The prevailing view in British scientific and theological communities when Charles Darwin began his inquiries was that species were fixed, immutable types, created by God. The complex and intricate design of organisms was taken as evidence of creation by a Designer. William Paley, a natural theologian, developed the argument, which Darwin studied during his education at Cambridge.

However, the idea that species are not immutable but can be transmuted had many advocates before Darwin. They can be found as far back as the pre-Socratics, but the view was greatly advanced in early nineteenth century scientific discussions by Jean-Baptiste-Pierre-Antoine de Monet de Lamarck in his Philosophie zoologique: Ou, Exposition des considérations relative à l’histoire naturelle des animaux (1809; Zoological Philosophy: An Exposition with Regard to the Natural History of Animals, 1914). What Lamarck and other early evolutionary theorists lacked was an adequate mechanism to explain how evolution occurred.

Charles Lyell, in his Principles of Geology (1830-1833), argued that geological formations had been shaped by the same physical forces that acted on them today, a doctrine known as uniformitarianism. Although Lyell was not a transmutationist until after On the Origin of Species by Means of Natural Selection, he did acknowledge in the volumes that Darwin read that changing environments could lead species to undergo accommodations, migrations, and extinctions.

By Darwin’s own testimony, the idea of natural selection as the mechanism of evolutionary change occurred to him upon rereading in 1838 An Essay on the Principle of Population, As It Affects the Future Improvement of Society (1798) by Thomas Malthus, a clergyman and political economist. “The doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms” is how Darwin described his theory of descent through modification by natural selection. Malthus asserted that because populations grow geometrically while crops grow arithmetically, there would be a struggle among creatures for survival. The fittest would prevail.

The thesis of On the Origin of Species by Means of Natural Selection is that species are created not by special acts of a Designer but by the process of evolution through natural selection. In each generation, more organisms are produced than can live to the point of reproducing. Those organisms that possess even a slight advantage in adapting to the demands of climate, food supply, and competition over fellow organisms (and other species of organisms) will have higher reproduction rates, will survive long enough to reproduce, and thus will pass their favored structures and instincts on to their offspring. Adaptations will accumulate over generations; internal organs, external structures, and behavior will be altered. New varieties, “incipient species” as Darwin called them, will appear and, over many generations, will come to be recognized as new species. Darwin does not explain the origin of life by his theory but rather how the diversity and complexity that is life came to be.

To speak persuasively for his theory, Darwin constructed what he described as “one long argument.” The book consists of arguments and evidence. The evidence is numerous illustrations of geological and biological facts about rocks, animals, and plants. The arguments and inferences support the superiority of natural selection and other natural causes of evolution as opposed to special creation as an explanation of the facts. The facts were not just those of the vast diversity of species, but also the innumerable details of their structures, behaviors, development, and interrelationships—facts that Darwin’s theory could integrate in a logical manner.

Darwin begins by discussing the variability that exists among members of a species, individual differences in structure and instinct, and the fact that some of these differences are inherited by offspring. What causes the variability is unknown to Darwin, although he speculates on changed environmental and life conditions that affect the organism both directly and through its reproduction system. These individual differences, the variability in structure and function, are used by breeders of domesticated animals to create new forms; they artificially select in a manner analogous to the way, Darwin would later argue, that nature selects. Breeders select for the qualities they desire: fancy tail feathers in pigeons, high milk yields in cows. Nature selects without regard to any particular quality, other than the ability to survive. The slight individual differences are the first small steps that lead to varieties, and varieties at the extreme cannot be distinguished from species. No rigid category lines or types exist.

Members of a species are competing among themselves as well as with other species in a struggle for life and for reproduction. The struggle for existence is created when more individuals are born than can survive under existing conditions. In this struggle, any slight advantage will increase the likelihood of living and, more important, the likelihood of leaving progeny. Thus, the ultimate struggle is not so much to live as it is to live long enough and well enough to leave offspring. Any advantage in doing so, to the degree that it is hereditary, will be preserved in subsequent generations and will accumulate to produce new forms of life, structure, and behavior. The process of preserving accumulated advantages is what Darwin called “natural selection,” which he regarded as the main force behind evolution.

If it were not for two other forces that Darwin named the “principle of divergence” and the “principle of geographical isolation,” natural selection would make individuals better suited to their environment but would not transform them. Because competition is most keen between those organisms that are most alike, the more individuals differ, the greater are their chances of survival, leading to greater and ever-increasing divergence in structure and instinct. Moreover, if members of a common species are separated from one another—as they naturally are by islands or mountain ranges—different features may be selected by the local conditions, leading in the course of time to different varieties and thus to new species. By these principles, Darwin said, small differences over the great expanse of geological time yield greater differences. New forms follow.

Natural selection is not the only cause of evolution, merely the most important. Darwin discussed several others that may lead to transmutation but may not necessarily produce changes in the organism that are adaptive. Sexual selection resulted from competition among males for mating with females. Males that failed in the competition to secure and attract female mates had no or fewer offspring. The development of fighting or defensive organs may be explained by sexual selection. In later works, Darwin added a second kind of sexual selection, female choice or preference, which he argued explained the beauty of the male peacock tail feathers or the generally more colorful male of most bird species. Other mechanisms of evolution Darwin discussed were the alteration of parts by use and disuse (the Larmarckian idea of the inheritance of acquired characteristics), spontaneous variation (later understood as genetic mutations), direct action of the environment, and correlated variation, in which a change in one part of a structure or instinct was necessarily accompanied by a change in another part.

Beginning with the first edition of On the Origin of Species by Means of Natural Selection, Darwin addressed difficulties with his theory, and in later editions he expanded his remarks to objections that had been raised to the theory by critics. Among the objections that Darwin answered were the sterility of species when first crossed compared to the fertility of varieties, the seeming difficulty of natural selection to construct complex organs in small steps (the eyes, for example), and the absence of intermediate or transitional forms among the fossils. (The answer he gave to the latter was that the geological record was necessarily incomplete and imperfect.) He also addressed how organisms acquire complex abilities, such as the ability of bees to build hives. Organisms show variability in fixed behavioral patterns, instincts, as they do in structure. Natural selection acts on behavior as it acts on structures and organs, Darwin argued. His discussion of instincts, a kind of primitive mentality, set the stage in later works for Darwin to argue for the evolutionary foundations of human mentality.

Darwin explained how the geographical distribution of plants and animals developed and why the nature of the distribution is compatible with natural selection but not special creation. He demonstrated that his theory yielded a better system of classification of organisms (taxonomy), one based on a genealogical arrangement that reflected the fact of common descent of species rather than one based on the logic of their superficial features. He also argued that the structures (morphology) of related species, their development as individuals until birth (embryology), and their possession of partially formed bodily parts (rudimentary organs) were all scattered facts of nature that were rendered intelligible by a theory of natural selection but not by the belief in special creation. Darwin’s theory is easily summarized with his own few key words: descent with modification through variation and natural selection.

On the Origin of Species by Means of Natural Selection may be the most important scientific book of the nineteenth century. It was the critical work of Darwin’s life and of all his books the one that most directly influenced nearly every field of philosophy. Darwin demonstrated that what appeared to be an object of design could be produced without a Designer. Not only was the tradition of natural theology and its argument for God’s existence by design threatened, but also, more important, mechanism was showed to be capable of yielding creativity. Mind could be produced by mindlessness. Darwin’s views also threatened essentialism, the doctrine, prominent since Plato and Aristotle, that each member of a class or species had some shared essence that made it a member and thereby defined the class. Darwin’s theory conceived of classes or categories as populations, in which class membership did not depend on an essence of whatever form but on members who would necessarily display variability in all of their identifying features. On the Origin of Species by Means of Natural Selection produced philosophical influences that were naturalistic, emphasizing development, function, and adaptation and that held selection as a powerful principle of change.

The influence of Darwin may be seen in the nineteenth century work of Charles Peirce, William James, and other American pragmatists who held that beliefs should be judged by their consequence; in essence, they are selected by their effects. John Dewey and his instrumentalism was also influenced by Darwinian ideas. Dewey conceived of thought as something that evolves to fit and solve a problem. Darwinism also led to a crass political philosophy, social Darwinism. The doctrine envisioned society as a struggle for survival of the fittest. The distribution of economic resources was in accord with natural selection processes, American sociologist William Graham Sumner said. Social welfare programs could only interfere. The view owed more to Herbert Spencer, a nineteenth century British social thinker, than to the work of Darwin. Karl Marx, who dedicated Das Kapital (1867, 1885, 1894; Capital: A Critique of Political Economy, 1886, 1907, 1909; better known as Das Kapital) to Darwin, perceived evolutionary theory as supporting his dialectical materialism. Other philosophers argued for a naturalistic, evolutionary-based ethic, one that would derive what was good from what was functional and natural. In the twentieth century, Daniel Dennett proclaimed the principle of natural selection as the solution to many philosophical puzzles, an idea that is still greatly resisted.

Appleman, Philip. Darwin. New York: W. W. Norton, 1970. A useful compilation of nineteenth and twentieth century responses to On the Origin of Species and to Darwinism as a new intellectual model.

Bowler, Peter J. Charles Darwin: The Man and His Influence. Cambridge, England: Cambridge University Press, 1996. Combines biography with cultural history. Bowler shows how Charles Darwin’s contemporaries were unable to comprehend the scientific importance of Darwin’s theory in the development of modern culture. Darwin’s relationships with other prominent scientists of the period are also portrayed.

Brackman, Arnold. A Delicate Arrangement: The Strange Case of Charles Darwin and Charles Russel Wallace. New York: Times Books, 1980. Brackman argues that Darwin and his friends conspired to deny Wallace credit for having first discovered the theory of biological evolution.

Clark, Ronald W. The Survival of Charles Darwin: A Biography of a Man and an Idea. New York: Random House, 1984. A study of Darwin’s life and work, concentrating on the genesis of evolutionary theory and its development after Darwin’s death.

Colp, Ralph, Jr. To Be an Invalid: The Illness of Charles Darwin. Chicago: University of Chicago Press, 1977. A detailed study of the various theories about what caused Darwin’s chronic, debilitating illness after the voyage of HMS Beagle.

De Beer, Gavin. Charles Darwin: A Scientific Biography. New York: Doubleday, 1965. The standard authorized biography of Darwin by an English scientist who enjoyed full access to the Darwin Papers at Cambridge University.

Depew, David J., and Bruce H. Weber, eds. Darwinism Evolving: Systems Dynamics and the Genealogy of Natural Selection. Cambridge, Mass.: MIT Press, 1995. Despite the technical terminology of the title, this work compiles a series of general articles on the origin, ideological contexts, and continuing reception of Darwin’s theory of natural selection.

Desmond, Adrian, and James Moore. Darwin: The Life of a Tormented Evolutionist. 1991. American ed. New York: W. W. Norton, 1994. Lively and enjoyable to read, this book has been hailed as the definitive biography of Darwin. The authors portray Darwin within the context of Victorian society and explain how he came to his momentous and controversial conclusion, which he kept secret for twenty years. Includes maps, photos, drawings, and extensive chapter notes.

Eiseley, Loren. Darwin’s Century: Evolution and the Men Who Discovered It. New York: Doubleday, 1958. A rigorous intellectual history of the concept of evolution and its antecedents, from Darwin’s precursors through the publication of On the Origin of Species and its reception.

Howard, Jonathan. Darwin. New York: Hill and Wang, 1982. A concise introduction to Darwin and his work, with chapters on Darwin’s biography, pre-evolutionary science, various aspects of Darwin’s theory, and an evaluation of Darwin’s stature as a scientist.

Irvine, William. Apes, Angels, and Victorians: The Story of Darwin, Huxley, and Evolution. New York: McGraw-Hill, 1955. A detailed cultural study of Darwinism and its impact on the Victorian mind.

Keynes, Randal. Darwin, His Daughter, and Human Evolution. New York: Riverhead, 2001. An examination of the spiritual crisis of the naturalist by his great, great grandson. Draws on family photographs and documents.

Stefoff, Rebecca. Charles Darwin and the Evolution Revolution. New York: Oxford University Press, 1996. This thoroughly researched biography emphasizes Darwin’s influence on and contributions to scientific, social, and political circles. Extensive photographs of family, colleagues, and reproductions of public notices and cartoons humanize the subject. Sidebars detail terms and concepts so as not to bog down the text.

Weiner, Jonathan. The Beak of the Finch. New York: Random House, 1994. A discussion of Darwin’s own anxieties over the lack of empirical evidence for the process of natural selection, as a prelude to compelling narrative of later research and the discovery of that evidence. Also includes thoughtful analysis of what natural selection means for the species that discovered it.

Young, Robert M. Darwin’s Metaphor: Nature’s Place in Victorian Culture. Cambridge, England: Cambridge University Press, 1985. A systematic study of Darwin’s chief metaphor of “selection.” Young explicates Darwinism’s place as the main scientific theory successfully to oppose an anthropocentric worldview and establish humanity as a part of nature.

Andrew J. Angyal Lisa A. Wroble