The Origin of Species

Charles Darwin's Origin of Species (publ. 1859) is a pivotal work in scientific literature and arguably the pivotal work in evolutionary biology. The book's full title is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. It introduced the theory that populations evolve over the course of generations through a process of natural selection. It was controversial because it contradicted religious beliefs which underlay the then current theories of biology. Darwin's book was the culmination of evidence he had accumulated on the voyage of the Beagle in the 1830s and added to through continuing investigations and experiments since his return.

The book is readable even for the non-specialist and attracted widespread interest on publication. The book was controversial, and generated much discussion on scientific, philosophical, and religious grounds. The scientific theory of evolution has itself evolved since Darwin first presented it, but natural selection remains the most widely accepted scientific model of how species evolve. Recently, American creationists have used the fact the Darwin's theory is "only a theory" to revive an at-times bitter and ill-informed creation-evolution controversy.

Theory in a nutshell
Darwin's theory is based on key observations and inferences drawn from them:


 * 1) Species have great fertility.  They make more offspring than can grow to adulthood.
 * 2) Populations remain roughly the same size, with modest fluctuations.
 * 3) Food resources are limited, but are relatively stable over time.
 * 4) An implicit struggle for survival ensues.
 * 5) In sexually reproducing species, generally no two individuals are identical.
 * 6) Some of these variations directly impact the ability of an individual to survive in a given environment.
 * 7) Much of this variation is inheritable.
 * 8) Individuals less suited to the environment are less likely to survive and less likely to reproduce, while individuals more suited to the environment are more likely to survive and more likely to reproduce.
 * 9) The individuals that survive are most likely to leave their inheritable traits to future generations.
 * 10) This slowly effected process results in populations that adapt to the environment over time, and ultimately, after interminable generations, the creations of new varieties, and ultimately, new species.

Background
The idea of biological evolution was around long before Darwin published The Origin, and was set out in Classical times by the Greek and Roman atomists, notably Lucretius. However, Christian thought in Medieval Europe involved complete faith in the ancient Biblical teachings of creation according to Genesis. Its concepts including "Created kinds" were interpreted by the priesthood as theology, then the Protestant Reformation widened access to the Bible and brought more literal interpretations. Natural history exploring the wonders of God's works made many discoveries and naturalists such as Carolus Linnaeus categorised an enormous number of species. A new belief developed that the original pair of every species had been brought into existence by God not so long ago. By the time of Darwin's birth in 1809, it was widely believed in England that both the natural world and the hierarchical social order were held stable, fixed by God's will, with nothing happening purely naturally and spontaneously. The idea that fossils were the remains of extinct species, first put forward by Robert Hooke in the mid seventeenth century, gradually gained acceptance and several competing theories of geology were put forward, notably James Hutton's uniformitarian theory of 1785 which envisioned gradual change over aeons of time. Some individuals put forward evolutionary concepts. By 1796 Charles Darwin's grandfather Erasmus Darwin had proposed ideas of common descent with organisms "acquiring new parts" in response to stimuli then passing these changes to their offspring. In 1809 Jean-Baptiste Lamarck developed a similar theory, with "needed" traits being acquired through use then passed on. At this time the word evolution (from the Latin word "evolutio", meaning "unroll like a scroll") was used to refer to an orderly sequence of events, particularly one in which the outcome was somehow contained within it from the start, so Lamarck avoided using this word for his concept in which traits were acquired during an organism's life, and the term transmutation came into use.

Such ideas were seen in Britain as attacking the social order, already threatened by the aftermath of the American and French Revolutions. In England, natural history was dominated by the universities which trained clergy for the Church of England in William Paley's natural theology which sought evidence of beneficial "design" by a Creator. British naturalists adopted Georges Cuvier's explanation of the fossil record by catastrophism, the concept that animals and plants were periodically annihilated as a result of natural catastrophes and that their places were taken by new species created ex nihilo (out of nothing), modifying it to support the biblical account of Noah's flood. However Lamarck's ideas were taken up by Radicals who wanted to overturn the establishment and extend the vote to the lower classes.

Inception of Darwin's theory
Charles Darwin's education at the University of Edinburgh gave him direct involvement in Robert Edmund Grant's evolutionist developments of the ideas of Erasmus Darwin and Jean-Baptiste Lamarck. Then at Cambridge University his theology studies convinced him of William Paley's argument of "design" by a Creator while his interest in natural history was increased by the botanist John Stevens Henslow and the geologist Adam Sedgwick, both of whom believed strongly in divine creation. During the voyage of the Beagle Charles Darwin became convinced by Charles Lyell's uniformitarianism, and puzzled over discrepancies between Lyell's uniformitarian idea that each species had its "centre of creation" and the evidence he saw. On his return Richard Owen showed that fossils Darwin had found were of extinct species related to current species in the same locality, and John Gould startlingly revealed that completely different birds from the Galápagos Islands were species of finches distinct to each island.

By early 1837 Darwin was speculating on transmutation in a series of secret notebooks. He investigated the breeding of domestic animals, consulting William Yarrell and reading a pamphlet by Yarrell's friend Sir John Sebright which commented that "A severe winter, or a scarcity of food, by destroying the weak and the unhealthy, has all the good effects of the most skilful selection." At the zoo in 1838 he had his first sight of an ape, and the orang-utan's antics impressed him as being "just like a naughty child" which from his experience of the natives of Tierra del Fuego made him think that there was little gulf between man and animals despite theological doctrines that only mankind possessed a soul.

In late September 1838 he began reading the 6th edition of Malthus's Essay on the Principle of Population which reminded him of Malthus's statistical proof that human populations breed beyond their means and compete to survive, at a time when he was primed to apply these ideas to animal species. Darwin applied to his search for the Creator's laws the Whig social thinking of struggle for survival with no hand-outs. By December 1838 he was seeing a similarity between breeders selecting traits and a Malthusian Nature selecting from variants thrown up by chance so that "every part of newly acquired structure is fully practised and perfected", thinking this "the most beautiful part of my theory".

First writings on the theory
Darwin was well aware of the implication the theory had for the origin of humanity and the real danger to his career and reputation as an eminent geologist of being convicted of blasphemy. He worked in secret to consider all objections and prepare overwhelming evidence supporting his theory. He increasingly wanted to discuss his ideas with his colleagues, and in January 1842 sent a tentative description of his ideas in a letter to Lyell, who was then touring America. Lyell, dismayed that his erstwhile ally had become a Transmutationist, noted that Darwin "denies seeing a beginning to each crop of species".

Despite problems with illness, Darwin formulated a 35 page "Pencil Sketch" of his theory in June 1842 then worked it up into a larger "essay". The botanist Joseph Dalton Hooker became Darwin's mainstay, and late in 1845 Darwin offered his "rough Sketch" for comments without immediate success, but in January 1847 when Darwin was particularly ill Hooker took away a copy of the "Sketch". After some delays he sent a page of notes, giving Darwin the calm critical feedback that he needed. Darwin made a huge study of barnacles which established his credentials as a biologist and provided more evidence supporting his theory.

The publication of the anonymous Vestiges of the Natural History of Creation (1844) paved the way for the acceptance of Origin.

Publication
In the spring of 1856 Lyell drew Darwin's attention to a paper on the "introduction" of species written by Alfred Russel Wallace, a naturalist working in Borneo, and urged Darwin to publish to establish priority. Darwin was now torn between the desire to set out a full and convincing account and the pressure to quickly produce a short paper. He ruled out exposing himself to an editor or counsel which would have been required to publish in an academic journal. On 14 May 1856 he began a "sketch" account and, by July, had decided to produce a full technical treatise on species.

Darwin pressed on, overworking, and was throwing himself into his work with his book on Natural Selection well under way, when on 18 June 1858 he received a parcel from Wallace enclosing about twenty pages describing an evolutionary mechanism, an unexpected response to Darwin's recent encouragement, with a request to send it on to Lyell. Darwin wrote to Lyell that "your words have come true with a vengeance,... forestalled" and he would, "of course, at once write and offer to send [it] to any journal" that Wallace chose, adding that "all my originality, whatever it may amount to, will be smashed". Lyell and Hooker agreed that a joint paper should be presented at the Linnean Society, and on 1 July 1858 the Wallace and Darwin papers entitled respectively On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection were read out, to surprisingly little reaction.

On 20 July 1858 Darwin started work on an "abstract" trimmed from his Natural Selection, writing much of it from memory. Lyell made arrangements with the publisher John Murray, who agreed to publish the manuscript sight unseen, and to pay Darwin two-thirds of the net proceeds. Darwin had initially decided to call his book An abstract of an Essay/on the/Origin/of/Species and Varieties/Through natural selection/, but with Murray's persuasion it was eventually changed to the snappier title: On the Origin of Species with the title page adding by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, a long book title as was common during the Victorian era.

Publication of The Origin
The Origin was first published on 24 November 1859, price fifteen shillings. The book was offered to booksellers at Murray's autumn sale on 22 November, and all available copies were taken up immediately. In total 1250 copies were printed, but after deducting presentation and review copies, and five for Stationers' Hall copyright, around 1,170 copies were available for sale. The second edition of 3,000 copies was quickly brought out on 7 January 1860, and added "by the Creator" into the closing sentence, so that from then on it read "There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved."

During Darwin's lifetime the book went through six editions, with cumulative changes and revisions to deal with counter-arguments raised. The third edition came out in 1861, and the fourth in 1866, each with an increasing number of sentences rewritten or added. The fifth edition published on 10 February 1869 incorporated more changes again, and for the first time included Herbert Spencer's phrase "survival of the fittest".

In January 1871 Mivart published On the Genesis of Species, the cleverest and most devastating critique of natural selection in Darwin's lifetime. Darwin took it personally and from April to the end of the year made extensive revisions to the Origin, using the word "evolution" for the first time and adding a new chapter to refute Mivart. He told Murray of working men in Lancashire clubbing together to buy the 5th edition at fifteen shillings, and he wanted a new cheap edition to make it more widely available.

The sixth edition was published by Murray on 19 February 1872 with "On" dropped from the title, at a price halved to 7s. 6d. by using minute print. Sales increased from 60 to 250 a month.

On the Origin of Species, as presented
After the words "On the Origin of Species" on page i, page ii shows quotations. The first, by William Whewell from his Bridgewater Treatise, sets out the idea that in natural theology events in the material world are brought about "by the establishment of general laws" rather than by individual miracles. The second by Francis Bacon from his Advancement of Learning argues that we should study both the the word of God in the Bible and the works of God in nature together, so that the works of God teach us how to interpret the word of God. From the second to sixth editions, a third quotation is included, from the Analogy of Revealed Religion by the eighteenth century bishop Joseph Butler. This describes natural as meaning "stated, fixed or settled" by "an intelligent agent" who can equally carry out single supernatural miracles.

These quotations relate theology to nature, and in the book Darwin includes various comments aiming to harmonise science and religion, in line with Isaac Newton's belief in the glory of a rational God who established a law-abiding cosmos rather than a capricious deity. The quotations are followed by the title page (as illustrated above), then the index. The book then begins with the Introduction, though from the 3rd edition onwards this is preceded by An Historical Sketch giving due credit to his predecessors in ideas of evolution and natural selection.

Introduction
WHEN on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.

Darwin starts with a reference to the distribution of rheas, Galapagos tortoises and mockingbirds inspiring doubts in species being fixed, and the close relationship of the giant fossils he found to their small modern relatives on the same continent. He then cites the question which John Herschel had raised in correspondence with Charles Lyell just before Darwin met Herschel in South Africa. Darwin mentions his years of work on his theory, and Wallace arriving at the same conclusion leading him to "publish this Abstract" of his incomplete work. He then outlines his ideas, and sets out its essence of his theory: As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Variation under domestication and under nature
Chapter I discusses the considerable amount of variation of plants and animals in conditions of domestication. Darwin partly attributes this to different conditions of life, and (incorrectly) to domestication itself as well as to changed habits producing an inherited effect. He discusses how domestication has been going on since the neolithic period, then turns in detail to his studies of domestic pigeons. "The diversity of the breeds is something astonishing", yet all show evidence of being descendants of the same species of rock pigeons. He describes breeding methods, and introduces the term artificial selection (though environmental changes, such as more food and protection from predators, were also factors).

In chapter II Darwin considers variation under nature, and shows that the nineteenth-century definition of species was chiefly a matter of opinion, since the discovery of new linking forms often degraded species to varieties.

Struggle for existence, and natural selection
At the start of chapter III on struggle for existence Darwin reiterates how this results in varieties, "which I have called incipient species", becoming distinct species, grouped into genera. Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.... I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.

In the 5th and 6th editions he added "But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient."

He discusses the universal struggle for existence as shown by De Candolle and Lyell, emphasising that he uses the term "in a large and metaphorical sense, including dependence of one being on another". The rate of increase in population which would follow if all offspring survived leads to a Malthusian struggle: "It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms". In reviewing checks to such increase he discusses the complex interdependencies which we now term ecology, including the effects of the introduction of new species by colonists. He notes that competition is most severe between closely related forms, "which fill nearly the same place in the economy of nature".

Chapter IV then turns in detail to natural selection under the "infinitely complex and close-fitting.. mutual relations of all organic beings to each other and to their physical conditions of life". Darwin takes as an example a country where a change in conditions leads to extinction of some species, possibly immigration of others more suited and, where suitable variations occur, descendants of species becoming increasingly adapted to the changing conditions. He does not suggest that every variation and every character must have a selection value, though it would be extremely rash to set down any characters as valueless to their owners. Importantly, he does not suggest that every individual with a favourable variation must be selected, or that the selected or favoured animals are better or higher, but merely that they are more adapted to their surroundings. Having no knowledge of Mendelian genetics, he tries to deal with anticipated blending of inherited characteristics.

Darwin then introduces what he calls sexual selection to explain seemingly non-funcional differences between sexes, as in beautiful plumage of birds. He draws attention to cross-breeding between varieties giving "vigour and fertility to the offspring", with close interbreeding having the opposite effect, in what he thinks may be a universal law. This explains features found in flowers which avoid self-fertilisation and attract insects to cross-pollinate. He thinks that natural selection leading to new species is most favoured by isolation of a population, or by open areas with large populations leading to increased numbers of variations. The effect of natural selection in forming species is expected to be very slow, and often intermittent, but given the effectiveness of artificial selection, he "can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection." With the aid of a tree diagram and calculations he indicates the "divergence of character" from original species into multiple new species and genera, branches stopping or falling off as extinction occurs, while fresh buds form new branches in "the great Tree of life... with its ever branching and beautiful ramifications."

Variation and heredity
One of the chief difficulties for Darwin and other naturalists in his time was that there was no agreed-upon model of heredity &mdash; in fact, the idea of heredity had not been completely separated conceptually from the idea of the development of the organism. Darwin himself saw variation and heredity as two essentially antagonistic forces, with most heredity working to preserve the fixity of a type rather than acting as the agent of species variability. Darwin's own model of heredity worked out in later works, which he dubbed "Pangenesis", was a mixture of a number of different ideas about heredity at the time. It contained what are now considered to be essentially Lamarckian aspects, whereby the effects of use and dis-use of different parts of the body in the parent could be transmitted to the child. Beyond this, it was essentially a model of "blended" heredity, by which the contributions of two parents (in the form of particles he called "gemmules") were roughly equal. Darwin was confident that even in this model, over long periods of time species would still be able to evolve.

It was not until the early 20th century that a model of heredity would become completely integrated with a model of variation, with the advent of the modern evolutionary synthesis known as neo-Darwinism. It is a common trope in the history of evolution and genetics written by scientists, rather than historians, to claim that Darwin's lack of an adequate model of heredity was the source of suspicion about his theory, but later historians of science have adequately documented the fact that this was not the source of most objections to Darwin, and that later scientists, such as Karl Pearson and the biometric school, could develop compelling models of evolution by natural selection with even a relatively simple "blending" model of heredity such as that used by Darwin.

Compatibility with Lamarckian inheritance
Contrarily to a common opinion, Darwin did not rule out at first the possibility of inherited acquired traits, and even mentions it explicitly in chapter 7:

When the first tendency was once displayed, methodical selection and the inherited effects of compulsory training in each successive generation would soon complete the work; and unconscious selection is still at work, as each man tries to procure, without intending to improve the breed, dogs which will stand and hunt best. On the other hand, habit alone in some cases has sufficed; no animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the young of the tame rabbit; but I do not suppose that domestic rabbits have ever been selected for tameness; and I presume that we must attribute the whole of the inherited change from extreme wildness to extreme tameness, simply to habit and long-continued close confinement.

Nevertheless, there is a distinction between Lamarck's theory and Darwin's one: while Darwin's theory stays valid whether acquired traits are transmitted or not, Lamarck's theory becomes inoperative if acquired traits cannot be transmitted.

Public reaction
Public reaction can be partitioned into three overlapping realms: scientific, religious, and philosophical.

At the time of publication, the educated public generally held the belief that science was a "friend of humanity" and that the natural world was orderly. This belief was based in part on advances made by Frenchman Louis Pasteur, who, in 1859, finally laid to rest the theory of spontaneous generation, and Isaac Newton's laws of motion and gravition, which were perceived as timeless and absolute.

After publication, Darwin's theories were discussed and debated extensively, in part due to Huxley's popular "working-men's lectures". With the publication of the 6th edition, the book's price was halved, increasing sales and disseminating Darwin's revolutionary ideas even more widely.

The book contradicted then-prevailing scientific doctrines, as well as widely held religious beliefs that held the Creator ordained not only the laws of nature but also directly created kinds. The idea of supernatural design in nature served two purposes; one scientific, and the other religious. Design made nature orderly, and hence made science possible. Supernatural design also gave sanction to "the moral and religious endeavors of man."



The religious controversy was fueled in part by one of Darwin's most vigorous defenders, Thomas Henry Huxley, who opined that Christianity is "a compound of some of the best and some of the worst elements of Paganism and Judaism, moulded in practice by the innate character of certain people of the Western World." Perhaps the most uncompromising of the evolutionist philosophers was the German Ernst Heinrick Haeckel, a professor of biology, who dogmatically affirmed that nothing spiritual exists, and instead asserted that all life descended from protoplasm that spontaneously combined from essential protoplasmic elements in antiquity. In "What is Darwinism?" the theologian Charles Hodge argued that Darwin's theories were tantamount to atheism. This is an argument that had been made by many almost immediately after Darwin's first publication. As Hodge pointed out, evolution does not seem to originate from a divine source, and some viewed God as a less powerful force in the universe. During an 1860 debate between Bishop Wilberforce and Thomas Huxley, the bishop of Oxford is reputed to have asked Huxley, "Is it on your grandfather's or your grandmother's side that you claim descent from a monkey?"

Darwin's revolutionary theory changed the way humans saw themselves and their world. If one accepted that humans were descended from animals, it became clear that humans also are animals. The natural world took on a darker tinge in the minds of many, as animals in the wild are understood to be in a constant state of deadly competition with one another. The world was also seen in a less permanent fashion; since the world was apparently much different millions of years ago, it dawned on many that the impact of human beings would lessen and perhaps disappear altogether over time.

The dizzying effect of Darwin's theory on the middle classes of Europe and United States was softened by the so-called Social Darwinists, such as Herbert Spencer, who promoted the virtues of social competition in fields outside biology. While religious controversies continue to this day, some scientific and religious thinkers dismiss apparent contradictions by simply rationalizing that not all questions that can be asked have answers "in terms of the alternatives that the questions themselves present." That is to say, historically, whenever questions are answered, be it in the realm of science or religion, answers invariably lead to additional unanswered questions. Better to consider science one method of gleaning natural truth, and scripture as a different method of building a relationship with and understanding of God.

While arguments against Darwin's theories were often vigourously presented in defense of religon, the ramifications of these arguments had scientific and philosophical applications. The American botanist and Darwin promoter Asa Gray tried to reconcile design doctrine with evolution by arguing that evolution is the secondary effect, or modus operandi, of the first cause, design.

Many of the debates did not center around Darwin's specifically proposed mechanism for evolution &mdash; natural selection &mdash; but rather on the concept of evolution in general. Though Darwin himself was too sickly to defend his work in public, four of his close scientific friends took up the cause of promoting Darwin's work and defending it against critics. Chief among these were Huxley, who argued for the evidence of evolution in anatomical morphology, and Joseph Dalton Hooker, the Royal botanist at Kew Gardens. In the United States, Asa Gray worked in close correspondence with Darwin to assure the theory's spread despite the opposition of one of the most prominent scientists in the country at the time, Louis Agassiz, and helped to facilitate American publication of the book.



Scientific reaction to Darwin's theory was mixed. Many well-respected members of the scientific community, such as the aforementioned Agassiz and the anatomist Richard Owen, came out strongly against Darwin's work. On the whole, though, Darwin was successful in convincing many scientists, especially of the younger generations, that evolution had happened in one form or another. Over the course of the next two decades, most scientists and educated lay-people would come to believe that evolution had occurred. Natural selection, though, did not find wide support, and was actively attacked and relatively unpopular until its revival during the creation of the modern evolutionary synthesis in the 1920s and 1930s. Similarly, Darwin's notion that evolution occurred gradually was also often attacked, and many of the evolutionary theories which flourished during what Peter J. Bowler has called the "eclipse of Darwinism" were forms of "saltationism", in which new species arose through "jumps" rather than gradual adaptation.

From the 1860s until the 1930s, Darwinian "selectionist" evolution was not universally accepted by scientists, while evolution of some form generally was (a variety of evolutionary theories competed for scientific approval, including neo-Darwinism, neo-Lamarckism, orthogenesis, and mutation theory). In the 1930s, the work of a number of biologists and statisticians (especially R. A. Fisher) created the modern synthesis of evolution, which merged Darwinian selection theory with sophisticated statistical understandings of Mendelian genetics.

Today, the overwhelming majority of scientists in the fields of earth and life sciences (over 99.8%) consider Darwin's theory correct. For example, Mark Isaak writes "Often, claims that scientists reject evolution or support creationism are exaggerated or fraudulent. Many scientists doubt some aspects of evolution, especially recent hypotheses about it.  All good scientists are skeptical about evolution (and everything else) and open to the possibility, however remote, that serious challenges to it may appear.  Creationists frequently seize such expressions of healthy skepticism to imply that evolution is highly questionable. They fail to understand that the fact that evolution has withstood many years of such questioning really means it is about as certain as facts can get." A significant proportion of non-scientists in the United States and a few other countries disagree, often on religious grounds.

Misconceptions, and comparison to Wallace's theory
Darwin was not the first to form evolutionary theories. The idea that species might evolve, i.e., that current species have arisen from previous ones, was already under discussion at the time, and several mechanisms had been proposed dating back to Anaximander's theory of aquatic descent in the sixth century B.C., as well as Jean-Baptiste Lamarck's 1809 hypothesis on the inheritance of acquired characteristics, which was well known at the time of publication.

Darwin's achievements were fourfold: Firstly, to propose a credible mechanism (natural selection); secondly, to provide a great deal of new evidence for evolution; thirdly, to present his ideas in a compelling book; and fourthly, to ally with other highly motivated and influential biologists and philosophers in a concerted effort to publicize and advocate his ideas. On every point, Darwin was successful.

Like many great scientists, Darwin did not invent his theory from the ground up. He seized upon earlier research to create a comprehensive and defendable theory. As he subsequently acknowledged, others before him published brief statements outlining the principle of natural selection, but he was not aware of these little known statements until after publication of the Origin. Instead, he and Wallace put forth the first convincing and coherent mechanism of evolution: natural selection. Darwin's work, through its long list of facts and its support by prominent naturalists, established for most that evolution of some form did occur&mdash;that there was no fixity of species&mdash;even if there was considerable disagreement on the mechanism. Also contrary to a common understanding, Darwin did not invent the phrase "survival of the fittest", but added this in the 5th edition of The Origin of Species, giving due credit to the philosopher Herbert Spencer (who had introduced the phrase in his Principles of Biology of 1864) and usually using the phrase "Natural Selection, or the Survival of the Fittest". Other aspects of Darwin's overall theory which themselves evolved over time were: common descent, sexual selection, gradualism, and pangenesis.

Darwin's explanation of natural selection was slightly different from that given by Wallace. Darwin used comparison to selective breeding and artificial selection as a means for understanding natural selection. No such connection between selective breeding and natural selection was made by Wallace; he expressed it simply as a basic process of nature and did not think the phenomena were in any way related. On Wallace's own first edition of The Origin of Species, he crossed out every instance of the phrase "natural selection" and replaced with it Spencer's "survival of the fittest." He also ruled out much of the ideas of Lamarckian inheritance present in Darwin's work, calling it "quite unnecessary." Darwin and Wallace would disagree on many substantive issues later in their lives especially, most bitterly on the question of whether human consciousness had itself evolved (to Darwin's horror, Wallace eventually turned against this and towards spiritualism).

Philosophical implications
According to Ernst Mayr, Darwin's evolutionary thinking rests on a rejection of essentialism, which assumes the existence of some perfect, essential form for any particular class of existent, and treats differences between individuals as imperfections or deviations away from the perfect essential form. Darwin embraced instead what Mayr calls "population thinking", which denies the existence of any essential form and holds that a class is the conceptualization of the numerous unique individuals. Individuals, in short, are real in an objective sense, while the class is an abstraction, an artifact of epistemology. This emphasis on the importance of individual differences is necessary if one believes that the mechanism of evolution, natural selection, operates on individual differences.

Mayr claims essentialism had dominated Western thinking for two thousand years, and that Darwin's theories thus represent an important and radical break from traditional Western philosophy. Ripples of Darwin's thought can now be seen in fields such as economics and complexity theory, suggesting that Darwin's influence extends well beyond the field of biology.

The historian of science Peter J. Bowler has suggested that many of the "implications" attributed to Darwinism had little to do with Darwin's theories themselves. Many of the so-called "Darwinists" of the late-nineteenth century, such as Herbert Spencer and Ernst Haeckel, were actually very non-Darwinian in many aspects of their thought and theory, and even the biggest supporters of Darwin, such as Thomas Henry Huxley, were suspicious as to whether natural selection was really what caused evolution. Nevertheless, Darwin became quickly identified with evolution in general and hailed as the figurehead of many conceptual changes in both science and society, whether or not all of these ideas were stated explicitly or at all in Darwin's work itself.

The philosopher Daniel Dennett recapitulated those philosophical implications in his book Darwin's Dangerous Idea.

Contemporaneous reviews

 * William Benjamin Carpenter Darwin on the Origin of Species. National Review 10: December 1859 188-214
 * Thomas Henry Huxley Time and life: Mr Darwin's "Origin of species." Macmillan's Magazine 1: 1859 142-148.
 * Richard Owen Review of Darwin's Origin of Species, Edinburgh Review, 3, April 1860, pp. 487-532.]
 * Thomas Henry Huxley Review of The Origin of Species, Westminster Review 17 (n.s.) April 1860, pp. 541-70.
 * Samuel Wilberforce (Review of) ‘On the origin of species’, Quarterly Review, June 1860, pp. 225-264.
 * Andrew Murray On Mr Darwin's theory of the origin of species. Proceedings of the Royal Society of Edinburgh 4: 1860 274-291.
 * Fleeming Jenkin 'Review of Darwin's The origin of species' The North British Review, June 1867, 46, pp. 277-318.