older brother, and the large Darwin and Wedgwood clans. He was keenly interested in specimen collecting and chemical investigations, but at the Shrewsbury school, where he was an uninspired student, the headmaster, Dr. Samuel Butler, stressed the classics and publicly rebuked Darwin for wasting his time with chemical experiments. At age 16 he was sent to study medicine at the University of Edinburgh, where he was repelled by surgery performed without anesthetics. During his two years in Scotland Darwin benefited from friendships with the zoologist Robert Grant, who introduced him to the study of marine animals, and the geologist Robert Jameson, who fed his growing interest in the history of the Earth.

Disappointed by Darwin's lack of enthusiasm for medicine, his father sent him to the University of Cambridge in 1827 to study divinity. At the time Darwin adhered to the conventional beliefs of the Church of England. His academic record at Christ's College was as undistinguished as it had been at Edinburgh. He socialized considerably with hunting, shooting, riding, and sporting friends. Cambridge did not yet offer a degree in the natural sciences, but, guided by his older cousin William Darwin Fox (an entomologist who inspired in him a lifelong passion for collecting beetles), Darwin met the circle of Cambridge scientists led by the cleric-botanist John Stevens Henslow. Soon a regular at Henslow's "open houses," Darwin accompanied him on daily walks and became known as "the man who walks with Henslow." Henslow encouraged Darwin's excitement about science and confidence in his own abilities.

On leaving Cambridge in the spring of 1831 Darwin, in preparation for a scientific trip to the Canary Islands, read Alexander von Humboldt's Personal Narrative of Travels to the Equinoctial Regions of the New Continent, a scientific travelogue of a journey to Central and the northern parts of South America. At Henslow's recommendation he accompanied Adam Sedgwick, Woodwardian professor of geology at Cambridge, on a three-week tour of North Wales to learn geologic fieldwork.

In August 1831, at Henslow's recommendation to the Admiralty, Darwin was invited to sail as the unpaid naturalist on HMS Beagle. The ship was to survey the east and west coasts of South America and continue to the Pacific islands to establish a chain of chronometric stations. Henslow suggested Darwin as both an acute observer and a companion for the aristocratic young captain, Robert FitzRoy. (The Beagle already had a naturalist-surgeon, but one whom FitzRoy found socially unsuitable.) Robert Darwin first refused permission on grounds that it was dangerous and would not advance Charles in his career. But upon the intercession of his brother-in-law, Josiah Wedgwood II, he changed his mind.

On Dec. 27, 1831, Charles Darwin sailed from Plymouth, Eng., on the Beagle, a 10-gun brig that had been refitted as a three-masted bark. The voyage, planned for two years, lasted five, during which Darwin kept meticulous notes and sent back geologic and biologic specimens.

The voyage of the "Beagle."

In a letter to FitzRoy accepting the post Darwin explained that he expected the voyage to be a "second birth." There is no doubt that the years he spent exploring the South American continent and the offshore islands of the Galapagos honed his skills as a collector, observer, and theorist. Often seasick, Darwin rested horizontally in a hammock during the worst motion and spent long periods of time ashore whenever the opportunity arose. He delighted in the exotica of the tropics. Adventurous, he braved his way through armed political rebellions, rode with the gauchos in Argentina, and on collecting and shooting expeditions justified his earlier devotion to sport. He joined the crew in towing the ship's boats upstream and once rescued the expedition by running to save a boat from a tidal wave. He seemed to relish danger and was sustained in the considerable discomfort by a lively curiosity. He wrote to one of his sisters, "We have in truth the world before us. Think of the Andes; the luxuriant forest of the Guayquil [sic]; the islands of the South Sea & New South Wales. How many magnificent & characteristic views, how many & curious tribes of men we shall see.--what fine opportunities for geology & for studying the infinite host of living beings: Is this not a prospect to keep up the most flagging spirit?"

Darwin had brought his own books to augment the ship's extensive library. The most important scientific work was the first volume of Charles Lyell's Principles of Geology, which Henslow had urged him to read though not to believe. Lyell argued that the face of the Earth had changed gradually over long periods of time through the continuing, cumulative effects of local disturbances, such as eruptions, earthquakes, erosion, and deposition. Such disturbances had existed in the distant past and could be observed in the present. This view differed dramatically from that held by most contemporary geologists, who hewed to the belief that changes in the face of the Earth resulted from short-lived events of great violence that could raise mountains or flood the entire planet. During the first months of his journey Darwin was converted to Lyell's views by his own observations.

About 1,800 miles southwest of the Canary Islands the Beagle visited Sco Tiago, a volcanic island in the Cape Verde Islands. From the harbour Darwin saw a band of white rock extending horizontally at a height of about 45 feet above the base of the sea cliffs. The formation was calcareous and contained numerous shells, almost all of which could be found on the coast. Darwin reasoned that a stream of lava from the ancient volcanoes had flowed over what had been ancient seabed, baking it to form the hard white rock. The whole island had subsequently been heaved up to make the sea cliff from the white band downward. Darwin also realized that the island's surface had been formed by a succession of volcanic events, not a single dramatic one. He discerned an initial subsidence, the settling of the surface around the original craters, its building up from new lava spills from different craters, and further subsidence and building up over a long period of time.

Later, in Chile, Darwin witnessed his first earthquake. He saw the land rise before his eyes. Then, after crossing the Andes in 1835, he wrote to his sister that he could understand "to a certain extent the description & manner of the force, which has elevated this great line of mountains." He had found fossil shells at an elevation of 12,000 feet, and he theorized that a chain of suboceanic volcanoes had poured forth enormous quantities of lava that formed the Andes through a further process of upheaval and fracturing. Darwin marveled at the whole South American continent, which he read as a vast testing ground for Lyell's ideas. To his cousin William Fox he wrote, "Everything in America is on such a grand scale. The same formations extend for 5 or 600 miles without the slightest change--for such geology one requires 6 league boots."

The data Darwin collected on the Beagle provided him with material for three books on South American geology. Although his theories of continental change have been superseded by the theory of plate tectonics, his descriptions in letters to Henslow, which Henslow excerpted and read before the Cambridge Philosophical Society and the Geological Society of London, brought him celebrity in scientific circles even before his return.

While still on the voyage he challenged Lyell's view of the formation of coral reefs by volcanic action. Darwin contended that the reefs were part of a process of gradual changes in the Earth's crust resulting from the subsidence of some landmasses and corresponding elevations elsewhere. He explained that coral, which only grows in shallow waters, forms a reef by building up on the seafloor as the floor subsides. He predicted that if a whole island sank below the ocean's surface, and the coral continued to grow, a reef would turn into an atoll around a lagoon. Lyell was convinced and supported Darwin's reinterpretation, which deep-sea borings in the 20th century have confirmed.

Darwin's geologic ponderings were important for geology and to his scientific development. Many of the rocks he examined contained fossils, and his constant exposure to the evidence of extinct species and the similarity of many of them to living species kept one problem at the fore: By what mechanism did new species replace extinct ones?

During the voyage Darwin developed confidence in his own observations as well as the ability to grasp a problem and work at it steadily. The isolation of the voyage, combined with the exposure to new phenomena, taught him to think for himself within the familiar scientific culture of his time. He developed a rare combination of strengths: a dedication to careful fact gathering and a propensity to theorize about the facts. His geologic pondering on a continental scale encouraged him to search for universal laws. The voyage of the Beagle transformed Darwin into an independent and adventurous scientist who had the courage to embrace the heretical idea of the transmutation of species.

"On the Origin of Species."

When Darwin returned to England in 1836 he was welcomed by the scientific fraternity as a colleague and was promptly made a fellow of the Geological Society. The next year he was elected to its governing council. In 1838 Darwin was elected to the Athenaeum, the exclusive club for men distinguished in literature, art, or science, and in 1839 he was elected to the Royal Society. Through his older brother, Erasmus, he met the historian Thomas Carlyle and the feminist Harriet Martineau. He was also a friend of Charles Babbage, whose computing machine was one of a host of scientific interests.

At this time, however, Darwin began to lead something of a double life. To the world he was busy preparing his Journal of Researches into the Geology and Natural History of the Various Countries Visited by H.M.S. Beagle, which was published in 1839. This book, modeled in part on von Humboldt's, established the lucid style enlivened by the sharp descriptions that makes all of Darwin's works both accessible and convincing. Darwin was also preparing his geology books and superintending the analysis and publication by specialists of The Zoology of the Voyage of H.M.S. Beagle (published between 1839 and 1843 with the help of a government grant). Privately Darwin had begun a remarkable series of notebooks in which he initiated a set of questions and answers about "the species problem." He proceeded to collect facts about species through letters and discussions with breeders, gardeners, naturalists, and zookeepers, as well as through extensive reading.

Darwin kept this interest secret while he gathered evidence to substantiate his theory of organic evolution. He was mindful of the fate of other unorthodox scientists. He jotted in his notebook, "Mention persecution of early astronomers--then add chief good of individual scientific men, is to push their science a few years in advance only of their age." Darwin's ideas were not only scientifically radical but also could have been interpreted as actionable under the laws governing blasphemy and sedition. England at the time was intensely evangelical, and the natural world was understood as one in which the spirit of God could be seen in the creation of new species of plants and animals that appeared to come into existence to replace those that became extinct. Darwin gradually became intellectually uncomfortable with this view of life as he confronted puzzling evidence.

Upon his return from the voyage Darwin had turned over his specimens to cataloging experts in Cambridge and London. In South America he had found fossils of extinct armadillos that were similar but not identical to the living animals. In Argentina he had seen species vary geographically; for example, the giant ostriches (rheas) on the pampas were replaced to the south in Patagonia by much smaller species, both of which were akin to but different from the African ostrich. He had been disturbed by the fact that the birds and tortoises of the Galapagos Islands off the western coast of Ecuador tended to resemble species found on the nearby continent, while inhabitants of similar neighbouring islands in the Galapagos had quite different animal populations. In London Darwin learned that the finches he had brought from the Galapagos belonged to different species, not merely different varieties, as he had originally believed. He also learned that the mockingbirds were of three distinct species and that the Galapagos tortoises represented at least two species and that, like many of the specimens from the archipelago, they were native to the islands but to neither of the American continents.

After Darwin received these reports, his doubts about the fixity of species crystallized into a belief in transmutation. In March 1837 he confided in his notebook that species changed from one place to another or from one era to the next. He continued analyzing his data, searching for a mechanism for this process. Then in October 1838 Darwin read Thomas Malthus' An Essay on the Principle of Population. Malthus argued that population growth is geometric, while the food supply increases only arithmetically, and thus that population increase is always checked by a limited food supply. Darwin recalled in his Autobiography his realization that given the struggle for existence everywhere, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. . . . The result of this would be the formation of new species. Here, then, I had at last got a theory"--the principle of natural selection--"by which to work."

Darwin's originality extended beyond perceiving the savagery of the natural world. Other scientists and philosophers had noted the brutality of species against species, of the lion devouring the lamb. Darwin saw competition between individuals of a single species. He recognized that within a local population the individual with, for example, the sharper beak, the longer horn, or the brighter feather might have a better chance to survive and reproduce than other individuals. If such advantageous traits were passed on to new generations, they would eventually be predominant in future populations. Darwin thus shifted the focus of evolutionary analysis from between to within species. He saw natural selection as the mechanism by which advantageous variations passed on to succeeding generations and by which the traits of individuals that were less competitive gradually disappeared from populations. (Later generations of biologists came to understand variations within a species as variations in the genes of its individual members, and they explained evolution as the action of natural selection upon genes responsible for advantageous traits.) After he had hit upon natural selection, Darwin was eager to verify it, and he stepped up his inquiries to plant and animal breeders. He hoped to learn from their experience with artificial selection how natural selection worked.

Darwin still faced the problem of divergence--that is, the evolutionary development of dissimilar characteristics in closely related species that have descended from a single organic ancestor. As he had observed during his voyage, divergent species appeared on different landmasses. Darwin solved this puzzle of geographic distribution by assigning the dissemination of populations of ocean islands to the power of wind and water. The theory of the evolution of species thus solved many puzzles in comparative anatomy, embryology, and paleontology.

The idea of organic evolution was not new. It had been suggested a generation earlier by Erasmus Darwin and in France by Buffon, Montesquieu, Maupertuis, Diderot, and most recently Lamarck. Lamarck had drawn the first evolutionary diagram--a ladder leading from unicellular organisms to man. But none of these earlier evolutionists had presented either a mechanism or persuasive evidence for the process. Lamarck offered the hypothesis that spontaneous generation occurs constantly, that organisms possess an "inner feeling" toward perfection, and that the traits an animal acquires to adapt to a changing environment are passed on to its descendants.

Though lack of an apparent mechanism of inheritance eventually prompted him to accept the latter idea, Darwin's theory was rooted in direct observation and an attempt to discover universal laws. His evolutionary sketch was a branching tree, not a single ladder. Above all, Darwin rejected the prevailing view that organisms are perfectly adapted to their environment. He viewed the natural world, instead, as caught in an incessant struggle between competing individuals that have different degrees of fitness. Others had seen struggles but always between species, never within them. By moving the battle from interspecies struggles to intraspecies competition, Darwin introduced the concept of populations--that is, localized groups consisting of members of a given species in which each individual differs from its sibling. He recognized that it is the competition within a species leading to the survival of individuals with adaptationally advantageous traits that eventually brings about the evolution of a new species.

By 1842 Darwin was confident enough in his theory to draft a short sketch, and in 1844 he composed a longer version, which he showed to his friend, the botanist Joseph Dalton Hooker. Wary of presenting his theory to the public, Darwin spent the next decade concentrating on a treatise on barnacles, in which he hinted but did not actually say that species were the product of natural selection. In the meantime the intellectual atmosphere in England altered and discussions about evolution became commonplace. Darwin still withheld publishing his thesis. When he would have determined that the time was ripe is impossible to know, but the decision was removed from him when on June 18, 1858, he received from Alfred Russel Wallace, a naturalist working in the Malay Archipelago, a paper that perfectly summarized the theory that Darwin had been elaborating for 20 years. Disheartened by this apparent preemption of his life's work, Darwin was saved by his friends and confidants, Lyell, Hooker, and T.H. Huxley, who arranged for a joint paper by Darwin and Wallace to be read to the Linnean Society of London on July 1, 1858.

Darwin then began work on what he called an "abstract" of the larger manuscript that he had begun two years earlier. This abstract, On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life, was published on Nov. 24, 1859. The first edition sold out immediately, and by 1872 the work had run through six editions. The theory was accepted quickly in most scientific circles. With the exception of holdouts like his old colleague Adam Sedgwick and individuals such as the biologist Richard Owen, who attacked Darwin personally, most opposition was from the clergy. They realized that the theory of evolution was inconsistent with a literal interpretation of the book of Genesis. Orthodox Christians felt threatened by the suggestion that the natural (or living) world worked according to laws as did the physical world. There was no place in Darwin's world for divine intervention, nor was mankind placed in a position of superiority vis-a-vis the rest of the animal world. Darwin saw man as part of a continuum with the rest of nature, not separated by divine injunction.

After the publication of the Origin, Darwin continued to write, while friends, especially Huxley, defended the theory before the public. In June 1860 at the Oxford meeting of the British Association for the Advancement of Science, Huxley confronted the bishop of Oxford, Samuel Wilberforce, who had been coached by Richard Owen. Wilberforce patronized Huxley, asking whether it was through his father or his mother that he was descended from an ape. Huxley replied that he was not ashamed of having descended from an ape but would be ashamed of an ancestor who used gifts of eloquence in the service of falsehoods. Huxley and Hooker annihilated Wilberforce's position at the Oxford debate and continued spreading what was tantamount to a gospel of evolution.

Darwin completed the elucidation of his theory in his next three books, which were all continuations of the Origin. In The Variation of Animals and Plants Under Domestication (1868), Darwin proposed his hypothesis of pangenesis (an ill-founded attempt to account for the acquisition of hereditary characteristics, a process eventually explained in the development of cell biology and genetics).

Darwin met the issue of human evolution head-on in The Descent of Man, and Selection in Relation to Sex (1871), in which he elaborated on the controversial subject only alluded to in the Origin. He expanded the scope of evolution to include moral and spiritual as well as physical traits and underscored man's psychological as well as physiological similarities to the great apes, predicting, "the time will before long come when it will be thought wonderful that naturalists, who were well acquainted with the comparative structure and development of man and other animals, should have believed that each was the work of a separate act of creation."

The second half of the book elaborated upon the theory of sexual selection. Darwin observed that in some species males battle other males for access to certain females. But in other species, such as peacocks, there is a social system in which the females select males according to such qualities as strength or beauty. Twentieth-century biologists have expanded this theory to the selection by females of males who can contribute toward the survival of their offspring; i.e., female selection secures traits that make the next generation more competitive. Although Darwin's description of female choice was roundly rejected by most scientists at the time, he adamantly defended this insight until the end of his life. While not universally accepted today, the theory of female choice has many adherents among evolutionary biologists.

The last of Darwin's sequels to the Origin, The Expression of the Emotions in Man and Animals (1872), was an attempt to erase the last barrier presumed to exist between human and nonhuman animals--the idea that the expression of such feelings as suffering, anxiety, grief, despair, joy, love, devotion, hatred, and anger is unique to human beings. Darwin connected studies of facial muscles and the emission of sounds with the corresponding emotional states in man and then argued that the same facial movements and sounds in nonhuman animals express similar emotional states. This book laid the groundwork for the study of ethology, neurobiology, and communication theory in psychology.

Later works.

Throughout his career Darwin wrote two kinds of books--those with a broad canvas, such as the evolution quartet, and those with a narrow focus, such as the treatise on barnacles. His interests shifted over the years from geology to zoology to botany. In these later works, however, he included theoretical interpretation, whereas his earlier works had contained mostly data. In On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects (1862) he demonstrated that plants exhibit complicated characteristics that are adaptive and that increase the survival of a species. One such characteristic, for example, is cross-pollination (the mechanism by which pollen is transferred from one flower to another). In explaining the interdependence of bees and orchids, Darwin noted that flowers that are pollinated by the wind have little colour, while those that need to attract insects have brightly coloured petals and sweet-smelling nectaries. In The Different Forms of Flowers on Plants of the Same Species (1877) he observed that flowers in some species differ in the lengths of their anthers and styles, which is another adaptation for cross-pollination. Darwin experimented in his garden at Down House in Kent where he raised two large beds of Linaria vulgaris, one from cross-pollinated and the other from self-pollinated seeds, both of which he obtained from the same parent plant. He observed, "To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilized ones." He continued horticultural experiments for another 12 years on 57 species and described his results in The Effects of Cross and Self Fertilisation in the Vegetable Kingdom (1876). Here he developed the theme that there are hereditary advantages in having two sexes in both the plant and animal kingdoms--to ensure cross-fertilization, which, as he knew from experiments, produced healthier, more vigorous offspring.

In "On the Movements and Habits of Climbing Plants" (1875) Darwin advanced an adaptive explanation for the tendency of the stems of certain plants to spiral toward heat and light, bending either clockwise or counterclockwise. Through experimentation he had discovered that a twining plant would not twine around an object larger than six inches in diameter. This characteristic Darwin interpreted as preventing a vine from climbing up a large tree where the shade from the upper branches would deprive it of sunlight. Still interested in the mechanism that enables some plants to climb and bend, Darwin continued experimenting and pinpointed "some matter in the upper part which is acted upon by light, and which transmits its effects to the lower part." He reported these researches in The Power of Movement in Plants (1880).

A chance observation of flies caught on the leaf of the common sundew initiated Darwin's investigation of carnivorous plants. He was especially impressed by the fact that the living cells of plants possess a similar capacity for irritability and response as the cells of animals. Darwin published these findings in Insectivorous Plants (1875). In his last botanical work, The Formation of Vegetable Mould, Through the Action of Worms, which appeared only six months before his death in 1881, he demonstrated the service that worms perform in digesting leaves and recirculating organic matter. It was a pioneering study in the field of quantitative ecology.

A man of his time.

Darwin worked alone at home, leading the life of an independent scientist (a privileged existence open to a fortunate few in Victorian England). Money from Robert Darwin made it unnecessary for Charles to seek employment. After his return from the voyage Darwin knew he would never become a clergyman like his mentor, Henslow. Nor would he remain a bachelor like his brother, Erasmus, who was a man-about-town. After drawing up lists of the benefits and drawbacks of marriage, he proposed to his first cousin Emma Wedgwood, whom he married on Jan. 29, 1839. She brought fortune, devotion, and considerable housewifely skills that enabled him to work in peace for the next 40 years. Newly married, the Darwins moved into a house on Gower Street in London, but within a few years Darwin's increasingly poor health prompted them to move to the country. In 1842 the Darwins moved into Down House in the village of Downe, Kent, only 16 miles from London but remote from easy access to the city.

Charles and Emma Darwin had 10 children: two died in infancy and a third, Anne, died at age 10. The surviving five sons went away to school. George, Francis, and Horace became distinguished scientists, and Leonard, a major in the royal army, was an engineer and eugenicist. William Erasmus was undistinguished, as were his sisters, who prepared at home to follow their mother into marriage. Henrietta married; Elizabeth remained single at Down. Darwin was devoted to his wife and daughters but treated them as children, obliging Emma to ask him for the only key to the drawers containing all the keys to cupboards and other locked depositories.

Darwin noted in The Descent that the young of both sexes resemble the adult female in most species and reasoned that males are more evolutionarily advanced than females. His attitude toward women coloured his scientific insights. "The female is less eager than the male," he wrote, "She is coy," and when she takes part in choosing a mate, she chooses "not the male which is most attractive to her, but the one which is least distasteful."

His medical school experience had left him sympathetic to the popular antivivisectionist movement, but he admonished women for their involvement in it in a letter to The Times of London on June 23, 1876: "Women, who from the tenderness of their hearts and from their profound ignorance are the most vehement opponents of such experiments, will I hope pause when they learn that a few such experiments performed under the influence of an anaesthetic, have saved and will save through all future time thousands of women from a dreadful and lingering death." All expressions of cruelty offended him, and he was an ardent opponent of slavery.

Comfortable in English society, Darwin treasured his place and feared alienating those who he knew would be offended by his theory. He benefited at the beginning of his career from the scientific fraternity in London, who helped him understand the specimens from the Beagle, and he appreciated his intellectual give and take with Henslow, Hooker, Lyell, and Huxley. He was a beneficiary of this conservative English society, and his fear of ostracism was one of the forces that prevented him from publishing his theory sooner. He also dreaded the hurt he knew that his ideas would inflict on his close friend Henslow and especially on Emma, both devout Christians, for whom his theory was heresy.

The conflict between his science and his realization of what publication would imply for the society he was so much a part of manifested itself in physical pain. The once adventurous young naturalist was a semi-invalid before his 40th year. Darwin's illness has been the subject of extensive speculation. Some of the symptoms--painful flatulence, vomiting, insomnia, palpitations--appeared in force as soon as he began his first transmutation notebook, in 1837. Although he was exposed to insects in South America and could possibly have caught Chagas' or some other tropical disease, a careful analysis of the attacks in the context of his activities points to psychogenic origins. Throughout the next decades Darwin's maladies waxed and waned. But during the last decade of his life, when he concentrated on botanical research and no longer speculated about evolution, he experienced the best health since his years at Cambridge.

Darwin made his home at Down into his laboratory, where he experimented in his garden and observed the local fauna. Dogs and cats were part of the Darwin household, which also was not without a child under school age between 1839 and 1856. By no means a recluse, Darwin often attended scientific meetings in London; he was away from home for about 2,000 days between 1842 and 1881. He was a member of 57 leading foreign learned societies and was no less a prominent figure in the village of Downe, treasurer of the Friendly Club and a Justice of the Peace. Darwin sent his children to village dances, and, even though he was a skeptical agnostic, he participated in church functions that were part of village life.

Darwin died at Down House on April 19, 1882. Within hours the news reached London, and a Parliamentary petition won him burial in Westminster Abbey. By this time the theory of evolution through natural selection was generally accepted. His ideas were modified by later developments in genetics and molecular biology, but his work remains central to modern evolutionary theory.

Excerpt from the Encyclopedia Britannica without permission.