The book aroused international interest and a widespread debate, with no sharp line between scientific issues and ideological, social and religious implications. Much of the initial reaction was hostile, but Darwin had to be taken seriously as a prominent and respected name in science. There was much less controversy than had greeted the 1844 publication Vestiges of Creation, which had been rejected by scientists, but had influenced a wide public readership into believing that nature and human society were governed by natural laws. The Origin of Species as a book of wide general interest became associated with ideas of social reform. Its proponents made full use of a surge in the publication of review journals, and it was given more popular attention than almost any other scientific work, though it failed to match the continuing sales of Vestiges. Darwin's book legitimised scientific discussion of evolutionary mechanisms, and the newly coined term Darwinism was used to cover the whole range of evolutionism, not just his own ideas. By the mid-1870s, evolutionism was triumphant.
With the exception of a brief hint in the final chapter, Darwin had avoided the subject of human evolution. Despite this, the first review claimed it made a creed of the "men from monkeys" idea from Vestiges. Human evolution became central to the debate and was strongly argued by Huxley who featured it in his popular "working-men's lectures". Darwin did not publish his own views on this until 1871.
The naturalism of natural selection conflicted with presumptions of purpose in nature and while this could be reconciled by theistic evolution, other mechanisms implying more progress or purpose were more acceptable. Herbert Spencer had already incorporated Lamarckism into his popular philosophy of progressive free market human society. He popularised the terms evolution and survival of the fittest, and many thought Spencer was central to evolutionary thinking.
Impact on the scientific community
Scientific readers were already aware of arguments that species changed through processes that were subject to laws of nature, but the transmutational ideas of Lamarck and the vague "law of development" of Vestiges had not found scientific favour. Darwin presented natural selection as a scientifically testable mechanism while accepting that other mechanisms such as inheritance of acquired characters were possible. His strategy established that evolution through natural laws was worthy of scientific study, and by 1875, most scientists accepted that evolution occurred but few thought natural selection was significant. Darwin's scientific method was also disputed, with his proponents favouring the empiricism of John Stuart Mill's A System of Logic, while opponents held to the idealist school of William Whewell's Philosophy of the Inductive Sciences, in which investigation could begin with the intuitive truth that species were fixed objects created by design. Early support for Darwin's ideas came from the findings of field naturalists studying biogeography and ecology, including Joseph Dalton Hooker in 1860, and Asa Gray in 1862. Henry Walter Bates presented research in 1861 that explained insect mimicry using natural selection. Alfred Russel Wallace discussed evidence from his Malay archipelago research, including an 1864 paper with an evolutionary explanation for the Wallace line.
Evolution had less obvious applications to anatomy and morphology, and at first had little impact on the research of the anatomist Thomas Henry Huxley. Despite this, Huxley strongly supported Darwin on evolution; though he called for experiments to show whether natural selection could form new species, and questioned if Darwin's gradualism was sufficient without sudden leaps to cause speciation. Huxley wanted science to be secular, without religious interference, and his article in the April 1860 Westminster Review promoted scientific naturalism over natural theology, praising Darwin for "extending the domination of Science over regions of thought into which she has, as yet, hardly penetrated" and coining the term "Darwinism" as part of his efforts to secularise and professionalise science. Huxley gained influence, and initiated the X Club, which used the journal Nature to promote evolution and naturalism, shaping much of late Victorian science. Later, the German morphologist Ernst Haeckel would convince Huxley that comparative anatomy and palaeontology could be used to reconstruct evolutionary genealogies.
The leading naturalist in Britain was the anatomist Richard Owen, an idealist who had shifted to the view in the 1850s that the history of life was the gradual unfolding of a divine plan. Owen's review of the Origin in the April 1860 Edinburgh Review bitterly attacked Huxley, Hooker and Darwin, but also signalled acceptance of a kind of evolution as a teleological plan in a continuous "ordained becoming", with new species appearing by natural birth. Others that rejected natural selection, but supported "creation by birth", included the Duke of Argyll who explained beauty in plumage by design. Since 1858, Huxley had emphasised anatomical similarities between apes and humans, contesting Owen's view that humans were a separate sub-class. Their disagreement over human origins came to the fore at the British Association for the Advancement of Science meeting featuring the legendary 1860 Oxford evolution debate. In two years of acrimonious public dispute that Charles Kingsley satirised as the "Great Hippocampus Question" and parodied in The Water-Babies as the "great hippopotamus test", Huxley showed that Owen was incorrect in asserting that ape brains lacked a structure present in human brains. Others, including Charles Lyell and Alfred Russel Wallace, thought that humans shared a common ancestor with apes, but higher mental faculties could not have evolved through a purely material process. Darwin published his own explanation in the Descent of Man (1871).
Impact outside Great Britain
Evolutionary ideas, although not natural selection, were accepted by German biologists accustomed to ideas of homology in morphology from Goethe's Metamorphosis of Plants and from their long tradition of comparative anatomy. Bronn's alterations in his German translation added to the misgivings of conservatives, but enthused political radicals. Ernst Haeckel was particularly ardent, aiming to synthesise Darwin's ideas with those of Lamarck and Goethe while still reflecting the spirit of Naturphilosophie. Their ambitious programme to reconstruct the evolutionary history of life was joined by Huxley and supported by discoveries in palaeontology. Haeckel used embryology extensively in his recapitulation theory, which embodied a progressive, almost linear model of evolution. Darwin was cautious about such histories, and had already noted that von Baer's laws of embryology supported his idea of complex branching.
Asa Gray promoted and defended Origin against those American naturalists with an idealist approach, notably Louis Agassiz who viewed every species as a distinct fixed unit in the mind of the Creator, classifying as species what others considered merely varieties. Edward Drinker Cope and Alpheus Hyatt reconciled this view with evolutionism in a form of neo-Lamarckism involving recapitulation theory.
French-speaking naturalists in several countries showed appreciation of the much modified French translation by Clémence Royer, but Darwin's ideas had little impact in France, where any scientists supporting evolutionary ideas opted for a form of Lamarckism. The intelligentsia in Russia had accepted the general phenomenon of evolution for several years before Darwin had published his theory, and scientists were quick to take it into account, although the Malthusian aspects were felt to be relatively unimportant. The political economy of struggle was criticised as a British stereotype by Karl Marx and by Leo Tolstoy, who had the character Levin in his novel Anna Karenina voice sharp criticism of the morality of Darwin's views.
Challenges to natural selection
There were serious scientific objections to the process of natural selection as the key mechanism of evolution, including Karl von Nägeli's insistence that a trivial characteristic with no adaptive advantage could not be developed by selection. Darwin conceded that these could be linked to adaptive characteristics. His estimate that the age of the Earth allowed gradual evolution was disputed by William Thomson (later awarded the title Lord Kelvin), who calculated that it had cooled in less than 100 million years. Darwin accepted blending inheritance, but Fleeming Jenkin calculated that as it mixed traits, natural selection could not accumulate useful traits. Darwin tried to meet these objections in the 5th edition. Mivart supported directed evolution, and compiled scientific and religious objections to natural selection. In response, Darwin made considerable changes to the sixth edition. The problems of the age of the Earth and heredity were only resolved in the 20th century.
By the mid-1870s, most scientists accepted evolution, but relegated natural selection to a minor role as they believed evolution was purposeful and progressive. The range of evolutionary theories during "the eclipse of Darwinism" included forms of "saltationism" in which new species were thought to arise through "jumps" rather than gradual adaptation, forms of orthogenesis claiming that species had an inherent tendency to change in a particular direction, and forms of neo-Lamarckism in which inheritance of acquired characteristics led to progress. The minority view of August Weismann, that natural selection was the only mechanism, was called neo-Darwinism. It was thought that the rediscovery of Mendelian inheritance invalidated Darwin's views.
Impact on economic and political debates
While some, like Spencer, used analogy from natural selection as an argument against government intervention in the economy to benefit the poor, others, including Alfred Russel Wallace, argued that action was needed to correct social and economic inequities to level the playing field before natural selection could improve humanity further. Some political commentaries, including Walter Bagehot's Physics and Politics (1872), attempted to extend the idea of natural selection to competition between nations and between human races. Such ideas were incorporated into what was already an ongoing effort by some working in anthropology to provide scientific evidence for the superiority of Caucasians over non white races and justify European imperialism. Historians write that most such political and economic commentators had only a superficial understanding of Darwin's scientific theory, and were as strongly influenced by other concepts about social progress and evolution, such as the Lamarckian ideas of Spencer and Haeckel, as they were by Darwin's work. Darwin objected to his ideas being used to justify military aggression and unethical business practices as he believed morality was part of fitness in humans, and he opposed polygenism, the idea that human races were fundamentally distinct and did not share a recent common ancestry.
The book produced a wide range of religious responses at a time of changing ideas and increasing secularisation. The issues raised were complex and there was a large middle ground. Developments in geology meant that there was little opposition based on a literal reading of Genesis, but defence of the argument from design and natural theology was central to debates over the book in the English-speaking world.
Natural theology was not a unified doctrine, and while some such as Louis Agassiz were strongly opposed to the ideas in the book, others sought a reconciliation in which evolution was seen as purposeful. In the Church of England, some liberal clergymen interpreted natural selection as an instrument of God's design, with the cleric Charles Kingsley seeing it as "just as noble a conception of Deity". In the second edition of January 1860, Darwin quoted Kingsley as "a celebrated cleric", and added the phrase "by the Creator" to the closing sentence, which from then on read "life, with its several powers, having been originally breathed by the Creator into a few forms or into one". While some commentators have taken this as a concession to religion that Darwin later regretted, Darwin's view at the time was of God creating life through the laws of nature, and even in the first edition there are several references to "creation".
Baden Powell praised "Mr Darwin's masterly volume [supporting] the grand principle of the self-evolving powers of nature". In America, Asa Gray argued that evolution is the secondary effect, or modus operandi, of the first cause, design, and published a pamphlet defending the book in terms of theistic evolution, Natural Selection is not inconsistent with Natural Theology. Theistic evolution became a popular compromise, and St. George Jackson Mivart was among those accepting evolution but attacking Darwin's naturalistic mechanism. Eventually it was realised that supernatural intervention could not be a scientific explanation, and naturalistic mechanisms such as neo-Lamarckism were favoured over natural selection as being more compatible with purpose.
Even though the book had barely hinted at human evolution, it quickly became central to the debate as mental and moral qualities were seen as spiritual aspects of the immaterial soul, and it was believed that animals did not have spiritual qualities. This conflict could be reconciled by supposing there was some supernatural intervention on the path leading to humans, or viewing evolution as a purposeful and progressive ascent to mankind's position at the head of nature. While many conservative theologians accepted evolution, Charles Hodge argued in his 1874 critique "What is Darwinism?" that "Darwinism", defined narrowly as including rejection of design, was atheism though he accepted that Asa Gray did not reject design. Asa Gray responded that this charge misrepresented Darwin's text. By the early 20th century, four noted authors of The Fundamentals were explicitly open to the possibility that God created through evolution, but fundamentalism inspired the American creation–evolution controversy that began in the 1920s. Some conservative Roman Catholic writers and influential Jesuits opposed evolution in the late 19th and early 20th century, but other Catholic writers, starting with Mivart, pointed out that early Church Fathers had not interpreted Genesis literally in this area. The Vatican stated its official position in a 1950 papal encyclical, which held that evolution was not inconsistent with Catholic teaching.