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It is commonly thought today that the theory of evolution originated from Charles Darwin in the nineteenth century. However, the idea that species mutate over time has been around for a long time in one form or another. Therefore, by Darwin’s time the idea that species change from one type into another was by no means new, but was rejected by most because the proponents of evolution could not come up with a satisfactory mechanism that would explain this change.
But how did Darwin come up with an acceptable theory of evolution, and how did Darwin’s proposal of natural selection impact the theory of evolution? The answer lies in the study of the works of others, and in the works of Darwin himself, through his theories, his travels and his scientific pioneering.
The most influential evolutionary theories prior to Darwin were those of Lamarck and Geoffrey St. Hilaire, developed between 1794 and 1830. Lamarck suggested that species evolve through the use or disuse of particular organs. In the classic example a giraffe that stretches its neck slightly to reach higher leaves will gain in neck length, and this small gain would be passed on to its offspring. (Poirier, McKee, 1999) St. Hilaire, on the other hand suggested that the change was discontinuous, large in magnitude, and occurred at the production of offspring. However, these theories of evolution were based on explanations that offered no demonstrated mechanism. (Bowler, 1990)
Darwin’s theory of evolution differs in that it is based on three easily verified observations. First, individuals within a species vary from one another in morphology, physiology, and behavior. Second, variation is in some part heritable so that variant forms have offspring that resemble them. Third, different variants leave different number of offspring(Wilson, 1999). Darwin than proceeded to elaborate on the mechanism of evolution by suggesting that in the universal struggle for life, nature “selects” those individuals who are best suited for the struggle, and these individuals in turn reproduce more than those who are less fit, thus changing the composition of the population. In addition to natural selection, Darwin also suggested that species also evolve through the complementary process of sexual selection.(Wilson, 1999)
According to Darwin, in sexual selection, one gender of a species develops a preference for individuals of the other gender who possess certain features. The individuals who possess these features will than have a reproductive advantage over others, resulting in a greater number of offspring, and thus, again, a change in the composition of the population.(Poirier, McKee, 1999) Therefore, it was Darwin who made the theory of evolution feasible by providing the mechanisms of natural and sexual selection. The possibility of a feasible theory of evolution made the idea more acceptable by the scientific community, although there were those that still opposed it.

Darwin’s Formative Years

Charles Darwin was born in England in 1809 and belonged to a wealthy and respectable family. His grandfather, Erasamus Darwin, was a noted botanical expert in his day who published two important books, Zoonomia, and the Botanic Garden. In these books, Erasamus speculated about various evolutionary ideas that were dismissed as too radical (i.e., the nose of the swine has become hard for the purpose of turning up the soil in search of insects and roots). Darwin who in his youth read his grandfather’s books with admiration, later commented that his grandfather “anticipated the views and erroneous grounds of opinion” of Lamarck. Nevertheless, Erasamus may have unconsciously influenced Darwin in preparing the way for evolution by natural selection.(Darwin, 1958)
In 1818, at the age of 9, Darwin entered the Shrewsbury school, which was ran by Dr. Butler. Darwin later recalled that “nothing could have been worse for the development of my mind than Dr. Butler’s school, as it was strictly classical, nothing else being taught , except a little ancient geography and history. The school as a means of education to me was simply a blank”(Darwin, 1958). He was removed from the school in 1825, and was sent to Edinburgh to study medicine. There he studied for two years before deciding that he didn’t like medicine. But before he left Edinburgh, he was introduced for the first time to the theories of Lamarck. According to Darwin at the time he was not very impressed with Lamarck’s ideas. (Bowler, 1990)
In 1828, at his father’s suggestion, Darwin entered Christ’s College in Cambridge to become a clergyman. To Darwin a good education meant instruction in the methods and logic of thought. Therefore, Just about the only thing he enjoyed studying there was Paley’s works on theology, because of their logic. For the rest, however, he judged Cambridge to be just as much a waste of time as Edinburgh and Shrewsbury. Nevertheless, in his spare time at Cambridge, Darwin became interested in various scientific endeavors, and became acquainted with and influenced by the scientific ideas of Henslow, Sedgwick, and Whewell (ironically Sedgwick later became a bitter opponent of Darwin’s theory). In addition, during his last year at Cambridge Darwin read two books which influenced him greatly, Herschel’s Preliminary Discourse on the Study of Natural Philosophy, and Von Humboldt’s Personal Narrative of Travels to the Equinoctial Regions of the New Continent. Darwin later confessed that these books inspired in him “a burning zeal to add even the most humble contribution to the noble structure of Natural Science”.(Darwin, 1958)
Darwin graduated from Cambridge in 1831, and as he was pondering his future he received a proposal to join a scientific expedition that would survey the southern coast of Tierra del Fuego. Darwin accepted the proposal, and sailed from England aboard the famed Beagle on December 27, 1831. His job was to collect and catalogue new species so that they could be sent back for further research in England. It is commonly thought that Darwin used the voyage to test his theory of evolution, but this is highly unlikely. At the time Darwin’s interests were purely geological as can be seen by his correspondence with his sister. For instance, writing about the fossils which he discovered he said, “All the interest which I individually feel about these fossils is their connection with the geology of the Pampas”(Darwin, 1958). Furthermore, Darwin himself confessed that he could not have appreciated the significance of his findings while on the voyage, because he lacked the necessary training in dissection and drawing as well as the knowledge of comparative anatomy. It was only much later when Darwin returned from the voyage, and when Owen identified the fossils, that Darwin began to examine them as zoological, rather then geological, phenomena.(Himmelfarb,1959)
The voyage turned out to be very productive for Darwin, who upon his return in 1836 began to work on the conversion of the diary, which he kept during the voyage, into a journal suitable for publication. The Journal was first published in 1839 under the title “Journal and Remarks”, as Volume III of the Narrative of the Surveying Voyages of H.M.S. Adventures and Beagle.(Vorzimmer, 1970) However, enough people thought that Darwin’s work was sufficiently important to warrant a separate publication, and in 1845 a second edition was published under the name Journal of Research into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the World .

Darwin “Discovers” Evolution
It appears to be that only sometime in 1837 did Darwin first start to entertain the idea of evolution seriously. (Himmelfarb, 1959) The proof for this lies in the notebook that he kept from July 1837 to February 1838. In particular, the following statement from the notebook provides valuable insight: “In July opened first notebook on transmutation of species. Had been greatly struck from about the previous March on character of South American fossils, and species of Galapagos Archipelago. These facts (especially latter), origin of all my views”. (Vorzimmer, 1970)
Therefore, it must have been at this time that Darwin’s ideas took this turn. Additionally, had the change occurred earlier, it would have shown up in Darwin’s writing in the Journal, which, more than half completed by March, shows no trace of it. (Vorzimmer, 1970)
Overall, with the notable exception of the idea of natural selection, most of what Darwin later wrote in On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (henceforth referred to as the Origin), was already sketched in that notebook. (Vorzimmer, 1970) It is important to note that Darwin’s thinking at this point was still distinctly teleological in character. He still believed that God had instituted the laws governing reproduction to maintain species in a state of perfect adaptation to their environment. Only after his full appreciation of the struggle for existence did he come to believe that a changed environment disturbs growth to produce random variation. Curiously, Darwin asserts that in originating his theory of evolution he was trying to follow “Baconian principles”, that is collect facts before theorizing. (Lewontin, 1974)
Specifically, in his autobiography he states “After my return to England it appeared to me that by following the example of Lyell in Geology, and by collecting all facts which bore in any way on th

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