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Evolution As A Theory Of Species Change

The belief in a changing or dynamic universe can be first seen in ancient Greek philosophy. Heracleitis (c. 540–c. 480 B.C.E.), also known as the "flux philosopher," believed that change was a fundamental property of the universe. His successor, Empedocles (c. 490–430 B.C.E.), first articulated a crude but dynamic theory that postulated that the origin of life had taken place in a manner that suggested evolution. With the philosophical worldview established by Aristotle (384–322 B.C.E.), the belief in a changing universe fell into disfavor. Aristotle and his numerous medieval and Renaissance translators, commentators, and supporters instead believed in a static universe which held that living organisms were created initially by a designer (the Demiurge of Plato's Timaeus or the biblical Creator) and then remained essentially unchanged. These ideal types or species were arranged hierarchically in what came to be known as the scala naturae, or the ladder of creation. Like the rungs of a ladder, each species was arranged hierarchically, with lower forms of life on the bottom and higher forms of life on the top. During the Renaissance, the ladder of creation gave way to the popular metaphor of the "great chain of being," which referred to a progression of living forms linked in an orderly chainlike arrangement. Extinction, the sudden disappearance of a species, in such a scheme was unthinkable since it meant that the chain would lose a vital link. Belief in the fixity of species and in the species characterized by an ideal type therefore dominated thinking about living entities and was most clearly demonstrated in the modern classification scheme that originated with Carolus Linnaeus (1707–1778).

Belief in species change, or more precisely what was termed "transmutationism," slowly began to emerge during the Enlightenment. One reason for this was the recognition that the earth was of greater antiquity than previously thought and that fossils, long held to be curiosities of nature that adorned the shelves of Renaissance collectors were in fact the remnants of once living organisms. The organic origin of fossils had been suggested earlier by Nicholas Steno (1638–1686) and others who were concerned with them.

Another reason was that the Enlightenment also saw the emergence of the belief in a progressive world, both scientific and social, at the same time that it was slowly realized that the earth itself had a long and tumultuous history of its own. The closing of the eighteenth century saw the beginnings of attempts to understand the history of the earth in terms of natural causes and processes. These geological theories suggested that fossils were of organic origin and that uniform or constant processes rather than catastrophic or one-time events had shaped the Earth's history. In the eighteenth century two rival schools of thought existed: the first, known as the catastrophists, upheld the belief in the uniqueness of geological events, while the second upheld the belief that geological processes were not unique or catastrophic but instead were part of a uniform and largely gradual process of natural change. The latter school was associated with a "uniformitarian" theory of geological change and its advocates known as uniformitarians.

The French naturalist Georges-Louis Leclerc, comte de Buffon (1707–1788), was one of the first to embrace a uniformitarian philosophy, to question the fixity of species, and to suggest a transmutationist theory for species change. Although he was a respected naturalist, writing a forty-four-volume treatise on the natural history of the world known as L'historie naturelle (Natural history), his theoretical explanations for the origin of life and of species change were not accepted during his time; he provided no cogent mechanism for such changes. Buffon's transmutationist ideas were also not accepted because they were undermined by the philosophical teachings of his successor, Georges Cuvier (1769–1832), an anti-uniformitarian who thought successive "revolutions" or catastrophes had shaped the pattern of diversity of life on earth. Cuvier was a pioneer of comparative anatomy and is generally regarded as the father of modern vertebrate paleontology. He upheld the fixity of species despite fossil evidence of species change. Ironically, although he opposed transmutationism strongly, Cuvier was the first to recognize the phenomenon of extinction, or the view that species had disappeared from the biological record. His system of classification placed living organisms into four distinct groupings or what he termed embranchements: the Vertebrata, Articulata (arthropods and segmented worms), Mollusca, and Radiata (echinoderms and cnidarians). The four "branches" were distinct from one another and could not share any evolutionary transformation. If any similarities existed, this was due to shared functional circumstances and not to any common ancestry. Cuvier's influence in zoology in particular and in French science generally was enormous and played a role in discrediting efforts to formulate transmutationist theories.

The first to suggest a viable theory of transmutation was the Frenchman Jean-Baptiste de Lamarck (1744–1829), a contemporary of Cuvier's who faced notable opposition from him. First an expert on botany, Lamarck was given the lowly task of organizing the invertebrate collections at the Musée National d'Histoire Naturelle (National Museum of Natural History). In the process of working with the little-known group (Lamarck coined the term invertebrate), he began to note progressive trends in the group. He became particularly interested in adaptation, or the manner and process by which organisms are able to adapt physiologically and morphologically to their environment, and he was especially interested in how well-adapted organs originated. His most celebrated example was the modification of the neck of the giraffe, which became elongated in response to stretching during feeding on the leaves of trees on the African plains. This and other examples were explored in works such as Philosophie zoologique (Zoological philosophy), published in 1809. According to Lamarck, the use, or in many cases the disuse, of such vital organs could lead to the origin of novel but well-adapted traits; the cumulative effect of these adaptations could eventually lead to new species. Lamarck never provided a cogent mechanism by which this physical transformation took place, however, though he did draw on contemporary theories from animal physiology to suggest that the body heat generated by physical exercise could lead to such structural transformation. Sometimes called "the inheritance of acquired characters," Lamarckian transmutationism, also later called Lamarckian evolution or "Lamarckism," was subsequently shown to be erroneous because changes acquired as a result of use and disuse were shown to be not heritable. The German experimental biologist Auguste Weismann (1834–1914) is generally credited with disproving Lamarckian inheritance through a number of experiments that included cutting off the tails of hundreds of mice, and through his famous theory that first made the distinction between germplasm (cells that passed on hereditary information) and somatic or bodily cells. The "Weismann Barrier," which eventually became one of the central dogmas of modern biology, postulated that hereditary information moves only from the genes to the somatic cells and not vice versa.

Lamarck's ideas were, however, very popular throughout much of the nineteenth century, especially among naturalists interested in adaptation, and continued to gain support in some communities well into the twentieth century, sometimes being associated with "neo-Lamarckian" theories of species change. Darwin himself relied heavily on the inheritance of acquired characters to explain many adaptations that he later outlined in laying out his own transmutationist theory as it finally appeared in 1859.

Transmutationism itself became increasingly acceptable in the early decades of the nineteenth century. It captured the interest of Darwin's own grandfather, Erasmus Darwin (1731–1802), who suggested that life had originated from "one living filament" in his two-volume work Zoonomia (1794–1796). Other transmutationists included the French anatomist Étienne Geoffroy Sainte Hilaire (1805–1861), who studied teratology, or the science of birth defects. He suggested that through such "monstrous births" new species might arise in a sudden or rapid process, a theory later challenged by modern genetics.

In the nineteenth century a series of scholars began to uphold not just transmutationist doctrines but theories suggestive of what eventually would become known as Darwinian natural selection. In 1813 William Wells delivered a paper to the Royal Society with the title "An Account of a Female in the White Race of Mankind." Wells suggested that new human races originated when groups moved into new territories where they encountered new conditions of life. In the process of adaptation to these new conditions, newer improved races of humans would emerge. In 1831 Patrick Matthew came even closer to formulating a view of natural selection in the appendix to an obscure treatise, On Naval Timber and Arboriculture. In this account Matthew invoked the extinction of species by catastrophic events, after which the survivors would diversify into new, better-adapted species that would remain stable for long periods of time. In 1835 yet another scholar, Edward Blyth (1810–1873), in a paper titled "An Attempt to Classify the Varieties of Animals," suggested a competitive process echoing natural selection whereby the elimination of the unfit groups would take place.

In 1844 the work of one transmutationist in particular drew the attention of wide Victorian audiences. Writing anonymously at first, Robert Chambers (1802–1871) outlined a transmutationist theory under the title Vestiges of the Natural History of Creation. The book became an instant sensation for its many readers, who were greatly entertained by the provocative—and indeed some thought scandalous—account of the origins of the solar system and of the origins of humanity, which postulated evolution from the apes. Though it was widely read and discussed, it received devastating criticism from scientists; this was so much the case that Charles Darwin, witnessing the controversy precipitated by Vestiges, is thought to have been dissuaded from publishing his own transmutationist views for nearly fifteen years.

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