Editing Alfred Russel Wallace (section)

== Theory of evolution ==

=== Early evolutionary thinking ===
Wallace began his career as a travelling naturalist who already believed in the transmutation of species. The concept had been advocated by [[Jean-Baptiste Lamarck]], [[Étienne Geoffroy Saint-Hilaire|Geoffroy Saint-Hilaire]], [[Erasmus Darwin]], and [[Robert Edmond Grant|Robert Grant]], among others. It was widely discussed, but not generally accepted by leading naturalists, and was considered to have [[radicalism (historical)|radical]], even revolutionary connotations.{{sfn|Larson|2004|p=73}}{{sfn|Bowler|Morus|2005|p=141}} Prominent anatomists and geologists such as [[Georges Cuvier]], [[Richard Owen]], [[Adam Sedgwick]], and Lyell attacked transmutation vigorously.{{sfn|McGowan|2001|pp=101, 154–155}}{{sfn|Larson|2004|pp=23–24, 37–38}} It has been suggested that Wallace accepted the idea of the transmutation of species in part because he was always inclined to favour radical ideas in politics, religion and science,{{sfn|Larson|2004|p=73}} and because he was unusually open to marginal, even fringe, ideas in science.{{sfn|Shermer|2002|p=54}}

Wallace was profoundly influenced by [[Robert Chambers (journalist)|Robert Chambers]]'s ''Vestiges of the Natural History of Creation'', a controversial work of popular science published anonymously in 1844. It advocated an evolutionary origin for the [[Solar System]], the Earth, and living things.{{sfn|Slotten|2004|p=31}} Wallace wrote to Henry Bates in 1845 describing it as "an ingenious hypothesis strongly supported by some striking facts and analogies, but which remains to be proven by ... more research".{{sfn|Shermer|2002|p=54}} In 1847, he wrote to Bates that he would "like to take some one family [of beetles] to study thoroughly, ... with a view to the theory of the origin of species."<ref>Wallace Family Archive, 11 October 1847, quoted in {{harvnb|Raby|2002|p=1}}.</ref>

Wallace planned fieldwork to test the evolutionary hypothesis that closely related species should inhabit neighbouring territories.{{sfn|Larson|2004|p=73}} During his work in the [[Amazon basin]], he came to realise that geographical barriers—such as the Amazon and its major tributaries—often separated the ranges of closely allied species. He included these observations in his 1853 paper "On the Monkeys of the Amazon". Near the end of the paper he asked the question, "Are very closely allied species ever separated by a wide interval of country?"{{sfn|Slotten|2004|p=94}}

In February 1855, while working in [[Sarawak]] on the island of [[Borneo]], Wallace wrote "On the Law which has Regulated the Introduction of New Species". The paper was published in the ''[[Journal of Natural History|Annals and Magazine of Natural History]]'' in September 1855.<ref>{{cite web |title=Wallace Collection – Wallace's 'Sarawak law' paper |url=http://www.nhm.ac.uk/nature-online/collections-at-the-museum/wallace-collection/closeup.jsp?itemID=138&theme=Evolution |publisher=[[Natural History Museum, London|Natural History Museum]] |year=2012 |access-date=14 February 2012 }}</ref> In this paper, he discussed observations of the geographic and geologic distribution of both living and fossil species, a field that became biogeography. His conclusion that "Every species has come into existence coincident both in space and time with a closely allied species" has come to be known as the "Sarawak Law", answering his own question in his paper on the monkeys of the Amazon basin. Although it does not mention possible mechanisms for evolution, this paper foreshadowed the momentous paper he would write three years later.<ref>{{cite web |last=Wallace |first=Alfred Russel |title=On the Law Which has Regulated the Introduction of Species |url=http://www.wku.edu/%7Esmithch/wallace/S020.htm |year=1855 |publisher=[[Western Kentucky University]] |access-date=8 May 2007 |archive-url=https://web.archive.org/web/20070428194531/http://www.wku.edu/~smithch/wallace/S020.htm |archive-date=28 April 2007 |url-status=live}}</ref>

The paper challenged Lyell's belief that species were immutable. Although Darwin had written to him in 1842 expressing support for transmutation, Lyell had continued to be strongly opposed to the idea. Around the start of 1856, he told Darwin about Wallace's paper, as did [[Edward Blyth]] who thought it "Good! Upon the whole!&nbsp;... Wallace has, I think put the matter well; and according to his theory the various domestic races of animals have been fairly developed into ''species''." Despite this hint, Darwin mistook Wallace's conclusion for the [[progressive creationism]] of the time, writing that it was "nothing very new&nbsp;... Uses my simile of tree [but] it seems all creation with him." Lyell was more impressed, and opened a notebook on species in which he grappled with the consequences, particularly for human ancestry. Darwin had already shown his theory to their mutual friend [[Joseph Dalton Hooker|Joseph Hooker]] and now, for the first time spelt out the full details of natural selection to Lyell. Although Lyell could not agree, he urged Darwin to publish to establish priority. Darwin demurred at first, but began writing up a ''species sketch'' of his continuing work in May 1856.{{sfn|Desmond|Moore|1991|p=438}}{{sfn|Browne|1995|pp=537–546}}

=== Natural selection and Darwin ===
{{see also|Publication of Darwin's theory|Natural selection}}

By February 1858, Wallace had been convinced by his biogeographical research in the Malay Archipelago that evolution was real. He later wrote in his autobiography that the problem was of how species change from one well-marked form to another.{{sfn|Wallace|1905a|p=361}} He stated that it was while he was in bed with a fever that he thought about Malthus's idea of positive checks on human population, and had the idea of natural selection. His autobiography says that he was on the island of [[Ternate]] at the time; but the evidence of his journal suggests that he was in fact on the island of [[Halmahera|Gilolo]].{{sfn|Slotten|2004|pp=144–145}} From 1858 to 1861, he rented a house on Ternate from the Dutchman [[Maarten Dirk van Renesse van Duivenbode]], which he used as a base for expeditions to other islands such as Gilolo.<ref>{{cite book |last=Heij |first=C. J. |year=2011 |title=Biographical Notes of [[Antonie Augustus Bruijn]] (1842–1890) |publisher=IBP Press |place=Bogor |isbn=978-979-493-294-0}}</ref>

Wallace describes how he discovered natural selection as follows:

{{blockquote|It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more quickly than does mankind, the destruction every year from these causes must be enormous to keep down the numbers of each species, since evidently they do not increase regularly from year to year, as otherwise the world would long ago have been crowded with those that breed most quickly. Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, why do some die and some live? And the answer was clearly, on the whole the best fitted live&nbsp;... and considering the amount of individual variation that my experience as a collector had shown me to exist, then it followed that all the changes necessary for the adaptation of the species to the changing conditions would be brought about&nbsp;... In this way every part of an animals organization could be modified exactly as required, and in the very process of this modification the unmodified would die out, and thus the definite characters and the clear isolation of each new species would be explained.{{sfn|Wallace|1905a|pp=361–362}} }}

[[File:Darwin-Wallace medal.jpg|thumb|upright|The [[Darwin–Wallace Medal]] was issued by the [[Linnean Society of London|Linnean Society]] on the 50th anniversary of the reading of Darwin and Wallace's papers on [[natural selection]]. Wallace received the only gold example.<ref>{{Cite web|title=The Darwin-Wallace Medal|url=https://wallacefund.myspecies.info/content/darwin-wallace-medal|website=The Wallace Website}}</ref>|alt=photograph of the Darwin-Wallace medal]]

Wallace had once briefly met Darwin, and was one of the correspondents whose observations Darwin used to support his own theories. Although Wallace's first letter to Darwin has been lost, Wallace carefully kept the letters he received.{{sfn|Marchant|1916|loc=[http://darwin-online.org.uk/content/frameset?itemID=F1593&viewtype=image&pageseq=116 p. 105]}} In the first letter, dated 1 May 1857, Darwin commented that Wallace's letter of 10 October which he had recently received, as well as Wallace's paper "On the Law which has regulated the Introduction of New Species" of 1855, showed that they thought alike, with similar conclusions, and said that he was preparing his own work for publication in about two years time.{{sfn|Darwin|2009|p=[http://darwin-online.org.uk/content/frameset?itemID=F1452.2&viewtype=text&pageseq=111 95]}} The second letter, dated 22 December 1857, said how glad he was that Wallace was theorising about distribution, adding that "without speculation there is no good and original observation" but commented that "I believe I go much further than you".{{sfn|Darwin|2009|p=[http://darwin-online.org.uk/content/frameset?itemID=F1452.2&viewtype=text&pageseq=124 108]}} Wallace believed this and sent Darwin his February 1858 essay, "[[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection|On the Tendency of Varieties to Depart Indefinitely From the Original Type]]", asking Darwin to review it and pass it to [[Charles Lyell]] if he thought it worthwhile.<ref name="tendency">{{cite web|last=Wallace |first=Alfred |title=On the Tendency of Varieties to Depart Indefinitely From the Original Type |url=http://www.wku.edu/~smithch/wallace/S043.htm |publisher=The Alfred Russel Wallace Page hosted by [[Western Kentucky University]] |access-date=22 April 2007 |archive-url=https://web.archive.org/web/20070429183411/http://www.wku.edu/~smithch/wallace/S043.htm |archive-date=29 April 2007 |url-status=live}}</ref> Although Wallace had sent several articles for journal publication during his travels through the Malay archipelago, the Ternate essay was in a private letter. Darwin received the essay on 18 June 1858. Although the essay did not use Darwin's term "natural selection", it did outline the mechanics of an evolutionary divergence of species from similar ones due to environmental pressures. In this sense, it was very similar to the theory that Darwin had worked on for 20 years, but had yet to publish. Darwin sent the manuscript to Charles Lyell with a letter saying "he could not have made a better short abstract! Even his terms now stand as heads of my chapters&nbsp;... he does not say he wishes me to publish, but I shall, of course, at once write and offer to send to any journal."{{sfn|Slotten|2004|pp=153–154}}{{sfn|Darwin|2009|loc=[http://darwin-online.org.uk/content/frameset?itemID=F1452.2&viewtype=text&pageseq=132 p. 116]}} Distraught about the illness of his baby son, Darwin put the problem to Charles Lyell and [[Joseph Dalton Hooker|Joseph Hooker]], who decided to publish the essay in a joint presentation together with unpublished writings which highlighted Darwin's priority. Wallace's essay was presented to the [[Linnean Society of London]] on 1 July 1858, along with excerpts from an essay which Darwin had disclosed privately to Hooker in 1847 and a letter Darwin had written to [[Asa Gray]] in 1857.{{sfn|Browne|2002|pp=33–42}}

Communication with Wallace in the far-off Malay Archipelago involved months of delay, so he was not part of this rapid publication. Wallace accepted the arrangement after the fact, happy that he had been included at all, and never expressed bitterness in public or in private. Darwin's social and scientific status was far greater than Wallace's, and it was unlikely that, without Darwin, Wallace's views on evolution would have been taken seriously. Lyell and Hooker's arrangement relegated Wallace to the position of co-discoverer, and he was not the social equal of Darwin or the other prominent British natural scientists. All the same, the joint reading of their papers on natural selection associated Wallace with the more famous Darwin. This, combined with Darwin's (as well as Hooker's and Lyell's) advocacy on his behalf, would give Wallace greater access to the highest levels of the scientific community.{{sfn|Shermer|2002|pp=148–150}} The reaction to the reading was muted, with the president of the Linnean Society remarking in May 1859 that the year had not been marked by any striking discoveries;{{sfn|Browne|2002|pp=40–42}} but, with Darwin's publication of ''[[On the Origin of Species]]'' later in 1859, its significance became apparent. When Wallace returned to the UK, he met Darwin. Although some of Wallace's opinions in the ensuing years would test Darwin's patience, they remained on friendly terms for the rest of Darwin's life.<ref name="Browne 2013">{{cite journal |last1=Browne |first1=Janet |author1-link=Janet Browne |title=Wallace and Darwin |journal=Current Biology |year=2013 |volume=23 |issue=24 |pages=R1071–R1072 |doi=10.1016/j.cub.2013.10.045 |pmid=24501768 |bibcode=2013CBio...23R1071B |s2cid=4281426 |url=https://www.cell.com/current-biology/pdf/S0960-9822(13)01320-1.pdf}}</ref>

Over the years, a few people have questioned this version of events. In the early 1980s, two books, one by [[Arnold Brackman]] and another by [[John Langdon Brooks]], suggested not only that there had been a conspiracy to rob Wallace of his proper credit, but that Darwin had actually stolen a key idea from Wallace to finish his own theory. These claims have been examined and found unconvincing by a number of scholars.{{sfn|Slotten|2004|pp=157–162}}{{sfn|Shermer|2002|loc=[https://web.archive.org/web/20081007080247/http://www.michaelshermer.com/darwins-shadow/excerpt/ Chapter 5: "A Gentlemanly Arrangement: Alfred Russel Wallace, Charles Darwin & the Scientific Priority Dispute"]}}<ref>{{cite web |last=Smith |first=Charles H. |author-link=Charles H. Smith (historian) |title=Responses to Questions Frequently Asked About Wallace: Did Darwin really steal material from Wallace to complete his theory of natural selection? |url=http://www.wku.edu/~smithch/wallace/FAQ.htm |publisher=The Alfred Russel Wallace Page hosted by [[Western Kentucky University]] |access-date=29 April 2008| archive-url= https://web.archive.org/web/20080509183747/http://www.wku.edu/~smithch/wallace/FAQ.htm |archive-date=9 May 2008 |url-status=live}}</ref> Shipping schedules show that, contrary to these accusations, Wallace's letter could not have been delivered earlier than the date shown in Darwin's letter to Lyell.<ref>{{cite journal |last1=van Wyhe |first1=John|author1-link=John van Wyhe |last2=Rookmaaker |first2=Kees |year=2012 |title=A new theory to explain the receipt of Wallace's Ternate Essay by Darwin in 1858 |journal=Biological Journal of the Linnean Society |volume=105 |pages=249–252 |doi=10.1111/j.1095-8312.2011.01808.x|doi-access= }}</ref><ref>{{cite journal |last=Ball |first=Philip |author-link=Philip Ball |title=Shipping timetables debunk Darwin plagiarism accusations |journal=Nature |url=http://www.nature.com/news/shipping-timetables-debunk-darwin-plagiarism-accusations-1.9613 |date=12 December 2011 |doi=10.1038/nature.2011.9613 |s2cid=178946874 |doi-access=free }}</ref>

==== Defence of Darwin and his ideas ====

After Wallace returned to England in 1862, he became one of the staunchest defenders of Darwin's ''On the Origin of Species''. In an incident in 1863 that particularly pleased Darwin, Wallace published the short paper "Remarks on the Rev. S. Haughton's Paper on the Bee's Cell, And on the Origin of Species". This rebutted a paper by a professor of geology at the University of Dublin that had sharply criticised Darwin's comments in the ''Origin'' on how hexagonal honey bee cells could have evolved through natural selection.{{sfn|Slotten|2004|pp=197–199}}
An even longer defence was a 1867 article in the ''[[Quarterly Journal of Science]]'' called "Creation by Law". It reviewed [[George Campbell, 8th Duke of Argyll|George Campbell]], the 8th Duke of Argyll's book, ''The Reign of Law'', which aimed to refute natural selection.<ref>{{cite web |last=Wallace |first=Alfred |title=Creation by Law (S140: 1867) |url=http://www.wku.edu/~smithch/wallace/S140.htm |publisher=The Alfred Russel Wallace Page hosted by [[Western Kentucky University]] |access-date=23 May 2007 |archive-url= https://web.archive.org/web/20070602121908/http://www.wku.edu/~smithch/wallace/S140.htm |archive-date= 2 June 2007 |url-status=live }}</ref>
After an 1870 meeting of the [[British Science Association]], Wallace wrote to Darwin complaining that there were "no opponents left who know anything of natural history, so that there are none of the good discussions we used to have".{{sfn|Slotten|2004|p=261}}

====Differences between Darwin and Wallace====

Historians of science have noted that, while Darwin considered the ideas in Wallace's paper to be essentially the same as his own, there were differences.<ref>{{Cite journal |last=Kutschera |first=Ulrich |author-link=Ulrich Kutschera |title=A comparative analysis of the Darwin–Wallace papers and the development of the concept of natural selection |journal=Theory in Biosciences |date=19 December 2003 |doi=10.1007/s12064-003-0063-6 |volume=122 |issue=4 |pages=343–359 |s2cid=24297627 }}</ref> Darwin emphasised competition between individuals of the same species to survive and reproduce, whereas Wallace emphasised environmental pressures on varieties and species forcing them to become adapted to their local conditions, leading populations in different locations to diverge.{{sfn|Larson|2004|p=75}}{{sfn|Bowler|Morus|2005|p=149}} The historian of science [[Peter J. Bowler]] has suggested that in the paper he mailed to Darwin, Wallace might have been discussing [[group selection]].{{sfn|Bowler|2013|pp=61–63}} Against this, Malcolm Kottler showed that Wallace was indeed discussing individual variation and selection.<ref>{{cite book |last=Kottler |first=Malcolm |year=1985 |chapter=Charles Darwin and Alfred Russel Wallace: Two decades of debate over natural selection |editor-last=Kohn |editor-first=David Kohn |title=The Darwinian Heritage |publisher=[[Princeton University Press]] |pages=367–432 |isbn=978-0691083568 }}</ref>

Others have noted that Wallace appeared to have envisioned natural selection as a kind of feedback mechanism that kept species and varieties adapted to their environment (now called 'stabilizing", as opposed to 'directional' selection).<ref name="Unfinished Business"/> They point to a largely overlooked passage of Wallace's famous 1858 paper, in which he likened "this principle ... [to] the [[centrifugal governor]] of the steam engine, which checks and corrects any irregularities".<ref name="tendency"/> The [[cybernetics|cybernetician]] and anthropologist [[Gregory Bateson]] observed in the 1970s that, although writing it only as an example, Wallace had "probably said the most powerful thing that'd been said in the 19th Century".<ref>{{cite web |last=Brand |first=Stewart |title=For God's Sake, Margaret |url=http://www.oikos.org/forgod.htm |publisher=CoEvolutionary Quarterly, June 1976 |access-date=4 April 2007 |archive-url=https://web.archive.org/web/20070415185352/http://www.oikos.org/forgod.htm |archive-date=15 April 2007 |url-status=dead}}</ref> Bateson revisited the topic in his 1979 book ''Mind and Nature: A Necessary Unity'', and other scholars have continued to explore the connection between natural selection and [[systems theory]].<ref name="Unfinished Business">{{cite journal |last=Smith |first=Charles H. |title=Wallace's Unfinished Business |journal=Complexity |volume=10 |issue=2 |year=2004 |doi=10.1002/cplx.20062 |doi-access=free }}</ref>

==== Warning coloration and sexual selection ====
{{further|The Colours of Animals}}

[[File:Darwinism 1889 Fig. 26 image of mimicry.jpg|thumb|upright|Illustration of [[Batesian mimicry]]: a wasp (top) mimicked by a beetle in Wallace's 1889 book ''Darwinism''|alt=see caption]]

[[Aposematism|Warning coloration]]<!--Brit. usage is colour, but coloration, see [[structural coloration]] for detailed explanation--> was one of Wallace's contributions to the evolutionary biology of [[Colours of animals|animal coloration]].{{sfn|Slotten|2004|pp=251–254}} In 1867, Darwin wrote to Wallace about a problem in explaining how some caterpillars could have evolved conspicuous colour schemes. Darwin had come to believe that many conspicuous animal colour schemes were due to sexual selection, but he saw that this could not apply to caterpillars. Wallace responded that he and Bates had observed that many of the most spectacular butterflies had a peculiar odour and taste, and that he had been told by [[John Jenner Weir]] that birds would not eat a certain kind of common white moth because they found it unpalatable. Since the moth was as conspicuous at dusk as a coloured caterpillar in daylight, it seemed likely that the conspicuous colours served as a warning to predators and thus could have evolved through natural selection. Darwin was impressed by the idea. At a later meeting of the Entomological Society, Wallace asked for any evidence anyone might have on the topic.<ref>{{Cite journal |last=Smith |first=Frederick |year=1867 |title=March 4, 1867 |journal=Transactions of the Royal Entomological Society of London |volume=15 |issue=7 |pages=509–566 |doi=10.1111/j.1365-2311.1967.tb01466.x}}</ref> In 1869, Weir published data from experiments and observations involving brightly coloured caterpillars that supported Wallace's idea.{{sfn|Slotten|2004|pp=253–254}}<!--mentioned also in Wallace's ''Darwinism'', ch. 9, and in [http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F1548.1&pageseq=322 a letter by Darwin to A. Weismann on 1 May 1875]--> Wallace attributed less importance than Darwin to sexual selection. In his 1878 book ''[https://www.gutenberg.org/ebooks/73563 Tropical Nature and Other Essays]'', he wrote extensively about the [[Animal coloration|coloration of animals]] and plants, and proposed alternative explanations for a number of cases Darwin had attributed to sexual selection.{{sfn|Slotten|2004|pp=353–356}} He revisited the topic at length in his 1889 book ''Darwinism''. In 1890, he wrote a critical review in ''[[Nature (journal)|Nature]]'' of his friend [[Edward Bagnall Poulton]]'s ''[[The Colours of Animals]]'' which supported Darwin on sexual selection, attacking especially Poulton's claims on the "aesthetic preferences of the insect world".<ref>{{cite journal |last=Wallace |first=Alfred Russel |title=[Review] The Colours of Animals |journal=Nature |volume=42 |pages=289–291 |date=24 July 1890 |issue=1082 |doi=10.1038/042289a0 |bibcode=1890Natur..42..289W |s2cid=27117910 |url=https://zenodo.org/record/1429319 }}</ref><ref name="Nature">{{cite web |url=https://people.wku.edu/charles.smith/wallace/S424.htm | title=The Colours of Animals |last=Wallace |first=Alfred Russel |website=The Alfred Russel Wallace Page |access-date=25 May 2022 }}</ref>

==== Wallace effect ====
{{further|Reinforcement (speciation)}}

In 1889, Wallace wrote the book ''Darwinism'', which explained and defended natural selection. In it, he proposed the hypothesis that natural selection could drive the reproductive isolation of two varieties by encouraging the development of barriers against hybridisation. Thus it might contribute to the development of new species. He suggested the following scenario: When two populations of a species had diverged beyond a certain point, each adapted to particular conditions, hybrid offspring would be less adapted than either parent form and so natural selection would tend to eliminate the hybrids. Furthermore, under such conditions, natural selection would favour the development of barriers to hybridisation, as individuals that avoided hybrid matings would tend to have more fit offspring, and thus contribute to the reproductive isolation of the two incipient species. This idea came to be known as the [[Reinforcement (speciation)|Wallace effect]],{{sfn|Wallace|1889|pp=174–179, 353}}{{sfn|Slotten|2004|pp=413–415}} later called reinforcement.<ref name="Speciation">{{cite book |last1=Coyne |first1=Jerry |author1-link=Jerry Coyne |last2=Orr |first2=H. Allen | title=Speciation | date=2004 | pages=353–381 | publisher=Sinauer Associates | isbn=978-0-87893-091-3 }}</ref> Wallace had suggested to Darwin that natural selection could play a role in preventing hybridisation in private correspondence as early as 1868, but had not worked it out to this level of detail.{{sfn|Slotten|2004|p=404}} It continues to be a topic of research in evolutionary biology today, with both computer simulation and empirical results supporting its validity.<ref>{{cite journal |last=Ollerton |first=J. |title=Speciation: Flowering time and the Wallace Effect |journal=Heredity |date=September 2005 |volume=95 |issue=3 |pages=181–182 |pmid=16077739 |doi=10.1038/sj.hdy.6800718 |s2cid=13300641}}</ref>

=== Application of theory to humans, and role of teleology in evolution ===
[[File:Wallace chimp.jpg|thumb|left|upright|An illustration from the chapter on the application of [[natural selection]] to humans in Wallace's 1889 book ''[[Darwinism (book)|Darwinism]]'' shows a chimpanzee.|alt=illustration of a chimpanzee from one of Wallace's books]]

In 1864, Wallace published a paper, "The Origin of Human Races and the Antiquity of Man Deduced from the Theory of 'Natural Selection{{' "}}, applying the theory to humankind. Darwin had not yet publicly addressed the subject, although [[Thomas Huxley]] had in ''[[Evidence as to Man's Place in Nature]]''. Wallace explained the apparent stability of the human stock by pointing to the vast gap in cranial capacities between humans and the [[Hominidae|great apes]]. Unlike some other Darwinists, including Darwin himself, he did not "regard modern primitives as almost filling the gap between man and ape".<ref name=eiseley>{{cite book |last=Eiseley |first=Loren |author-link=Loren Eiseley |title=Darwin's Century |publisher=[[Anchor Books]] |year=1958 |pages=305–306}}</ref> He saw the evolution of humans in two stages: achieving a bipedal posture that freed the hands to carry out the dictates of the brain, and the "recognition of the human brain as a totally new factor in the history of life".<ref name=eiseley/> Wallace seems to have been the first evolutionist to see that the human brain effectively made further specialisation of the body unnecessary.<ref name=eiseley/> Wallace wrote the paper for the [[Anthropological Society of London]] to address the debate between the supporters of [[monogenism]], the belief that all human races shared a common ancestor and were one species, and the supporters of [[polygenism]], who held that different races had separate origins and were different species. Wallace's anthropological observations of Native Americans in the Amazon, and especially his time living among the [[Dyaks|Dayak people]] of Borneo, had convinced him that human beings were a single species with a common ancestor. He still felt that natural selection might have continued to act on mental faculties after the development of the different races; and he did not dispute the nearly universal view among European anthropologists of the time that Europeans were intellectually superior to other races.{{sfn|Slotten|2004|pp=207–213}}{{sfn|Shermer|2002|pp=218–221}} According to political scientist [[Adam Jones (Canadian scholar)|Adam Jones]], "Wallace found little difficulty in reconciling the extermination of native peoples with his progressive political views".<ref>{{Cite book |last=Jones |first=Adam |url=https://www.worldcat.org/oclc/672333335 |title=Genocide : a comprehensive introduction |date=2011 |publisher=Routledge |isbn=978-0-203-84696-4 |edition=2nd |location=London |pages=209–210 |oclc=672333335}}</ref> In 1864, in the aforementioned paper, he stated "It is the same great law of the preservation of favored races in the struggle for life, which leads to the inevitable extinction of all those low and mentally undeveloped populations with which Europeans come in contact."<ref>{{Cite journal |last=Wallace |first=Alfred |date=2010-01-01 |title=The Origin of Human Races and the Antiquity of Man Deduced From the Theory of "Natural Selection" (1864) |url=https://digitalcommons.wku.edu/dlps_fac_arw/6 |journal=Alfred Russel Wallace Classic Writings |pages=164}}</ref> He argued that the natives die out due to an unequal struggle.<ref>{{Cite book |last=Brantlinger |first=Patrick |url=https://www.jstor.org/stable/10.7591/j.ctt1287f39 |title=Dark Vanishings: Discourse on the Extinction of Primitive Races, 1800–1930 |date=2003 |publisher=Cornell University Press |isbn=978-0-8014-3809-7 |edition=1st |pages=185–186 |jstor=10.7591/j.ctt1287f39 |quote=...The red Indian in North America and in Brazil; the Tasmanian, Australian, and New Zealander in the southern hemisphere, die out, not from any one special cause, but from the inevitable effects of an unequal mental and physical struggle. The intellectual and moral, as well as the physical qualities of the European are superior; ...}}</ref>

Shortly afterwards, Wallace became a [[Spiritualism (movement)|spiritualist]]. At about the same time, he began to maintain that natural selection could not account for mathematical, artistic, or musical genius, metaphysical musings, or wit and humour. He stated that something in "the unseen universe of Spirit" had interceded at least three times in history: the creation of life from inorganic matter; the introduction of consciousness in the higher animals; and the generation of the higher mental faculties in humankind. He believed that the [[wikt:raison d'être|raison d'être]] of the universe was the development of the human spirit.{{sfn|Wallace|1889|p=477}}

While some historians have concluded that Wallace's belief that natural selection was insufficient to explain the development of consciousness and the higher functions of the human mind was directly caused by his adoption of spiritualism, other scholars have disagreed, and some maintain that Wallace never believed natural selection applied to those areas.{{sfn|Shermer|2002|pp=157–160}}<ref>{{cite web |last=Smith |first=Charles H. |title=Alfred Russel Wallace: Evolution of an Evolutionist Chapter Six. A Change of Mind? |url=http://www.wku.edu/~smithch/wallace/chsarw6.htm |publisher=[[Western Kentucky University]] |access-date=29 April 2007 |archive-date=18 June 2009 |archive-url=https://web.archive.org/web/20090618030426/http://wku.edu/~smithch/wallace/chsarw6.htm |url-status=dead }}</ref> Reaction to Wallace's ideas on this topic among leading naturalists at the time varied. Lyell endorsed Wallace's views on human evolution rather than Darwin's.{{sfn|Larson|2004|p=100}}{{sfn|Shermer|2002|p=160}} Wallace's belief that human consciousness could not be entirely a product of purely material causes was shared by a number of prominent intellectuals in the late 19th and early 20th centuries.{{sfn|Shermer|2002|pp=231–233}} All the same, many, including Huxley, Hooker, and Darwin himself, were critical of Wallace's views.{{sfn|Slotten|2004|pp=280–296}}

As the historian of science and sceptic [[Michael Shermer]] has stated, Wallace's views in this area were at odds with two major tenets of the emerging Darwinian philosophy. These were that evolution was not [[teleological]] (purpose-driven), and that it was not [[anthropocentric]] (human-centred).{{sfn|Shermer|2002|pp=208–209}} Much later in his life Wallace returned to these themes, that evolution suggested that the universe might have a purpose, and that certain aspects of living organisms might not be explainable in terms of purely materialistic processes. He set out his ideas in a 1909 magazine article entitled ''The World of Life'', later expanded into a book of the same name.<ref>{{cite web |url=https://people.wku.edu/charles.smith/wallace/S669.htm |title=The World of Life: As Visualised and Interpreted by Darwinism (S669: 1909) |last=Wallace |first=Alfred Russel |publisher=[[Western Kentucky University]] |access-date=25 May 2022}}</ref> Shermer commented that this anticipated ideas about design in nature and directed evolution that would arise from religious traditions throughout the 20th century.{{sfn|Shermer|2002|pp=231–233}}

=== Assessment of Wallace's role in history of evolutionary theory ===
{{further|History of evolutionary thought}}

In many accounts of the development of evolutionary theory, Wallace is mentioned only in passing as simply being the stimulus to the publication of Darwin's own theory.{{sfn|Slotten|2004|p=6}} In reality, Wallace developed his own distinct evolutionary views which diverged from Darwin's, and was considered by many (especially Darwin) to be a leading thinker on evolution in his day, whose ideas could not be ignored. One historian of science has pointed out that, through both private correspondence and published works, Darwin and Wallace exchanged knowledge and stimulated each other's ideas and theories over an extended period.{{sfn|Shermer|2002|p=149}} Wallace is the most-cited naturalist in Darwin's ''[[Descent of Man]]'', occasionally in strong disagreement.{{sfn|Slotten|2004|pp=289–290}} Darwin and Wallace agreed on the importance of natural selection, and some of the factors responsible for it: competition between species and geographical isolation. But Wallace believed that evolution had a purpose ("teleology") in maintaining species' fitness to their environment, whereas Darwin hesitated to attribute any purpose to a random natural process. Scientific discoveries since the 19th century support Darwin's viewpoint, by identifying additional mechanisms and triggers such as mutations triggered by environmental radiation or mutagenic chemicals.<ref name="Hamilton 2008">{{cite magazine  |last=Hamilton |first=Garry |title=Viruses: The unsung heroes of evolution |magazine=[[New Scientist]] |date=27 August 2008 |url=https://www.newscientist.com/article/mg19926711-600-viruses-the-unsung-heroes-of-evolution}}</ref> Wallace remained an ardent defender of natural selection for the rest of his life. By the 1880s, evolution was widely accepted in scientific circles, but natural selection less so. Wallace's 1889 ''Darwinism'' was a response to the scientific critics of natural selection.{{sfn|Slotten|2004|p=409}} Of all Wallace's books, it is the most cited by scholarly publications.{{sfn|Shermer|2002|p=18}}