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About Charles Darwin.
Charles Darwin. Charles Robert Darwin was an English naturalist, eminent as a collector and geologist, who proposed and provided scientific evidence that all species of life have evolved over time from common ancestors through the process he called natural selection. The characterization of species and the limits of variation were integral to this quest. Alexander von Humboldt, the polymath German explorer and naturalist who influenced both Darwin and fellow naturalist Alfred Russel Wallace in profound ways see Sloan , urged the detailed study of biogeography.
Beyond merely cataloging species and varieties of different locales, Henslow sought out and highlighted insidual variations. He focused attention on how the limits of variation relate to species boundaries, and undertook experiments to probe the limits of morphological variation of species. Darwin was thus very much aware of the burning questions of natural philosophy. Yet explicit reflections on the nature of species and the meaning of their geographical distributions are few in his Beagle diary. Darwin later asserted e.
This is true, but he realized the deeper significance of his observations in retrospect—it took the analysis of specialists to lead him to see their true importance. Anatomist Richard Owen later in described these fossils as extinct giant quadrupeds that nonetheless belonged to recognizable extant South American groups. Darwin's biogeographical observations in South America seem to have had a more immediate effect on him.
Title page of the first edition of On the Origin of Species. Reproduced with permission from John van Wyhe ed. Now what would the Dis believer say to this? Would any two workmen ever hit on so beautiful, so simple and yet so artificial a contrivance? It cannot be thought so. Darwin seemed to glimpse the significance of these birds, but he did not yet realize that the finches of those equatorial islands were even more remarkable, nor that the tortoises told a similar story Sulloway a , b. Richard Owen received Darwin's fossil mammals sometime between late December and early January Historians largely agree that Gould's analysis was most likely the final factor convincing Darwin of transmutation Sulloway b , Browne Between the findings of Owen and Gould, then, we have the crucial juxtaposition of temporal patterns of species relationship in the fossil record with spatial biogeographic patterns.
The idea of transmutation prompted Darwin to consider again the nature of species and varieties. He did so largely in his private diary and notebooks, but in his public writing he said little to reveal the ferment of ideas he was developing. His first evolutionary musings in this notebook, probably dating to the time of his meeting with Gould in March , pertain to how species might change, prompted by observations of, for example, the contiguous ranges of the two South American rheas.
Ostriches; bigger one encroaches on smaller. These facts origin especially latter of all my views. Realizing the reality of transmutation immediately raised a host of pressing questions to Darwin: Does change occur quickly, or slowly? Does it occur according to some fixed law that places limits on how different species can become, or on how many different species can exist at any one time in a given taxonomic group?
What is the significance of islands? What are the environmental or geological factors? And, of course, What causes change?
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It was not until the fall of that Darwin hit upon the mechanism of natural selection. Domestication is so compelling an analogy to the natural process of species change that Darwin later seemed to see it as the initial inspiration for his ideas on both transmutation and natural selection. As we shall see, reflections on domesticates may have preceded Darwin's insight from Malthus, but they did not present an analogy in quite the straightforward manner he later imagined.
Arguing from domestication to a natural process of species change was more problematic for Darwin than one might assume.
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Nonetheless, production of domestic varieties assumed great importance to Darwin. Although scholars are not in agreement as to precisely how or to what extent domestication served as an analogy for a natural process of species change for Darwin e. It is clear, too, that domestication played a role in the derivation of his general concept of selection as an agent of change applicable to both domestication and nature Gildenhuys Darwin's C Notebook dated from about March through June bears entries pertaining to domestic varieties that most likely reflect Darwin's reading of key agricultural breeding tracts.
Darwin began to scrutinize Sebright's work and other publications on breeding in around March , around the time he opened the C Notebook. His reading of the agriculturists may have prompted him to seek out a mechanism of change in nature parallel or equivalent to the process employed by breeders, ultimately showing that the same process operates in both contexts e. In this view, while an appreciation of the production of domestic varieties precedes Darwin's reading of Malthus, domestication as an analogical process became fully apparent to Darwin over a period of time.
This is supported by notebook entries pertaining to domestic varieties in the months between March and September bracketed by his reading of Sebright and other breeders in March and his crucial reading of Malthus in September , as well as in the months after reading Malthus. Some of these entries are worth examining. Darwin's Malthusian insight comes about three-quarters of the way through the D Notebook, in passages dating to September Kohn suggests that before this point, Darwin saw domesticated varieties as analogous to natural species in a broad sense, but not produced by analogous causes per se.
Then, by the time of this entry, Darwin sees them as causally analogous, and subsequently inverts the presentation of his argument to proceed from the compelling domestication analogy.
Note that this represents an inversion of the order of his actual insights. After formulating the principle of natural selection, Darwin continued to struggle with issues such as crossing, heritability, reversion, and the extent and cause of variability.
The Foundations of the Origin of Species Two Essays written in 1842 & 1844
He voraciously read scientific treatises and journals Vorzimmer , sent lists of questions to contacts in the agricultural community, and even distributed a questionnaire to breeders in the spring of That first brief sketch of 35 pages was written in , followed by a detailed page essay in see Darwin F In both of these preliminary works, a basic, three-part presentation is evident, following an outline that is thought to date to about Vorzimmer , Hodge :.
The possible and probable application of these same principles to wild animals and consequently the possible and probable production of wild races, analogous to the domestic ones of plants and animals. The reasons for and against believing that such races really have been produced, forming what are called species. Darwin may have structured the exposition of his theory in this way as a conscious effort to present a philosophical argument.
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In this regard, the influence of John Herschel and, to a perhaps lesser extent, that of philosopher William Whewell is evident Ruse , Snyder Darwin personally knew these philosophers and read their works, and was familiar with their ideas on the nature of the scientific enterprise—notably, their thoughts on identifying verae causae in nature. In Herschelian logic, the case for a vera causa is best made by demonstrating or arguing plausibly for the existence of a mechanism, the causal adequacy or competence of that mechanism, and the responsibility of the mechanism in explaining observed phenomena Hodge , Hull , Waters The responsibility case resonates with Whewell's concept of consilience of inductions, which holds that a vera causa can be demonstrated through the concordance of otherwise disparate observations.
Snyder noted that the consilience aspect of Darwin's theory was especially potent: Beyond merely explaining facts, his theory tied together many classes of facts with a single causal explanation. The philosophical roadmap provided by the existence, adequacy, and responsibility or consilience argument model sheds light on the three-part outline Darwin used in the and Sketch and Essay , as well as in the Origin itself: the creation of domestic varieties by artificial selection as an analog to divergence of natural varieties and species, arguing for the existence and adequacy of transmutation by natural selection.
Selection, the causal agent of change under domestication, must also occur in nature as a logical result of abundant variability and severe struggle, the outcome of which depends in large part on that variability. Accordingly, chapter 1 of the Origin presents the domestication analogy, and chapters 2—4 set forth the logical argument for selection in nature being based on abundant, heritable natural variation, with differential survival and reproduction linked to that variation. These comprise the existence and adequacy cases. Darwin then turns to disciplines as diverse as hybridism, the fossil record, instinct, biogeography, morphology, and embryology to argue that his model of transmutation by natural selection is consistent with the observed facts, underscoring the responsibility of natural selection as causal mechanism.
Darwin's evolutionary ideas continued to develop as he read, experimented, sent endless queries to naturalists the world over, and pondered. The publication of the metaphysical evolutionary work Vestiges of the Natural History of Creation , and its wholesale rejection by the scientific establishment, may have reinforced Darwin's conviction that he needed to continue to amass data and observations for a complete and well-substantiated theory. Though he came to the essential components and logical structure of the theory by the early s, other elements came to him over his years of study and experimentation.
The most important of these later insights was undoubtedly the principle of divergence, which became the centerpiece of his grand vision for not just transmutation but also diversification. This principle lies at the heart of the only illustration found in the Origin : the divergence of character diagram of chapter 4. Divergence of character is a process, Darwin envisioned, by which natural selection acts on varieties of a species to enhance their competitiveness, an important outcome of which is the differential survival and reproduction of the most divergent varieties on average insofar as the most divergent varieties compete least.
ecanreelisys.tk This leads to a de facto ecological sision of labor—niche partitioning, in modern terms—yielding an ever-ramifying divergence pattern when iterated over time: the tree of life Tammone Browne showed that Darwin's divergence principle was fully formulated as late as spring , whereupon he inserted it into his already-complete chapter on natural selection in the summer of As we shall see, Darwin finally initiated a book on descent with modification in ; see Stauffer . Its genesis was far earlier, however, with roots that trace to Darwin's extensive barnacle studies, which resulted in four monographs published between and Among other insights, his studies of fossil and extant barnacles helped to impress upon Darwin the abundance of natural variation in virtually all points of structure, and to clarify the difficulties inherent in delineating species and varieties.
Darwin's version of botanical arithmetic looked at the creative activity of natural selection on a more local level, in a process of species splitting that at once explained ever-increasing competitiveness as well as species diversification, extinction, coexistence, and, by extension, the hierarchical classification system Browne The scope of Darwin's botanical arithmetic study was considerable. He first tallied from some dozen botanical tomes such data as whether genera with many varieties or with wide ranges also boasted many species—to him, a gauge of active generation of new species.
But then his friend and neighbor John Lubbock showed him that this approach was simplistic, suggesting a far better one that used proportional ratios to calculate observed and expected numbers of varieties associated with species of precisely defined large and small genera. Darwin then scrambled to redo all of his calculations of the previous 20 months. To his amazement and delight, he found that species in small genera having one to three species consistently exhibited fewer varieties than expected, compared with those in larger genera with four or more species.
This new approach crystallized for him the way in which natural selection drives diversification and grows the tree of life. It is often unappreciated just how central these botanical studies, and the divergence principle they inspired, were to the development of Darwin's overall theory. He mentioned the importance of his principle of divergence in a letter to a friend, the botanist Joseph Dalton Hooker, in August , and described it in some detail the following month in a letter to another confidante, the American botanist Asa Gray:.
One other principle, which may be called the principle of divergence plays, I believe, an important part in the origin of species. The same spot will support more life if occupied by very diverse forms: we see this in the many generic forms in a square yard of turf… or in the plants and insects, on any little uniform islet, belonging almost to as many genera and families as to species. Now every single organic being, by propagating so rapidly, may be said to be striving its utmost to increase in numbers.
So it will be with the offspring of any species after it has broken into varieties or sub-species or true species. And it follows, I think, from the foregoing facts that the varying offspring of each species will try only few will succeed to seize on as many and as diverse places in the economy of nature, as possible.
Each new variety or species, when formed will generally take the places of and so exterminate its less well-fitted parent. This, I believe, to be the origin of the classification or arrangement of all organic beings at all times. These always seem to branch and sub-branch like a tree from a common trunk; the flourishing twigs destroying the less vigorous—the dead and lost branches rudely representing extinct genera and families. Burkhardt et al. This insight soon came, and Darwin had completed only some 10 chapters when he was forestalled by the arrival of Wallace's brilliant Ternate essay of Stauffer The first of July, , marked the presentation of both Wallace's essay and extracts of Darwin's writings on the subject to the Linnean Society, but this was only the beginning.
On the Origin of Species may have lacked the extensive footnoted references, tables of supporting data, and numerous argument-buttressing examples and observations that his longer, more traditional, scientific treatise would have had, but it perhaps reads better as a narrative unburdened by the distractions of footnotes and tables. Significantly, Darwin planned to follow the Origin with three works dedicated to a fuller exposition of his theory, one for each component of his three-part logical argument.
Accordingly, what was a page chapter on variation under domestication in the Origin became the two-volume Variation of Animals and Plants under Domestication. He reiterated in the introduction to Variation , too, that he intended to present his theory in three works, of which Variation was the first.