This result damaged Hilbert's programme for foundations of mathematics whereby 'infinitary' theories — such as that of PM — were to be proved consistent from finitary theories, with the aim that those uneasy about 'infinitary methods' could be reassurred that their use should provably not result in the derivation of a contradiction.
On the face of it, this damages the logicist programme also, albeit only for those already doubtful concerning 'infinitary methods'. Some believe programmes derived from logicism remain valid because the incompleteness theorems are proved with logic just like any other theorems. However, that conclusion fails to acknowledge any distinction between theorems of first-order logic and those of higher-order logic. Therefore, the claim that logicism remains a valid programme may commit one to holding that a system of proof based on the existence and properties of the natural numbers is less convincing than one based on some particular formal system.
Logicism — especially through the influence of Frege on Russell and Wittgenstein  and later Dummett — was a significant contributor to the development of analytic philosophy during the twentieth century. Ivor Grattan-Guinness states that the French word 'Logistique' was "introduced by Couturat and others at the International Congress of Philosophy ', and was used by Russell and others from then on, in versions appropriate for various languages.
Apparently the first and only usage by Russell appeared in his Apart from the mis-representation which Russell partly rectified by explaining his own view of the role of arithmetic in mathematics , the passage is notable for the word which he put in quotation marks, but their presence suggests nervousness, and he never used the word again, so that 'logicism' did not emerge until the later s" G-G About the same time as Carnap , but apparently independently, Fraenkel used the word: Carnap used a slightly different word 'Logistik'; Behmann complained about its use in Carnap's manuscript so Carnap proposed the word 'Logizismus', but he finally stuck to his word-choice 'Logistik' G-G Ultimately "the spread was mainly due to Carnap, from onwards.
The overt intent of Logicism is to derive all of mathematics from symbolic logic Frege, Dedekind, Peano, Russell. As contrasted with algebraic logic Boolean logic that employs arithmetic concepts, symbolic logic begins with a very reduced set of marks non-arithmetic symbols , a few "logical" axioms that embody the "laws of thought", and rules of inference that dictate how the marks are to be assembled and manipulated — for instance substitution and modus ponens ie from  A materially implies B and  A, one may derive B.
Logicism also adopts from Frege's groundwork the reduction of natural language statements from "subject predicate" into either propositional "atoms" or the "argument function" of "generalization"—the notions "all", "some", "class" collection, aggregate and "relation". In a logicist derivation of the natural numbers and their properties, no "intuition" of number should "sneak in" either as an axiom or by accident. The goal is to derive all of mathematics, starting with the counting numbers and then the real numbers, from some chosen "laws of thought" alone, without any tacit hidden assumptions of "before" and "after" or "less" and "more" or to the point: But "It was only [Russell's] Principia Mathematica that full use was made of the new method for actually deriving large parts of mathematics from a very few logical concepts and axioms.
In addition, the young science was enriched by a new instrument, the abstract theory of relations" p. Kleene states it this way: Dedekind and Frege , , were engaged in defining mathematical notions in terms of logical ones, and Peano , — in expressing mathematical theorems in a logical symbolism" p. Frege describes his intent in the Preface to his Begriffsschrift: He started with a consideration of arithmetic: He believed that in the "foundations of the simplest science; viz.
Russell describes his intent in the Preface to his Principles of Mathematics:. The epistemology of Dedekind and Frege is not as well-defined as that of the philosopher Russell, but both seem accepting as a priori the customary "laws of thought" concerning simple propositional statements usually of belief ; these laws would be sufficient in themselves if augmented with theory of classes and relations e.
Dedekind's "free formations of the human mind" rebels against the strictures of Kronecker: Dedekind's argument begins with "1. In what follows I understand by thing every object of our thought"; we humans use symbols to discuss these "things" of our minds; "A thing is completely determined by all that can be affirmed or thought concerning it" p. In a subsequent paragraph Dedekind discusses what a "system S is: Indeed he awaits Kronecker's "publishing his reasons for the necessity or merely the expediency of these limitations" p.
Leopold Kronecker , famous for his assertion that "God made the integers, all else is the work of man"  had his foes, among them Hilbert. Hilbert called Kronecker a " dogmatist , to the extent that he accepts the integer with its essential properties as a dogma and does not look back"  and equated his extreme constructivist stance with that of Brouwer's intuitionism , accusing both of "subjectivism": To found it I do not need God, as does Kronecker. Russell's Realism served him as an antidote to British Idealism ,  with portions borrowed from European Rationalism and British empiricism.
For Russell, tables are real things that exist independent of Russell the observer. Rationalism would contribute the notion of a priori knowledge,  while empiricism would contribute the role of experiential knowledge induction from experience. To say that logic and arithmetic are contributed by us does not account for this" And in this Russell's epistemology seems different from that of Dedekind's belief that "numbers are free creations of the human mind" Dedekind But his epistemology about the innate he prefers the word a priori when applied to logical principles, cf. He would strongly, unambiguously express support for the Platonic "universals" cf.
Where did Russell derive these epistemic notions? He tells us in the Preface to his Principles of Mathematics. Note that he asserts that the belief: In Russell discovered a "vicious circle" Russell's paradox in Frege's Grundgesetze der Arithmetik , derived from Frege's Basic Law V and he was determined not to repeat it in his Principles of Mathematics. In two Appendices added at the last minute he devoted 28 pages to both a detailed analysis of Frege's theory contrasted against his own, and a fix for the paradox. But he was not optimistic about the outcome:.trampolina.pro/components/parametre/zaru-logiciel-despionnage.php
History of science
How did Russell arrive in this situation? But he observes that "when he started on a concrete problem, the objects to be analyzed e. In an observation pertinent to Russell's brand of logicism, Perry remarks that Russell went through three phases of realism -- extreme, moderate and constructive Perry In he was in his extreme phase; by he would be in his moderate phase.
In a few years he would "dispense with physical or material objects as basic bits of the furniture of the world. He would attempt to construct them out of sense-data" in his next book Our knowledge of the External World " Perry The logicism of Frege and Dedekind is similar to that of Russell, but with differences in the particulars see Criticisms, below.
Overall, the logicist derivations of the natural numbers are different from derivations from - for example - Zermelo's axioms for set theory 'Z'. Whereas, in derivations from Z, one definition of "number" uses an axiom of that system — the axiom of pairing - that leads to the definition of "ordered pair" - no overt number axiom exists in the various logicist axiom systems allowing the derivation of the natural numbers. Note that the axioms needed to derive the definition of a number may differ between axiom systems for set theory in any case.
For instance, in ZF and ZFC, the axiom of pairing, and hence ultimately the notion of an ordered pair is derivable from the Axiom of Infinity and the Axiom of Replacement and is required in the definition of the Von Neumann numerals but not the Zermelo numerals , whereas in NFU the Frege numerals may be derived in an analogous way to their derivation in the Grundgesetze.
Rather, logicism begins its construction of the numbers from "primitive propositions" that include "class", "propositional function", and in particular , "relations" of "similarity" "equinumerosity": Kleene observes the following. The importance to the logicist programme of the construction of the natural numbers derives from Russell's contention that "That all traditional pure mathematics can be derived from the natural numbers is a fairly recent discovery, though it had long been suspected" One derivation of the real numbers derives from the theory of Dedekind cuts on the rational numbers, rational numbers in turn being derived from the naturals.
While an example of how this is done is useful, it relies first on the derivation of the natural numbers. So, if philosophical difficulties appear in a logicist derivation of the natural numbers, these problems should be sufficient to stop the program until these are resolved see Criticisms, below. One attempt to construct the natural numbers is summarized by Bernays — For Dedekind, Frege and Russell, collections classes are aggregates of "things" specified by proper names, that come about as the result of propositions utterances about one or more things that assert a fact about that thing or things.
Russell analysed this general notion. He begins with "terms" in sentences - which he analyses as follows:. For Russell, "terms" are either "things" or "concepts": This, then, is the widest word in the philosophical vocabulary. I shall use as synonymous with it the words, unit, individual, and entity. The first two emphasize the fact that every term is one, while the third is derived from the fact that every term has being, i.
A man, a moment, a number, a class, a relation, a chimaera, or anything else that can be mentioned, is sure to be a term; and to deny that such and such a thing is a term must always be false" Russell Things are indicated by proper names; concepts are indicated by adjectives or verbs: Among concepts, again, two kinds at least must be distinguished, namely those indicated by adjectives and those indicated by verbs" Concept-adjectives are "predicates"; concept-verbs are "relations": The notion of a "variable" subject appearing in a proposition: It is a characteristic of the terms of a proposition that anyone of them may be replaced by any other entity without our ceasing to have a proposition.
Thus we shall say that "Socrates is human" is a proposition having only one term; of the remaining component of the proposition, one is the verb, the other is a predicate.. Predicates, then, are concepts, other than verbs, which occur in propositions having only one term or subject. In other words, a "term" can be place-holder that indicates denotes one or more things that can be put into the placeholder.
Suppose Russell were to point to an object and utter: If by investigation of the utterance and correspondence with "fact", Russell discovers that Emily is a rabbit, then his utterance is considered "false"; if Emily is a female human a female "featherless biped" as Russell likes to call humans, following Diogenes Laertius 's anecdote about Plato , then his utterance is considered "true". Thus if Russell were to assert "All Emilys are women", then the "All" is a tipoff that the utterance is about all entities "Emily" in correspondence with a concept labeled "woman" and a methodical examination of all creatures with human names would have to commence.
Classes can be specified by extension listing their members or by intension, i. But "if we take extension pure, our class is defined by enumeration of its terms, and this method will not allow us to deal, as Symbolic Logic does, with infinite classes.
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Thus our classes must in general be regarded as objects denoted by concepts, and to this extent the point of view of intension is essential. Extensional versus intensional definition of a class: Class may be defined either extensionally or intensionally. That is to say, we may define the kind of object which is a class, or the kind of concept which denotes a class: Rather, people suggested that another planet influenced Uranus' orbit—and this prediction was indeed eventually confirmed.
Kitcher agrees with Popper that "There is surely something right in the idea that a science can succeed only if it can fail. He insists we view scientific theories as an "elaborate collection of statements", some of which are not falsifiable, while others—those he calls "auxiliary hypotheses", are. According to Kitcher, good scientific theories must have three features: Like other definitions of theories, including Popper's, Kitcher makes it clear that a theory must include statements that have observational consequences.
But, like the observation of irregularities in the orbit of Uranus, falsification is only one possible consequence of observation. The production of new hypotheses is another possible and equally important result. The concept of a scientific theory has also been described using analogies and metaphors. For instance, the logical empiricist Carl Gustav Hempel likened the structure of a scientific theory to a "complex spatial network: Its terms are represented by the knots, while the threads connecting the latter correspond, in part, to the definitions and, in part, to the fundamental and derivative hypotheses included in the theory.
The whole system floats, as it were, above the plane of observation and is anchored to it by the rules of interpretation. These might be viewed as strings which are not part of the network but link certain points of the latter with specific places in the plane of observation. By virtue of these interpretive connections, the network can function as a scientific theory: From certain observational data, we may ascend, via an interpretive string, to some point in the theoretical network, thence proceed, via definitions and hypotheses, to other points, from which another interpretive string permits a descent to the plane of observation.
Michael Polanyi made an analogy between a theory and a map:. A theory is something other than myself. It may be set out on paper as a system of rules, and it is the more truly a theory the more completely it can be put down in such terms. Mathematical theory reaches the highest perfection in this respect. But even a geographical map fully embodies in itself a set of strict rules for finding one's way through a region of otherwise uncharted experience.
Indeed, all theory may be regarded as a kind of map extended over space and time. A scientific theory can also be thought of as a book that captures the fundamental information about the world, a book that must be researched, written, and shared. In , Galileo Galilei wrote:. It is written in the language of mathematics, and its characters are triangles, circles, and other geometrical figures, without which it is humanly impossible to understand a single word of it; without these, one is wandering around in a dark labyrinth. The book metaphor could also be applied in the following passage, by the contemporary philosopher of science Ian Hacking:.
I myself prefer an Argentine fantasy. God did not write a Book of Nature of the sort that the old Europeans imagined. He wrote a Borgesian library, each book of which is as brief as possible, yet each book of which is inconsistent with every other. No book is redundant. For every book there is some humanly accessible bit of Nature such that that book, and no other, makes possible the comprehension, prediction and influencing of what is going on…Leibniz said that God chose a world which maximized the variety of phenomena while choosing the simplest laws.
In physics , the term theory is generally used for a mathematical framework—derived from a small set of basic postulates usually symmetries—like equality of locations in space or in time, or identity of electrons, etc. A good example is classical electromagnetism , which encompasses results derived from gauge symmetry sometimes called gauge invariance in a form of a few equations called Maxwell's equations.
The specific mathematical aspects of classical electromagnetic theory are termed "laws of electromagnetism," reflecting the level of consistent and reproducible evidence that supports them. Within electromagnetic theory generally, there are numerous hypotheses about how electromagnetism applies to specific situations. Many of these hypotheses are already considered to be adequately tested, with new ones always in the making and perhaps untested.
An example of the latter might be the radiation reaction force. As of , its effects on the periodic motion of charges are detectable in synchrotrons , but only as averaged effects over time. Some researchers are now considering experiments that could observe these effects at the instantaneous level i. Note that many fields of inquiry do not have specific named theories, e. Scientific knowledge outside a named theory can still have a high level of certainty, depending on the amount of evidence supporting it. Also note that since theories draw evidence from many different fields, the categorization is not absolute.
From Wikipedia, the free encyclopedia. For a broader coverage of this topic, see theory. Game theory Decision theory. Information theory Systems theory Control theory. Physics Classical Modern Applied. Mechanics classical analytical continuum fluid solid. Molecular Atomic Nuclear Particle. Special relativity General relativity. Inorganic Organic Analytical Physical. Applied physics Artificial intelligence Bioethics Bioinformatics Biomedical engineering Biostatistics Cognitive science Complex systems Computational linguistics Cultural studies Cybernetics Environmental science Environmental social science Environmental studies Ethnic studies Evolutionary psychology.
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A View from the National Academy of Sciences 2nd ed. The Stanford Encyclopedia of Philosophy. National Academy of Sciences. Reprinted in Theodore Schick ed.
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Stanford Encyclopedia of Philosophy. Retrieved 1 August The Nature of Science". This explanation did not require any modification of the theory, but rather modification of the hypothesis that there were only seven planets in the Solar System. Le Verrier , in French , "Lettre de M.
Fisher , Ernst Mayr , J. Haldane , and many others. Dreams of a Final Theory: A treatise on electricity and magnetism. The electroweak force is the unification of electromagnetism and the weak force. All observed causal interactions are understood to take place through one or more of these three mechanisms, although most systems are far too complicated to account for these except through the successive approximations offered by other theories.
Sidney Coleman, Sheldon L. B —52, found here . An overview can be found here. Cambridge University Press, , pp. McComas 30 December The Language of Science Education: Retrieved 25 March The University of Chicago Press. Retrieved 14 February Introductory Topics in the Philosophy of Natural Science. Empirical Model-Building and Response Surfaces. 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 ,  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. 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.
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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.
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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. 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.
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