Darwin discovered fossils resembling huge armadillos , and noted the geographical distribution of modern species in hope of finding their "centre of creation". Richard Owen showed that fossils of extinct species Darwin found in South America were allied to living species on the same continent. At the zoo he had his first sight of an ape, and was profoundly impressed by how human the orangutan seemed. In late September , 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.
Darwin now had the basic framework of his theory of natural selection, but he was fully occupied with his career as a geologist and held back from compiling it until his book on The Structure and Distribution of Coral Reefs was completed. Darwin continued to research and extensively revise his theory while focusing on his main work of publishing the scientific results of the Beagle voyage. In November , 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, [41] but he carefully reviewed his own arguments after leading scientists, including Adam Sedgwick, attacked its morality and scientific errors.
While few naturalists were willing to consider transmutation, Herbert Spencer became an active proponent of Lamarckism and progressive development in the s. Hooker was persuaded to take away a copy of the "Essay" in January , 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. Darwin's barnacle studies convinced him that variation arose constantly and not just in response to changed circumstances.
In , he completed the last part of his Beagle -related writing and began working full-time on evolution. He now realised that the branching pattern of evolutionary divergence was explained by natural selection working constantly to improve adaptation. 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 , his theory was much more sophisticated, with a mass of supporting evidence.
In his autobiography, Darwin said he had "gained much by my delay in publishing from about , when the theory was clearly conceived, to ; and I lost nothing by it". Various biographers have proposed 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.
A more recent study by science historian John van Wyhe has determined that the idea that Darwin delayed publication only dates back to the s, 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 that he could work on it full-time. His estimate that writing his "big book" would take five years proved optimistic. An 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.
Darwin was torn between the desire to set out a full and convincing account and the pressure to quickly produce a short paper. He met Lyell, and in correspondence with Joseph Dalton Hooker affirmed that he did not want to expose his ideas to review by an editor as would have been required to publish in an academic journal. He began a "sketch" account on 14 May , and by July had decided to produce a full technical treatise on species as his "big book" on Natural Selection.
His theory including the principle of divergence was complete by 5 September when he sent Asa Gray a brief but detailed abstract of his ideas. Darwin was hard at work on the manuscript for his "big book" on Natural Selection , when on 18 June 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. 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.
Soon after the meeting, Darwin decided to write "an abstract of my whole work" in the form of one or more papers to be published by the Linnean Society , but was concerned about "how it can be made scientific for a Journal, without giving facts, which would be impossible. By early October, he began to "expect my abstract will run into a small volume, which will have to be published separately.
By mid March Darwin's abstract had reached the stage where he was thinking of early publication; Lyell suggested the publisher John Murray , and met with him to find if he would be willing to publish. On 28 March Darwin wrote to Lyell asking about progress, and offering to give Murray assurances "that my Book is not more un -orthodox, than the subject makes inevitable.
Murray's response was favourable, and a very pleased Darwin told Lyell on 30 March that he would "send shortly a large bundle of M. He bowed to Murray's objection to "abstract" in the title, though he felt it excused the lack of references, but wanted to keep "natural selection" which was "constantly used in all works on Breeding", and hoped "to retain it with Explanation, somewhat as thus",— Through Natural Selection or the preservation of favoured races.
On 5 April, Darwin sent Murray the first three chapters, and a proposal for the book's title. On the Origin of Species was first published on Thursday 24 November , priced at fifteen shillings with a first printing of copies. In total, 1, copies were printed but after deducting presentation and review copies, and five for Stationers' Hall copyright, around 1, copies were available for sale. The third edition came out in , 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 , [85] while the fourth in had further revisions.
The fifth edition, published on 10 February , 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 In January , George Jackson Mivart 's On the Genesis of Species listed detailed arguments against natural selection, and claimed it included false metaphysics.
The sixth edition was published by Murray on 19 February as The Origin of Species , 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 7 s 6 d by printing in a smaller font. It includes a glossary compiled by W.
Book sales increased from 60 to per month. In the United States, botanist Asa Gray , an American colleague of Darwin, 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. In a May letter, Darwin mentioned a print run of 2, copies, but it is not clear if this referred to the first printing only as there were four that year. 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.
He welcomed the distinguished elderly naturalist and geologist Heinrich Georg Bronn , but the German translation published in 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.
Darwin corresponded with Royer about a second edition published in and a third in , but he had difficulty getting her to remove her notes and was troubled by these editions. By , it had appeared in an additional 18 languages. Page ii contains quotations by William Whewell and Francis Bacon on the theology of natural laws , [] harmonising science and religion in accordance with Isaac Newton 's belief in a rational God who established a law-abiding cosmos. WHEN on board HMS 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.
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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. 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. Starting with the third edition, Darwin prefaced the introduction with a sketch of the historical development of evolutionary ideas. 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.
Ancon sheep with short legs , and 2 ubiquitous small differences example: 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".
Darwin and Wallace made variation among individuals of the same species central to understanding the natural world. 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 "; [] 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.
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.
He notes that both A. 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: Chapter IV details natural selection under the "infinitely complex and close-fitting 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. Historians have remarked that here Darwin anticipated the modern concept of an ecological niche.
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. 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 In Darwin's time there was no agreed-upon model of heredity ; [] in Chapter I Darwin admitted, "The laws governing inheritance are quite unknown. 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.
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.
More detail was given in Darwin's 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, [] and Darwin's concept of selection working on a population with a range of small variations was workable.
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. As Darwin noted, "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?
Another difficulty, related to the first one, is the absence or rarity of transitional varieties in time. Darwin commented that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth?
Why do species exist? The chapter then 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. 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?
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. But I can find out no such case. In a section on "organs of little apparent importance", Darwin discusses the difficulty of explaining various seemingly trivial traits with no evident adaptive function, and outlines some possibilities such as correlation with useful features.
He accepts that we "are profoundly ignorant of the causes producing slight and unimportant variations" which distinguish domesticated breeds of animals, [] and human races.
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He suggests that sexual selection might explain these variations: I might have adduced for this same purpose the differences between the races of man, which are so strongly marked; I may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious details my reasoning would appear frivolous. Chapter VII of the first edition addresses the evolution of instincts. His examples included two he had investigated experimentally: 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. 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.
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. Chapter IX deals with the fact that the geological 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. Combining this with an estimate of recent rates of sedimentation and erosion, Darwin calculated that erosion of The Weald had taken around million years. 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.
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. 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.
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.
Neglected fossil evidence
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: 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. 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; Chapter XIII starts by observing that classification depends on species being grouped together in a Taxonomy , a multilevel system of groups and sub groups based on varying degrees of resemblance.
Darwin Online: On the Origin of Species
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, 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? 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 "Recapitulation and Conclusion" reviews points from earlier chapters, and Darwin concludes by hoping that his theory might produce revolutionary changes in many fields of natural history. 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.
As discussed under religious attitudes , Darwin added the phrase "by the Creator" from the second edition onwards, so that the ultimate sentence began "There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one". Darwin's aims were twofold: 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. The Examiner review of 3 December 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. 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.
Quammen advised that later editions were weakened by Darwin making concessions and adding details to address his critics, and recommended the first edition. 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.
From his early transmutation notebooks in the late s onwards, Darwin considered human evolution as part of the natural processes he was investigating, [] and rejected divine intervention. In the final chapter of On the Origin of Species , " Recapitulation and Conclusion ", Darwin briefly highlights the human implications of his theory:. In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history. Discussing this in January , Darwin assured Lyell that "by the sentence [Light will be thrown on the origin of man and his history] I show that I believe man is in same predicament with other animals. Some other statements in the book are quietly effective at pointing out the implication that humans are simply another species, evolving through the same processes and principles affecting other organisms.
For example, [] in Chapter III: Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates, [] with races of humans differing over ideas of beauty. A Fragment , he called this effect sexual selection. When Darwin published The Descent of Man, and Selection in Relation to Sex twelve years later, he said that he had not gone into detail on human evolution in the Origin as he thought that would "only add to the prejudices against my views".
He had not completely avoided the topic: It seemed to me sufficient to indicate, in the first edition of my 'Origin of Species,' that by this work 'light would be thrown on the origin of man and his history;' and this implies that man must be included with other organic beings in any general conclusion respecting his manner of appearance on this earth. He also said that he had "merely alluded" in that book to sexual selection differentiating human races. Passages such as the following excerpt from an letter to Joseph Hooker have been of particular importance in the debate on this issue:.
Here, Darwin seems to be suggesting that the dream of determining the necessary and sufficient conditions for membership in the species category is unrealizable. Regardless of whether he did or did not believe in the reality of the species category, remarks such as these are important for our purposes because they provide valuable insight into his views about the process of biological classification. Crucially, however, he did not believe that the process of determining whether a particular specimen should be ranked as a species or a variety is entirely haphazard: Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.
Darwin , pp. That is, when we attempt to determine whether a specimen belongs to a particular species, the judgment we reach will likely be based on morphological comparisons with previously described members of the species. If the investigator believes that the unidentified specimen is sufficiently similar to known varieties, it will be classified as a member of the described species. If it is not deemed sufficiently similar, it will be treated as a new species. With this in mind, we can turn our attention back to transitional forms and, in particular, how Darwin would go about classifying them.
Casually speaking, a transitional or intermediate form is a species in which a quantitative trait of interest represents an approximate morphological median between two other species. If a hypothetical transitional form is labeled B , we should expect it to be similar to a species A in some regards, but in other ways, it should resemble a species C.
This is the way transitional forms were understood during the nineteenth century and, arguably, the same way we tend to conceive them today. The fact that transitional forms should have been coveted by Darwin appears obvious, in part because he held that evolutionary change is a very gradual process. As Eldredge has recently discussed, Darwin believed that species initially form continuous populations.
Over time, however, populations begin to diverge geographically, leading to subpopulations being subjected to different selective pressures. Specialist forms with extreme morphologies are favored over generalist ancestral types, and as time passes, the latter are eventually driven to extinction.
If, as Darwin suggested, evolutionary change occurs in this gradual manner rather than through rapid saltations, the gaps we observe in the fossil record must have formerly been filled by morphological intermediates. Because his detractors repeatedly criticized him for failing to produce evidence that transitional forms once existed, why did he not attempt to rebut these charges by pointing to the existence of an animal such as Archaeopteryx?
One reason is that it would be extremely difficult for someone with his views about classification to declare that a particular specimen is transitional. Darwin's thoughts on intermediate forms are most fully developed in the following selection from the Origin , which deserves to be quoted at length: It is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties; they grant some little variability to each species, but when they meet with a somewhat greater amount of difference between any two forms, they rank both as species, unless they are enabled to connect them together by close intermediate gradations.
And this from the reasons just assigned we can seldom hope to effect in any one geological section. Supposing B and C to be two species, and a third, A, to be found in an underlying bed; even if A were strictly intermediate between B and C, it would simply be ranked as a third and distinct species, unless at the same time it could be most closely connected with either one or both forms by intermediate varieties.
Nor should it be forgotten, as before explained, that A might be the actual progenitor of B and C, and yet might not at all necessarily be strictly intermediate between them in all points of structure. So that we might obtain the parent-species and its several modified descendants from the lower and upper beds of a formation, and unless we obtained numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled to rank them all as distinct species.
According to Darwin, members of any given species vary in certain ways, but the amount of variation we allow within a taxon is finite. Again, this means that if the organism under consideration cannot be connected to previously known varieties, it will be classified as a new species. If we follow Darwin and accept that there is no touchstone standard by which species can be distinguished from varieties, we are forced to maintain that most classifications are made in the manner just described.
However, if systematic analyses are carried out exclusively by means of comparison with known forms, the task of classifying paleontological specimens becomes very difficult. As Darwin mentioned in conjunction with his pessimistic arguments about the geological record, fine gradations of form are not found because a fossilization is extremely rare and b phenomena such as erosion and the movement of the Earth's crust are constantly destroying the geological strata. To understand why the incompleteness of the geological record made the identification of transitional forms difficult for Darwin, it is necessary to note that he regarded well-marked varieties as incipient species.
Given this equivalence principle, the process of determining whether a particular fossil should be classified as a transitional form is similar to the procedure by which a specimen is judged to be a species or a variety. According to Darwin, the transitional designation can only be conferred upon a specimen if it can be connected to two seemingly distinct lineages by means of a series of intermediate varieties.
Given that such varieties are not preserved in the fossil record, he was unable to class Archaeopteryx , Compsognathus , or any of the other fossils that have been discussed as transitional forms. Just as an unidentified specimen of contemporary origin that cannot be linked to known varieties will be classified as a new species, a paleontological specimen that cannot be linked to purported ancestral and descendent types by means of varieties will be treated as a new monotypic lineage. Therefore, it can be said that the incompleteness of the geological record and Darwin's views about species and classification conspired to prevent him from supporting his theory by citing the existence of certain well-known transitional-form candidates.
At this point, enough has been said about Darwin's treatment of the transitional-form candidates see figure 2 for a summary of the relevant events , but one very important question remains unanswered: There is some indication that the positive evidence Darwin desired was a graded succession of forms. Evidence of this type would have done little to appease a persistent critic who relishes in playing the missing-link game, and, of course, Darwin himself did not need to be persuaded of the truth of his theory.
However, such a discovery would have undoubtedly helped to sway objective researchers who had previously been unconvinced by the data and accompanying discussion in the Origin. The fact that Darwin hoped paleontological evidence would be discovered is shown in his praise of the American paleontologist Othniel Marsh's work on horses and Cretaceous toothed birds The former research provided quantitative evidence of the evolutionary development of the horse from Orohippus , the multi-toed mountain horse of the Eocene, to single-toed Quaternary animals that closely resemble the modern domestic horse figure 3.
What made Marsh's work on birds noteworthy is the fact that he managed to acquire the remains of over one hundred animals from two very different genera. Because these animals were toothed, and varied in morphology within and across genera, they provided important new insight into the evolutionary history of the birds, and their relationship to dinosaurs. In a letter to Marsh, Darwin made his thoughts on the importance of these discoveries absolutely clear: America has afforded the best support to the theory of evolution, which has appeared within the last 20 years.
Each equine form Marsh discovered could be quantitatively connected to an ancestral and descendent form with minimal difficulty, and likewise, in conjunction with previously known specimens such as Archaeopteryx , the birds he discovered provided preliminary insight into how the transition from the ancestral dinosaurian to the derived avian form actually occurred. The crucial point is that in both cases, the animals he described graded into ancestral and descendent lineages without a substantial amount of conjecture needed to fill in the gaps.
Image from Marsh's paper on fossil horses of North America, depicting the transition from a multi-toed to single-toed condition. As I mentioned earlier, Huxley eventually suggested that specimens such as Archaeopteryx and Hypsilophodon could be used to support the theory of evolution, but he often seemed hesitant to use isolated forms as evidence in favor of the Darwinian hypothesis. Although his work was well received by prominent evolutionists of the era, Marsh's findings only became widely known in the last quarter of the nineteenth century, and as such, one does not find mention of his studies in the Origin.
However, we have good reason to believe that Darwin regarded Marsh's discoveries as positive evidence that could be used to combat the charge that the theory of evolution was unsupported by the geological record. It seems that Darwin did not rely on fossil evidence to support his theory in the Origin simply because the isolated specimens known at the time were not the type of evidence he sought. Audience members at the Museum of Natural History in Toulouse also provided valuable suggestions.
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