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.
Geology books
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 £1,000 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.
After a drawing by Darwin's former shipmate, Philip King
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.
Return from the voyage
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. 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 Galápagos 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 Galápagos 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.
Continued analyzing
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.
Problem of divergence
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. (For further discussion of the details of Darwin's theory, see evolution; for details of the evolution of humans, see human evolution.)
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.
The Preservation of Favoured Races in the Struggle
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. 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.
Darwin kept this interest secret while he gathered evidence to substantiate his theory of organic evolution
There was no place in Darwin's world for divine intervention, nor was mankind placed in a position of superiority vis-à-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.
Completed the elucidation of his theory
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.
Female choice was rejected
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.