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WIKIMAG n. 7 - Giugno 2013
The Origin of Species
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On the Origin of Species, published on 24 November 1859, is a
work of
scientific literature by
Charles Darwin which is considered to be the foundation of
evolutionary biology. Its full title was On the Origin of Species by
Means of Natural Selection, or the Preservation of Favoured Races in the
Struggle for Life. For the sixth edition of 1872, the short title was
changed to The Origin of Species. Darwin's book introduced the
scientific theory that populations
evolve
over the course of generations through a process of
natural selection. It presented a body of evidence that
the
diversity of life arose by
common descent through a
branching pattern of evolution. Darwin included evidence that he had
gathered on
the Beagle expedition in the 1830s and his subsequent findings from
research, correspondence, and experimentation.
Various evolutionary ideas had already been proposed to explain
new findings in biology. There was growing support for such ideas among
dissident anatomists and the general public, but during the first half of the
19th century the English scientific establishment was closely tied to the
Church of England, while science was part of
natural theology. Ideas about the
transmutation of species were controversial as they conflicted with the
beliefs that species were unchanging parts of a designed hierarchy and that
humans were unique, unrelated to other animals. The political and theological
implications were intensely debated, but transmutation was not accepted by the
scientific mainstream.
The book was written for non-specialist readers and attracted widespread
interest upon its publication. As Darwin was an eminent scientist, his findings
were taken seriously and the evidence he presented generated scientific,
philosophical, and religious discussion. The debate over the book contributed to
the campaign by
T.H. Huxley and his fellow members of the
X Club to
secularise
science by promoting
scientific naturalism. Within two decades there was widespread scientific
agreement that evolution, with a branching pattern of common descent, had
occurred, but scientists were slow to give natural selection the significance
that Darwin thought appropriate. During the "eclipse
of Darwinism" from the 1880s to the 1930s, various other mechanisms of
evolution were given more credit. With the development of the
modern evolutionary synthesis in the 1930s and 1940s, Darwin's concept of
evolutionary
adaptation
through natural selection became central to modern evolutionary theory, now the
unifying concept of the
life sciences.
Summary of
Darwin's theory
Darwin pictured shortly before publication
Darwin's theory of evolution is based on key
facts and the
inferences
drawn from them, which biologist
Ernst Mayr
summarised as follows:[3]
-
- Every
species is fertile enough that if all offspring survived to
reproduce the population would grow (fact).
- Despite periodic fluctuations, populations remain roughly the same
size (fact).
- Resources such as food are limited and are relatively stable over
time (fact).
- A struggle for survival ensues (inference).
- Individuals in a population vary significantly from one another
(fact).
- Much of this variation is
inheritable (fact).
- Individuals less suited to the environment are less likely to
survive and less likely to reproduce; individuals more suited to the
environment are more likely to survive and more likely to reproduce and
leave their inheritable traits to future generations, which produces the
process of
natural selection (inference).
- This slowly effected process results in populations changing to
adapt to their environments, and ultimately, these variations accumulate
over time to form new species (inference).
Background
Developments before Darwin's theory
In later editions of the book, Darwin traced evolutionary ideas as far back
as Aristotle;[4]
the text he cites is a summary by Aristotle of the ideas of the earlier Greek
philosopher
Empedocles.[5]
Early
Christian
Church Fathers and
Medieval European scholars interpreted the
Genesis creation narrative allegorically rather than as a literal historical
account;[6]
organisms were described by their mythological and heraldic significance as well
as by their physical form. Nature was widely believed to be unstable and
capricious, with monstrous births from union between species, and
spontaneous generation of life.[7]
Cuvier's 1799 paper on living and fossil elephants helped establish
the reality of
extinction.
The
Protestant Reformation inspired
a literal interpretation of the Bible, with concepts of creation that
conflicted with the findings of an
emerging science seeking explanations congruent with the
mechanical philosophy of
René Descartes and the
empiricism
of the
Baconian method. After the turmoil of the
English Civil War, the
Royal
Society wanted to show that science did not threaten religious and political
stability. John
Ray developed an influential
natural theology of rational order; in his
taxonomy,
species were static and fixed, their adaptation and complexity designed by
God, and
varieties showed minor differences caused by local conditions. In God's
benevolent design, carnivores caused mercifully swift death, but the suffering
caused by
parasitism was a
puzzling problem. The
biological classification introduced by
Carolus Linnaeus in 1735 also viewed species as fixed according to the
divine plan. In 1766,
Georges Buffon suggested that some similar species, such as horses and
asses, or lions, tigers, and leopards, might be varieties descended from a
common ancestor. The
Ussher chronology of the 1650s had calculated creation at 4004 BC, but by
the 1780s geologists assumed a much older world.
Wernerians thought
strata were
deposits from
shrinking seas, but
James
Hutton proposed a self-maintaining infinite cycle, anticipating
uniformitarianism.[8]
Charles Darwin's grandfather
Erasmus Darwin outlined a hypothesis of
transmutation of species in the 1790s, and
Jean-Baptiste Lamarck published a more developed theory in 1809. Both
envisaged that
spontaneous generation produced simple forms of life that progressively
developed greater complexity, adapting to the environment by inheriting changes
in adults caused by use or disuse. This process was later called
Lamarckism.
Lamarck thought there was an inherent progressive tendency driving organisms
continuously towards greater complexity, in parallel but separate lineages with
no extinction.[9]
Geoffroy contended that
embryonic development recapitulated transformations of organisms in past
eras when the
environment acted on embryos, and that animal structures were determined by a
constant plan as demonstrated by
homologies.
Georges Cuvier strongly disputed such ideas, holding that unrelated, fixed
species showed similarities that reflected a design for functional needs.[10]
His
palæontological work in the 1790s had established the reality of extinction,
which he explained by local
catastrophes, followed by repopulation of the affected areas by other
species.[11]
In Britain,
William Paley's Natural Theology saw adaptation as evidence of
beneficial
"design" by the Creator acting through natural laws. All naturalists in
English universities were
Church of England clergymen, and science became a search for these laws.[12]
Geologists adapted catastrophism to show repeated worldwide annihilation and
creation of new fixed species adapted to a changed environment, initially
identifying the most recent catastrophe as the
biblical flood.[13]
Some anatomists such as
Robert Grant were influenced by Lamarck and Geoffroy, but most naturalists
regarded their ideas of transmutation as a threat to divinely appointed social
order.[14]
Inception of
Darwin's theory
Darwin went to
Edinburgh University in 1825 to study medicine. In his second year he
neglected his medical studies for
natural history and spent four months assisting
Robert Grant's research into
marine invertebrates. Grant revealed his enthusiasm for the transmutation of
species, but Darwin rejected it.[15]
At
Cambridge University starting in 1827, Darwin learnt science as
natural theology from botanist
John Stevens Henslow, and read
Paley,
John
Herschel and
Alexander von Humboldt. Filled with zeal for science, he studied
catastrophist
geology with
Adam
Sedgwick.[16][17]
In mid-July 1837 Darwin started his "B" notebook on Transmutation
of Species, and on page 36 wrote "I think" above his first
evolutionary tree.
In December 1831, he joined the
Beagle expedition as a geologist and naturalist. He read
Charles Lyell's
Principles of Geology and from the first stop ashore, at
St. Jago, found
Lyell's uniformitarianism a key to the geological history of landscapes.
Darwin discovered fossils resembling
huge
armadillos, and noted the geographical distribution of modern species in
hope of finding their "centre of creation".[18]
The three
Fuegian missionaries the expedition returned to
Tierra del Fuego were friendly and civilised, yet to Darwin their relatives
on the island seemed "miserable, degraded savages",[19]
and he no longer saw an unbridgeable gap between humans and animals.[20]
As the Beagle neared England in 1836, he noted that species might not be
fixed.[21]
Richard Owen showed that fossils of extinct species Darwin found in South
America were allied to living species on the same continent. In March 1837,
ornithologist
John Gould
announced that
Darwin's Rhea was a separate species from the previously described
rhea
(though their territories overlapped), that
mockingbirds collected on the
Galápagos Islands represented three separate species each
unique to a
particular island, and that several distinct birds from those islands were all
classified as
finches.[22]
Darwin began speculating, in a series of notebooks, on the possibility that "one
species does change into another" to explain these findings, and around July
sketched a
genealogical branching of a single
evolutionary tree, discarding Lamarck's independent
lineages progressing to higher forms.[23]
Unconventionally, Darwin asked questions of
fancy
pigeon and
animal breeders as well as established scientists. At the zoo he had his
first sight of an ape, and was profoundly impressed by how human the
orangutan
seemed.[24]
In late September 1838, he started reading
Thomas Malthus's
An Essay on the Principle of Population with its statistical argument
that human populations, if unrestrained, breed beyond their means and struggle
to survive. Darwin related this to the struggle for existence among wildlife and
botanist
de Candolle's "warring of the species" in plants; he immediately envisioned
"a force like a hundred thousand wedges" pushing well-adapted variations into
"gaps in the economy of nature", so that the survivors would pass on their form
and abilities, and unfavourable variations would be destroyed.[25][26]
By December 1838, he had noted a similarity between the act of breeders
selecting traits and a Malthusian Nature selecting among variants thrown up by
"chance" so that "every part of newly acquired structure is fully practical and
perfected".[27]
Darwin now had the framework of his theory of natural selection "by which to
work",[28]
but he was fully occupied with his career as a geologist and held off writing a
sketch of his theory until his book on
The Structure and Distribution of Coral Reefs was completed in May 1842.[29][30]
Further development
Darwin continued to research and extensively revise his theory while focusing
on his main work of publishing the scientific results of the Beagle
voyage.[29]
He tentatively wrote of his ideas to Lyell in January 1842;[31]
then in June he roughed out a 35-page "Pencil Sketch" of his theory.[32]
Darwin began correspondence about his theorising with the botanist
Joseph Dalton Hooker in January 1844, and by July had rounded out his
"sketch" into a 230-page "Essay", to be expanded with his research results and
published if he died prematurely.[33]
Darwin researched how the skulls of different pigeon breeds varied,
as shown in his Variation of Plants and Animals Under
Domestication of 1868.
In November 1844, the anonymously published
popular science book
Vestiges of the Natural History of Creation, written by Scottish
journalist
Robert Chambers, widened public interest in the concept of transmutation of
species. Vestiges used evidence from the fossil record and embryology to
support the claim that living things had progressed from the simple to the more
complex over time. But it proposed a linear progression rather than the
branching common descent theory behind Darwin's work in progress, and it ignored
adaptation.
Darwin read it soon after publication, and scorned its amateurish geology and
zoology,[34]
but he carefully reviewed his own arguments after leading scientists, including
Adam Sedgwick, attacked its morality and scientific errors.[35]
Vestiges had significant influence on public opinion, and the intense
debate helped to pave the way for the acceptance of the more scientifically
sophisticated Origin by moving evolutionary speculation into the
mainstream. While few naturalists were willing to consider transmutation,
Herbert Spencer became an active proponent of Lamarckism and progressive
development in the 1850s.[36]
Hooker was persuaded to take away a copy of the "Essay" in January 1847, and
eventually sent a page of notes giving Darwin much needed feedback. Reminded of
his lack of expertise in
taxonomy, Darwin began an eight-year study of
barnacles,
becoming the leading expert on their classification. Using his theory, he
discovered
homologies showing that slightly changed body parts served different
functions to meet new conditions, and he found an
intermediate stage in the evolution of
distinct
sexes.[37]
Darwin's barnacle studies convinced him that variation arose constantly and
not just in response to changed circumstances. In 1854, he completed the last
part of his Beagle-related writing and began working full-time on
evolution. His thinking changed from the view that
species formed in isolated populations only, as on islands, to an emphasis
on
speciation without isolation; that is, he saw increasing specialisation
within large stable populations as continuously exploiting new
ecological niches. He conducted empirical research focusing on difficulties
with his theory. He studied the developmental and anatomical differences between
different breeds of many domestic animals, became actively involved in
fancy
pigeon breeding, and experimented (with the help of his son
Francis) on ways that plant seeds and animals might disperse across oceans
to colonise distant islands. By 1856, his theory was much more sophisticated,
with a mass of supporting evidence.[37][38]
Publication
Events leading
to publication
A photograph of Alfred Russel Wallace (1823–1913) taken in
Singapore in 1862
An 1855 paper on the "introduction" of species, written by
Alfred Russel Wallace, claimed that patterns in the geographical
distribution of living and fossil species could be explained if every new
species always came into existence near an already existing, closely related
species.[39]
Charles Lyell recognised the implications of Wallace's paper and its
possible connection to Darwin's work, although Darwin did not, and in the spring
of 1856 Lyell urged Darwin to publish his theory to establish priority. Darwin
was torn between the desire to set out a full and convincing account and the
pressure to quickly produce a short paper. He decided he did not want to expose
his ideas to review by an editor as 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.[40]
Darwin was hard at work on his "big book" on
Natural Selection, when on 18 June 1858 he received a parcel from
Wallace, who stayed on the
Maluku Islands (Ternate
and Gilolo). It enclosed twenty pages describing an evolutionary mechanism, a
response to Darwin's recent encouragement, with a request to send it on to Lyell
if Darwin thought it worthwhile. The mechanism was similar to Darwin's own
theory.[40]
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".[41]
Lyell and Hooker agreed that a joint paper should be presented at the
Linnean Society, and on 1 July 1858, the papers entitled
On the Tendency of Species to form Varieties; and on the Perpetuation of
Varieties and Species by Natural Means of Selection, by Wallace and
Darwin respectively, were read out but drew little reaction. While Darwin
considered Wallace's idea to be identical to his concept of natural selection,
historians have pointed out differences. Darwin described natural selection as
being analogous to the
artificial selection practised by animal breeders, and emphasised
competition between individuals; Wallace drew no comparison to
selective breeding, and focused on ecological pressures that kept different
varieties adapted to local conditions.[42][43][44]
After the meeting, Darwin decided to write "an abstract of my whole work".[45]
He started work on 20 July 1858, while on holiday at
Sandown,[46]
and wrote parts of it from memory.[47]
Lyell discussed arrangements with publisher
John Murray III, of the publishing house John Murray,[48]
who responded immediately to Darwin's letter of 31 March 1859[49]
with an agreement to publish the book without even seeing the manuscript, and an
offer to Darwin of 2⁄3 of
the profits.[50]
(eventually Murray paid £180 to Darwin for the 1st edition and by Darwin's death
in 1882 the book was in its 6th edition, earning Darwin nearly £3000.[51])
Darwin had initially decided to call it 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.[1]
Here the term "races"
is used as an alternative for "varieties"
and does not carry the modern connotation of
human races—the first use in the book refers to "the several races, for
instance, of the cabbage" and proceeds to a discussion of "the hereditary
varieties or races of our domestic animals and plants".[52]
Time taken to publish
Darwin had his basic theory of natural selection "by which to work" by
December 1838, yet almost twenty years later, when Wallace's letter arrived on
18 June 1858, Darwin was still not ready to publish his theory. It was long
thought that Darwin avoided or delayed making his ideas public for personal
reasons. Reasons suggested have included fear of religious persecution or social
disgrace if his views were revealed, and concern about upsetting his clergymen
naturalist friends or his pious wife Emma.
Charles Darwin's illness caused repeated delays. His paper on
Glen Roy
had proved embarrassingly wrong, and he may have wanted to be sure he was
correct.
David
Quammen has suggested all these factors may have contributed, and notes
Darwin's large output of books and busy family life during that time.[53]
A more recent study by science historian
John
van Wyhe has determined that the idea that Darwin delayed publication only
dates back to the 1940s, and Darwin's contemporaries thought the time he took
was reasonable. Darwin always finished one book before starting another. While
he was researching, he told many people about his interest in transmutation
without causing outrage. He firmly intended to publish, but it was not until
September 1854 that he could work on it full-time. His estimate that writing his
"big book" would take five years was optimistic.[54]
Publication and subsequent editions
On the Origin of Species was first published on Thursday 24 November
1859, priced at fifteen
shillings.
The book had been offered to booksellers at Murray's autumn sale on Tuesday 22
November, and all available copies had been taken up immediately. In total,
1,250 copies were printed but after deducting presentation and review copies,
and five for
Stationers' Hall copyright, around 1,170 copies were available for sale.[1]
Significantly, 500 were taken by
Mudie's Library, ensuring that the book promptly reached a large number of
subscribers to the library.[55]
The second edition of 3,000 copies was quickly brought out on 7 January 1860,[56]
and incorporated numerous corrections as well as a response to religious
objections by the addition of a new epigraph on page ii, a quotation from
Charles Kingsley, and the phrase "by the Creator" amended to the closing
sentence.[57]
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, with a number of sentences rewritten or added and an
introductory appendix, An Historical Sketch of the Recent Progress of Opinion
on the Origin of Species,[58]
while the fourth in 1866 had further revisions. The fifth edition, published on
10 February 1869, incorporated more changes and for the first time included the
phrase "survival
of the fittest", which had been coined by the philosopher
Herbert Spencer in his Principles of Biology (1864).[59]
In January 1871,
George Jackson Mivart's On the Genesis of Species listed detailed
arguments against natural selection, and claimed it included false
metaphysics.[60]
Darwin made extensive revisions to the sixth edition of the Origin (this
was the first edition in which he used the word "evolution"
which had commonly been associated with
embryological development, though all editions concluded with the word
"evolved"[61]),
and added a new chapter VII, Miscellaneous objections, to address
Mivart's arguments.[62]
The sixth edition was published by Murray on 19 February 1872 with "On" dropped
from the title. Darwin had told Murray of working men in
Lancashire
clubbing together to buy the 5th edition at fifteen shillings and wanted it made
more widely available; the price was halved to 7s
6d
by printing in a smaller
font. It includes a
glossary compiled by W.S. Dallas. Book sales increased from 60 to 250 per month.[62]
Publication
outside Great Britain
American botanist Asa Gray (1810–1888)
In the United States,
Asa Gray
negotiated with a Boston publisher for publication of an authorised American
version, but learnt that two New York publishing firms were already planning to
exploit the absence of international
copyright
to print Origin.[63]
Darwin was delighted by the popularity of the book, and asked Gray to keep any
profits.[64]
Gray managed to negotiate a 5% royalty with
Appleton's of New York,[65]
who got their edition out in mid January 1860, and the other two withdrew. In a
May letter, Darwin mentioned a print run of 2,500 copies, but it is not clear if
this referred to the first printing only as there were four that year.[1][66]
The book was widely translated in Darwin's lifetime, but problems arose with
translating concepts and metaphors, and some translations were biased by the
translator's own agenda.[67]
Darwin distributed presentation copies in France and Germany, hoping that
suitable applicants would come forward, as translators were expected to make
their own arrangements with a local publisher. He welcomed the distinguished
elderly naturalist and geologist
Heinrich Georg Bronn, but the German translation published in 1860 imposed
Bronn's own ideas, adding controversial themes that Darwin had deliberately
omitted. Bronn translated "favoured races" as "perfected races", and added
essays on issues including the origin of life, as well as a final chapter on
religious implications partly inspired by Bronn's adherence to
Naturphilosophie.[68]
In 1862, Bronn produced a second edition based on the third English edition and
Darwin's suggested additions, but then died of a heart attack.[69]
Darwin corresponded closely with
Julius Victor Carus, who published an improved translation in 1867.[70]
Darwin's attempts to find a translator in France fell through, and the
translation by
Clémence Royer published in 1862 added an introduction praising Darwin's
ideas as an alternative to religious revelation and promoting ideas anticipating
social Darwinism and
eugenics,
as well as numerous explanatory notes giving her own answers to doubts that
Darwin expressed. Darwin corresponded with Royer about a second edition
published in 1866 and a third in 1870, but he had difficulty getting her to
remove her notes and was troubled by these editions.[69][71]
He remained unsatisfied until a translation by Edmond Barbier was published in
1876.[1]
A Dutch translation by
Tiberius Cornelis Winkler was published in 1860.[72]
By 1864, additional translations had appeared in Italian and Russian.[67]
In Darwin's lifetime, Origin was published in Swedish in 1871,[73]
Danish in 1872, Polish in 1873, Hungarian in 1873–1874, Spanish in 1877 and
Serbian in 1878. By 1977, it had appeared in an additional 18 languages.[74]
Content
Title pages and
introduction
John Gould's illustration of
Darwin's Rhea was published in 1841. The existence of two rhea
species with overlapping ranges influenced Darwin.
Page ii contains quotations by
William Whewell and
Francis Bacon on the theology of
natural
laws,[75]
harmonising science and religion in accordance with
Isaac
Newton's belief in a rational God who established a law-abiding cosmos.[76]
In the second edition, Darwin added an epigraph from
Joseph Butler affirming that God could work through scientific laws as much
as through
miracles, in a nod to the religious concerns of his oldest friends.[57]
The Introduction establishes Darwin's credentials as a naturalist and
author,[77]
then refers to
John
Herschel's letter suggesting that the origin of species "would be found to
be a natural in contradistinction to a miraculous process":[78]
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.[79]
Darwin refers specifically to the distribution of the species
rheas, and to that of the
Galápagos tortoises and
mockingbirds. He mentions his years of work on his theory, and the arrival
of Wallace at the same conclusion, which led him to "publish this Abstract" of
his incomplete work. He outlines his ideas, and sets out the 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.[80]
Starting with the third edition, Darwin prefaced the introduction with a
sketch of the historical development of evolutionary ideas.[81]
In that sketch he acknowledged that
Patrick Matthew had, unknown to Wallace or himself, anticipated the concept
of natural selection in an appendix to a book published in 1831;[82]
in the fourth edition he mentioned that
William Charles Wells had done so as early as 1813.[83]
Variation under domestication and under nature
Chapter I covers
animal husbandry and
plant breeding, going back to
ancient Egypt. Darwin discusses contemporary opinions on the origins of
different breeds under cultivation to argue that many have been produced from
common ancestors by
selective breeding.[84]
As an illustration of
artificial selection, he describes
fancy
pigeon breeding,[85]
noting that "[t]he diversity of the breeds is something astonishing", yet all
were descended from one species of
rock pigeon.[86]
Darwin saw two distinct kinds of variation: (1) rare abrupt changes he called
"sports" or "monstrosities" (example: ancon sheep with short legs), and (2)
ubiquitous small differences (example: slightly shorter or longer bill of
pigeons).[87]
Both types of hereditary changes can be used by breeders. However, for Darwin
the small changes were most important in evolution.
In Chapter II, Darwin specifies that the distinction between
species and
varieties is arbitrary, with experts disagreeing and changing their
decisions when new forms were found. He concludes that "a well-marked variety
may be justly called an incipient species" and that "species are only strongly
marked and permanent varieties".[88]
He argues for the ubiquity of variation in nature.[89]
Historians have noted that naturalists had long been aware that the individuals
of a species differed from one another, but had generally considered such
variations to be limited and unimportant deviations from the
archetype
of each species, that archetype being a fixed ideal in the mind of God. Darwin
and Wallace made variation among individuals of the same species central to
understanding the natural world.[85]
Struggle for existence, natural selection, and divergence
In Chapter III, Darwin asks how varieties "which I have called incipient
species" become distinct species, and in answer introduces the key concept he
calls "natural
selection";[90]
in the fifth edition he adds, "But the expression often used by Mr.
Herbert Spencer, of the
Survival of the Fittest, is more accurate, and is sometimes equally
convenient."[91]
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.[90]
He notes that both
A. P. de Candolle and
Charles Lyell had stated that all organisms are exposed to severe
competition. Darwin emphasizes that he used the phrase "struggle for existence"
in "a large and metaphorical sense, including dependence of one being on
another"; he gives examples ranging from plants struggling against drought to
plants competing for birds to eat their fruit and disseminate their seeds. He
describes the struggle resulting from population growth: "It is the doctrine of
Malthus applied with manifold force to the whole animal and vegetable kingdoms."
He discusses checks to such increase including complex
ecological
interdependencies, and notes that competition is most severe between closely
related forms "which fill nearly the same place in the economy of nature".[92]
Chapter IV details 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".[93]
Darwin takes as an example a country where a change in conditions led to
extinction of some species, immigration of others and, where suitable variations
occurred, descendants of some species became adapted to new conditions. He
remarks that the artificial selection practised by animal breeders frequently
produced sharp divergence in character between breeds, and suggests that natural
selection might do the same, saying:
But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently, from the simple circumstance
that the more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better enabled
to seize on many and widely diversified places in the polity of nature, and
so be enabled to increase in numbers.[94]
Historians have remarked that here Darwin anticipated the modern concept of
an
ecological niche.[95]
He did not suggest that every favourable variation must be selected, nor that
the favoured animals were better or higher, but merely more adapted to their
surroundings.
This tree diagram, used to show the divergence of species, is the
only illustration in the Origin of Species.
Darwin proposes
sexual selection, driven by competition between males for mates, to explain
sexually dimorphic features such as lion manes, deer antlers, peacock tails,
bird songs, and the bright plumage of some male birds.[96]
He analysed sexual selection more fully in
The Descent of Man, and Selection in Relation to Sex (1871). Natural
selection was expected to work very slowly in forming new species, but given the
effectiveness of artificial selection, he could "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". Using a
tree diagram and calculations, he indicates the "divergence of character"
from original species into new species and genera. He describes branches falling
off as extinction occurred, while new branches formed in "the great
Tree of life ... with its ever branching and beautiful ramifications".[97]
Variation and heredity
In Darwin's time there was no agreed-upon model of
heredity;[98]
in Chapter I Darwin admitted, "The laws governing inheritance are quite
unknown."[99]
He accepted a version of the
inheritance of acquired characteristics (which after Darwin's death came to
be called
Lamarckism), and Chapter V discusses what he called the effects of use and
disuse; he wrote that he thought "there can be little doubt that use in our
domestic animals strengthens and enlarges certain parts, and disuse diminishes
them; and that such modifications are inherited", and that this also applied in
nature.[100]
Darwin stated that some changes that were commonly attributed to use and disuse,
such as the loss of functional wings in some island dwelling insects, might be
produced by natural selection. In later editions of Origin, Darwin
expanded the role attributed to the inheritance of acquired characteristics.
Darwin also admitted ignorance of the source of inheritable variations, but
speculated they might be produced by environmental factors.[101][102]
However, one thing was clear: whatever the exact nature and causes of new
variations, Darwin knew from observation and experiment that breeders were able
to select such variations and produce huge differences in many generations of
selection.[87]
The observation that selection works in domestic animals is not destroyed
by lack of understanding of the underlying hereditary mechanism.
Breeding of animals and plants showed related varieties varying in similar
ways, or tending to revert to an ancestral form, and similar patterns of
variation in distinct species were explained by Darwin as demonstrating
common descent. He recounted how
Lord Morton's mare apparently demonstrated
telegony, offspring inheriting characteristics of a previous mate of the
female parent, and accepted this process as increasing the variation available
for natural selection.[103]
More detail was given in Darwin's 1868 book on
The Variation of Animals and Plants under Domestication, which tried to
explain heredity through his hypothesis of
pangenesis.
Although Darwin had privately questioned
blending inheritance, he struggled with the theoretical difficulty that
novel individual variations would tend to blend into a population. However,
inherited variation could be seen,[104]
and Darwin's concept of selection working on a population with a range of small
variations was workable.[105]
It was not until the
modern evolutionary synthesis in the 1930s and 1940s that a model of
heredity became completely integrated with a model of variation.[106]
Difficulties for
the theory
Chapter VI begins by saying the next three chapters will address possible
objections to the theory, the first being that often no intermediate forms
between closely related species are found, though the theory implies such forms
must have existed. Darwin attributed this to the competition between different
forms, combined with the small number of individuals of intermediate forms,
often leading to extinction of such forms.[107]
The rest of the chapter deals with whether natural selection could produce
complex specialised structures, and the behaviours to use them, when it would be
difficult to imagine how intermediate forms could be functional. Darwin said:
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modification of
some animal with wholly different habits? Can we believe that natural
selection could produce, on the one hand, organs of trifling importance,
such as the tail of a giraffe, which serves as a fly-flapper, and, on the
other hand, organs of such wonderful structure, as the eye, of which we
hardly as yet fully understand the inimitable perfection?[108]
His answer was that in many cases animals exist with intermediate structures
that are functional. He presented
flying squirrels, and
flying lemurs as examples of how bats might have evolved from non-flying
ancestors.[109]
He discussed various simple eyes found in invertebrates, starting with nothing
more than an optic nerve coated with pigment, as examples of how the
vertebrate eye could have evolved. Darwin concludes: "If it could be
demonstrated that any complex organ existed, which could not possibly have been
formed by numerous, successive, slight modifications, my theory would absolutely
break down. But I can find out no such case."[110]
Chapter VII (of the first edition) addresses the evolution of instincts. His
examples included two he had investigated experimentally: slave-making ants and
the construction of hexagonal cells by honey bees. Darwin noted that some
species of slave-making ants were more dependent on slaves than others, and he
observed that many ant species will collect and store the pupae of other species
as food. He thought it reasonable that species with an extreme dependency on
slave workers had evolved in incremental steps. He suggested that bees that make
hexagonal cells evolved in steps from bees that made round cells, under pressure
from natural selection to economise wax. Darwin concluded:
Finally, it may not be a logical deduction, but to my imagination it is
far more satisfactory to look at such instincts as the young cuckoo ejecting
its foster-brothers, —ants making slaves, —the larvæ of ichneumonidæ feeding
within the live bodies of caterpillars, —not as specially endowed or created
instincts, but as small consequences of one general law, leading to the
advancement of all organic beings, namely, multiply, vary, let the strongest
live and the weakest die.[111]
Chapter VIII addresses the idea that species had special characteristics that
prevented hybrids from being fertile in order to preserve separately created
species. Darwin said that, far from being constant, the difficulty in producing
hybrids of related species, and the viability and fertility of the hybrids,
varied greatly, especially among plants. Sometimes what were widely considered
to be separate species produced fertile hybrid offspring freely, and in other
cases what were considered to be mere varieties of the same species could only
be crossed with difficulty. Darwin concluded: "Finally, then, the facts briefly
given in this chapter do not seem to me opposed to, but even rather to support
the view, that there is no fundamental distinction between species and
varieties."[112]
In the sixth edition Darwin inserted a new chapter VII (renumbering the
subsequent chapters) to respond to criticisms of earlier editions, including the
objection that many features of organisms were not adaptive and could not have
been produced by natural selection. He said some such features could have been
by-products of adaptive changes to other features, and that often features
seemed non-adaptive because their function was unknown, as shown by his book on
Fertilisation of Orchids that explained how their elaborate structures
facilitated pollination by insects. Much of the chapter responds to
George Jackson Mivart's criticisms, including his claim that features such
as baleen
filters in whales,
flatfish
with both eyes on one side and the camouflage of
stick
insects could not have evolved through natural selection because
intermediate stages would not have been adaptive. Darwin proposed scenarios for
the incremental evolution of each feature.[113]
Geologic record
Chapter IX deals with the fact that the
geologic record appears to show forms of life suddenly arising, without the
innumerable
transitional fossils expected from gradual changes. Darwin borrowed
Charles Lyell's argument in
Principles of Geology that the record is extremely imperfect as
fossilisation
is a very rare occurrence, spread over vast periods of time; since few areas had
been geologically explored, there could only be fragmentary knowledge of
geological formations, and fossil collections were very poor. Evolved local
varieties which migrated into a wider area would seem to be the sudden
appearance of a new species. Darwin did not expect to be able to reconstruct
evolutionary history, but continuing discoveries gave him well founded hope that
new finds would occasionally reveal transitional forms.[114][115]
To show that there had been enough time for natural selection to work slowly, he
again cited Principles of Geology and other observations based on
sedimentation and erosion, including an estimate that erosion of
The Weald had taken 300 million years.[116]
The initial appearance of entire groups of well developed organisms in the
oldest fossil-bearing layers, now known as the
Cambrian explosion, posed a problem. Darwin had no doubt that earlier seas
had swarmed with living creatures, but stated that he had no satisfactory
explanation for the lack of fossils.[117]
Fossil evidence of
pre-Cambrian life has since been found, extending the history of life back
for billions of years.[118]
Chapter X examines whether patterns in the fossil record are better explained
by common descent and branching evolution through natural selection, than by the
individual creation of fixed species. Darwin expected species to change slowly,
but not at
the same rate – some organisms such as
Lingula were unchanged since the earliest fossils. The pace of natural
selection would depend on variability and change in the environment.[119]
This distanced his theory from
Lamarckian
laws of inevitable progress.[114]
It has been argued that this anticipated the
punctuated equilibrium hypothesis,[115][120]
but other scholars have preferred to emphasise Darwin's commitment to
gradualism.[121]
He cited
Richard
Owen's findings that the earliest members of a class were a few simple and
generalised species with characteristics intermediate between modern forms, and
were followed by increasingly diverse and specialised forms, matching the
branching of common descent from an ancestor.[114]
Patterns of extinction matched his theory, with related groups of species having
a continued existence until extinction, then not reappearing. Recently extinct
species were more similar to living species than those from earlier eras, and as
he had seen in South America, and
William Clift had shown in Australia, fossils from recent geological periods
resembled species still living in the same area.[119]
Geographic
distribution
Chapter XI deals with evidence from
biogeography, starting with the observation that differences in flora and
fauna from separate regions cannot be explained by environmental differences
alone; South America, Africa, and Australia all have regions with similar
climates at similar latitudes, but those regions have very different plants and
animals. The species found in one area of a continent are more closely allied
with species found in other regions of that same continent than to species found
on other continents. Darwin noted that barriers to migration played an important
role in the differences between the species of different regions. The coastal
sea life of the Atlantic and Pacific sides of Central America had almost no
species in common even though the
Isthmus of Panama was only a few miles wide. His explanation was a
combination of migration and descent with modification. He went on to say: "On
this principle of inheritance with modification, we can understand how it is
that sections of genera, whole genera, and even families are confined to the
same areas, as is so commonly and notoriously the case."[122]
Darwin explained how a volcanic island formed a few hundred miles from a
continent might be colonised by a few species from that continent. These species
would become modified over time, but would still be related to species found on
the continent, and Darwin observed that this was a common pattern. Darwin
discussed ways that species could be dispersed across oceans to colonise
islands, many of which he had investigated experimentally.[123]
Chapter XII continues the discussion of biogeography. After a brief
discussion of freshwater species, it returns to oceanic islands and their
peculiarities; for example on some islands roles played by mammals on continents
were played by other animals such as flightless birds or reptiles. The summary
of both chapters says:
... I think all the grand leading facts of geographical distribution are
explicable on the theory of migration (generally of the more dominant forms
of life), together with subsequent modification and the multiplication of
new forms. We can thus understand the high importance of barriers, whether
of land or water, which separate our several zoological and botanical
provinces. We can thus understand the localisation of sub-genera, genera,
and families; and how it is that under different latitudes, for instance in
South America, the inhabitants of the plains and mountains, of the forests,
marshes, and deserts, are in so mysterious a manner linked together by
affinity, and are likewise linked to the extinct beings which formerly
inhabited the same continent ... On these same principles, we can
understand, as I have endeavoured to show, why oceanic islands should have
few inhabitants, but of these a great number should be endemic or
peculiar; ...[124]
Classification, morphology, embryology, rudimentary organs
Chapter XIII starts by observing that classification depends on species being
grouped together in a multilevel system of groups and sub groups based on
varying degrees of resemblance. After discussing classification issues, Darwin
concludes:
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification; that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, and, in
so far, all true classification is genealogical; that community of descent
is the hidden bond which naturalists have been unconsciously seeking, ...[125]
Darwin discusses
morphology, including the importance of
homologous structures. He says, "What can be more curious than that the hand
of a man, formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be constructed
on the same pattern, and should include the same bones, in the same relative
positions?"[126]
He notes that animals of the same class often have extremely similar
embryos.
Darwin discusses rudimentary organs, such as the wings of flightless birds and
the rudiments of pelvis and leg bones found in some snakes. He remarks that some
rudimentary organs, such as teeth in
baleen whales, are found only in embryonic stages.
The final chapter reviews points from earlier chapters, and Darwin concludes
by hoping that his theory might produce revolutionary changes in many fields of
natural history. Although he avoids the controversial topic of human origins in
the rest of the book so as not to prejudice readers against his theory, here he
ventures a cautious hint that psychology would be put on a new foundation and
that "Light will be thrown on the origin of man".[127]
Darwin ends with a passage that became well known and much quoted:
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent on each other in so complex a manner, have all been
produced by laws acting around us ... Thus, from the war of nature, from
famine and death, the most exalted object which we are capable of
conceiving, namely, the production of the higher animals, directly follows.
There is grandeur in this view of life, with its several powers, having been
originally breathed into a few forms or into one; and that, whilst this
planet has gone cycling 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.[128]
Structure and style
Nature and structure of Darwin's argument
Darwin's aims were twofold: to show that species had not been separately
created, and to show that
natural selection had been the chief agent of change.[129]
He knew that his readers were already familiar with the concept of transmutation
of species from
Vestiges, and his introduction ridicules that work as failing to provide
a viable mechanism.[130]
Therefore the first four chapters lay out his case that selection in nature,
caused by the struggle for existence, is analogous to the selection of
variations under domestication, and that the accumulation of adaptive variations
provides a scientifically testable mechanism for evolutionary
speciation.[131][132]
Later chapters provide evidence that evolution has occurred, supporting the
idea of branching, adaptive evolution without directly proving that selection is
the mechanism. Darwin presents supporting facts drawn from many disciplines,
showing that his theory could explain a myriad of observations from many fields
of natural history that were inexplicable under the alternate concept that
species had been individually created.[132][133][134]
The structure of Darwin's argument showed the influence of
John
Herschel, whose philosophy of science maintained that a mechanism could be
called a vera causa (true cause) if three things could be demonstrated:
its existence in nature, its ability to produce the effects of interest, and its
ability to explain a wide range of observations.[135]
Literary style
The Examiner review of 3 December 1859 commented, "Much of Mr.
Darwin's volume is what ordinary readers would call 'tough reading;' that is,
writing which to comprehend requires concentrated attention and some preparation
for the task. All, however, is by no means of this description, and many parts
of the book abound in information, easy to comprehend and both instructive and
entertaining."[130][136]
While the book was readable enough to sell, its dryness ensured that it was
seen as aimed at specialist scientists and could not be dismissed as mere
journalism or imaginative fiction. Unlike the still-popular Vestiges, it
avoided the narrative style of the historical novel and cosmological
speculation, though the closing sentence clearly hinted at cosmic progression.
Darwin had long been immersed in the literary forms and practices of specialist
science, and made effective use of his skills in structuring arguments.[130]
David
Quammen has described the book as written in everyday language for a wide
audience, but noted that Darwin's literary style was uneven: in some places he
used convoluted sentences that are difficult to read; in other places his
writing was beautiful. Quammen advised that later editions were weakened by
Darwin making concessions and adding details to address his critics, and
recommended the first edition.[137]
James T. Costa said that because the book was an abstract produced in haste in
response to Wallace's essay, it was more approachable than the big book on
natural selection Darwin had been working on, which would have been encumbered
by scholarly footnotes and much more technical detail. He added that some parts
of Origin are dense, but other parts are almost lyrical, and the case
studies and observations are presented in a narrative style unusual in serious
scientific books, which broadened its audience.[138]
Reception
British cartoonists presented Darwin's theory in an unthreatening
way. In the 1870s iconic caricatures of Darwin with an
ape or
monkey
body emphasised his significance in transforming ideas, and
contributed to widespread identification of
evolutionism with
Darwinism. [139]
The book aroused international interest[140]
and a widespread debate, with no sharp line between scientific issues and
ideological, social and religious implications.[141]
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,[140]
but had influenced a wide public readership into believing that nature and human
society were governed by natural laws.[25]
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.[142]
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.[141]
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.[143]
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.[144]
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.[145]
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.[146]
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.[147]
Huxley used illustrations to show that humans and apes had the
same basic skeletal structure. [148]
Evolution had less obvious applications to
anatomy and
morphology, and at first had little impact on the research of the anatomist
Thomas Henry Huxley.[149]
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,[150][151]
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.[152]
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.[149][153]
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.[154]
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.[155]
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.[156]
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.[157]
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).[158]
Impact outside
Great Britain
Haeckel showed a main trunk leading to mankind with minor branches
to various animals, unlike Darwin's branching evolutionary tree. [159]
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.[68][160]
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.[159]
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.[160]
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.[71]
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.[67]
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.[60][161]
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.[162][163]
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.[164]
Religious attitudes
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,[165]
but defence of the
argument from design and
natural theology was central to debates over the book in the English
speaking world.[166][167]
The liberal theologian
Baden Powell defended evolutionary ideas by arguing that the
introduction of new species should be considered a natural rather
than a miraculous process. [168]
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.[165]
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".[169][170]
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".[171]
While some commentators have taken this as a concession to religion that Darwin
later regretted,[57]
Darwin's view at the time was of God creating life through the laws of nature,[172][173]
and even in the first edition there are several references to "creation".[174]
Baden Powell praised "Mr Darwin's masterly volume [supporting] the grand
principle of the self-evolving powers of nature".[175]
In America, Asa
Gray argued that evolution is the secondary effect, or modus operandi,
of the first cause, design,[176]
and published a pamphlet defending the book in terms of
theistic evolution, Natural Selection is not inconsistent with Natural
Theology.[169][177][178]
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.[165]
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.[165]
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.[179][180]
Asa Gray responded that this charge misrepresented Darwin's text.[181]
By the early 20th century, four noted authors of
The Fundamentals were explicitly open to the possibility that God
created through evolution,[182]
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.[183]
The Vatican
stated its
official position in a
1950 papal encyclical, which held that evolution was not inconsistent with
Catholic teaching.[184][185]
Modern influence
Various alternative evolutionary mechanisms favoured during "the
eclipse of Darwinism" became untenable as more was learned about inheritance
and mutation.
The full significance of
natural selection was at last accepted in the 1930s and 1940s as part of the
modern evolutionary synthesis. During that synthesis biologists and
statisticians, including
R. A. Fisher,
Sewall Wright and
J.B.S. Haldane, merged Darwinian selection with a statistical understanding
of
Mendelian genetics.[163]
Modern
evolutionary theory continues to develop. Darwin's theory of evolution by
natural selection, with its tree-like model of branching
common descent, has become the unifying theory of the
life sciences. The theory explains the diversity of living organisms and
their adaptation to the environment. It makes sense of the
geologic record, biogeography, parallels in
embryonic
development,
biological homologies,
vestigiality,
cladistics,
phylogenetics and other fields, with unrivalled explanatory power; it has
also become essential to applied sciences such as
medicine and agriculture.[186][187]
Despite the scientific consensus, a religion-based political
controversy has developed over how evolution is taught in schools,
especially in the United States.[188]
Interest in Darwin's writings continues, and scholars have generated an
extensive literature, the
Darwin Industry, about his life and work. The text of Origin itself
has been subject to much analysis including a
variorum,
detailing the changes made in every edition, first published in 1959,[189]
and a
concordance, an exhaustive external index published in 1981.[190]
Worldwide commemorations of the 150th anniversary of the publication of On
the Origin of Species and the bicentenary of Darwin's birth were scheduled
for 2009.[191]
They celebrated the ideas which "over the last 150 years have revolutionised our
understanding of nature and our place within it".[192]
See also
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