Darwin refers specifically to the distribution of the species rheas

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: Sony VAIO VPCF11C5E battery 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. Sony VPCZ12X9E/X battery 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.Sony VPCZ12M9E/B battery 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. Chapter I covers animal husbandry and plant breeding, going back to ancient Egypt. Sony VPCZ11Z9E/B battery 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 describesfancy 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] Sony VPCZ11X9E/B battery 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. Sony VPCY11S1E battery 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] Sony VPCY11S1E/S battery 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. Sony VPCY21S1E/G battery 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";^[90] in the fifth edition he adds, Sony VPCY21S1E/L battery "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, Sony VPCY21S1E/SI battery 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. Sony VPCY22Z5008B battery 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";Sony VPCYA1V9E/B battery 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." Sony VPCYA1S1E/B battery 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 ... Sony VPCW11S1E/P battery 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. Sony VPCW11S1E/T battery 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, Sony VPCW11S1E/W battery 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. Sony VPCW12S1E/P battery 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. Sony VPCW12S1E/T battery 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). Sony VPCW12S1E/W battery 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". Sony VPCW12S1E/WZ battery 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".Sony VPCW21C7E battery 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; Sony VPCW21M2E/WI battery 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, Sony VAIO PCG-7148L battery 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. Sony VAIO PCG-7151L battery 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.he observation that selection works in domestic animals is not destroyed by lack of understanding of the underlying hereditary mechanism. Sony VAIO PCG-7152L battery 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, Sony VAIO PCG-7153L battery 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. Sony VAIO PCG-7154L battery 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. Sony VAIO PCG-7161L battery 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. 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, Sony VAIO PCG-7162L battery 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.The rest of the chapter deals with whether natural selection could produce complex specialised structures, Sony VAIO PCG-7171L battery 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? Sony VAIO PCG-7172L battery 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. Sony VAIO PCG-7173L battery 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: Sony VAIO PCG-7174L battery "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. Sony VAIO PCG-7181L battery 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. Sony VAIO PCG-7182L battery 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: Sony VAIO PCG-7183L battery 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, Sony VAIO PCG-7184L battery 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. Chapter VIII addresses the idea that species had special characteristics that prevented hybrids from being fertile in order to preserve separately created species. Sony VAIO PCG-7185L battery 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. Sony VAIO PCG-381L battery 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, Sony VAIO PCG-382L battery 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, Sony VAIO PCG-383L battery as shown by his book onFertilisation 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, fSony VAIO PCG-384L battery latfish with both eyes on one side and the camouflage of stick insectscould 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 geologic record appears to show forms of life suddenly arising, Sony VAIO PCG-391L battery 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. Sony VAIO PCG-393L battery 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. Sony VAIO PCG-394L battery 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, Sony VPCEH26FJ/W battery 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 ofpre-Cambrian life has since been found, extending the history of life back for billions of years.^[118] Sony VPCEH27FG/W battery 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. Sony VPCEH28FA/B battery The pace of natural selection would depend on variability and change in the environment.^[119] This distanced his theory from Lamarckianlaws 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] Sony VPCEH28FF/B battery 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] Sony VPCEH28FG/B battery 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 Clifthad shown in Australia, fossils from recent geological periods resembled species still living in the same area. Sony VPCEH28FG/P battery