History Weblecture for Unit 42
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The topic of evolution is a controversial one for many people, touching on their personal religious beliefs as well as on their ideas of how science should be done. Yahoo! lists about about 50 links to both pro-creation and pro-evolution sites, some of which are very good; the others range from the merely provacative to distorted representations of historical facts and the theories themselves. My own opinions on the subject are on my Evolution FAQ page, which is part of the Scholars Online Biology Class website. I don't expect all members of the class to share this position; your job is to determine and explain your own, and to learn to accurately and charitably restate historical events, the contents of the theory, and the opinions of others in terms they would accept. This is harder than it sounds.
For the purposes of this course, I am going to present the theory of evolution as it developed and now stands, without debating it. While many people will argue that "evolution is a fact", in this course we take as definitional that a theory is a provisional explanation of perceived events, and as such is subject to many limitations, including those of its source (scientific methodologies), which do not necessarily encompass the whole of how man learns and interacts with his natural environment.
From Aristotle's time onward, the classical and medieval philosophers conceived a "great chain of being" as an organizing principle of the different kinds of matter. At the bottom was dumb matter, lifeless and inert. At the top was man, the most complex of living creatures. Stretching from the bottom to the top, starting with the simplest life forms and gradually increasing in complexity, were all the other living creatures. This was not an "evolutionary" ladder--it said nothing about the development of the different species. It merely categorized different groups of living things according to Aristotle's sense of their complexity. Species were "fixed" and could not gain or lose the characteristics that gave them a certain position on the chain. All relationships were vertical: one species is higher than some and lower than others, not by moral or aesthetic standards, but simply in terms of physical organization.
This idea captured the imagination of the NeoPlatonists, and the medieval mind, both of whom extendend the great chain of being from the center of earth out through the orders of angels to God as the highest form of "being" in the cosmos. Dante's Divine Comedy reflects the influence of this idean through the circles of hell, purgatory, and heaven. Likewise the interlocking spheres of the Ptolemaic and Aristotelian world models enforce this sense of continuity of all of nature in a vertical hierarchical relationship.
Take a brief glance at the diagram of the medieval chain of being at the top of this web page. It is similar to the medieval world view C. S. Lewis describes in The Discarded Image.
The voyages of exploration in the 16th and 17th centuries made the Europeans aware of new plants, new animals, and new geological formations, as well as more fossils. As they gathered information, the French Enlightenment philosophers realized that any theory on the origin of life would have to account for three primary observations:
The title page to Linnaeus' work on plant classification from the British Museum exhibit on the Enlightment,
29 June 2007 ©2007 Christe Ann McMenomy
Following Newton and Aristotle, the Swedish botanist Carolus Linnaeus developed a more complex theory of the fixity of species, in which each species was the special creation of God. Like Aristotle, Linneaus was an astute observer of actual living things, although he concentrated on plants where Aristotle had been fascinated by sealife. Linnaeus classified over 18,000 species of plants and animals into a hierarchy using class, order, genus and species.
During the 18th century, various French philosophers proposed the idea that different species lived together without regard to a specific hierarchy. Those which were more suited to their environments survived; those which were not suited to the environments, or which could not adapt to catastrophic changes in the environment, died out. This theory would explain the existence of fossils, bones for animals which no longer lived in a given area, or even anywhere in the world. Charles Bonnet, a Swiss naturalist, discovered that female aphids can reproduce without fertilization (a form of parthenogenesis), and speculated that all females carry within them all future generations in some miniature form, subject to change only by great catastrophes, such as the biblical Flood. The French philosopher and encylopaidist Jean-Baptist-Rene Robinet proposed that the Great Chain was itself the embodiment of this progression stretching from non-being to divine being, with each species "higher" on the chain an example of a more complex and more complete form of life, none of which was purely one thing or another, but each of which encompassed both nutrition and elimination, both creative and destructive activity. Although they differed in their mechanisms of how change was accomplished, both men reflected the humanism of the Enlightenment which fostered a new view in which over time, all things progressed toward perfection.
Lamarck's statue in the Jardin des Plantes, near the Museum de'Histoire Naturalle where he worked in Paris. Lamarck was responsible for creating a garden containing examples of all the plants that he could find and grow.
© 2007 Christe Ann McMenomy
Lamarck was one of the earliest to propose that the "great chain of being" described a sequence of events, not merely a way of organizing related species. Rather than a single "chain", Lamarck proposed a branched tree of relationships between different organisms, with simpler organisms dependent on their environment for heat and electrical energy, but the more complex organisms able to generate their own heat, and therefore to some extent independent of their environment. He was influenced by Robinet and by Erasmus Darwin (Charles' grandfather), who had already published Zoonomia, a work that speculated whether habits acquired by an animal during its lifetime could be passed on to its offspring. Erasmus Darwin's theories were based on simple observation rather than systematic research and his radical reputation hindered acceptance of his theory. However, Larmarck developed Erasmus' idea more fully into the theory of acquired characteristics.
According to this theory, simple individual animals adapted to their environment, learning how to survive, perhaps growing or developing new features. These new features were then passed on to the individual's descendents, who were better able to survive. Lamarck conceived of evolution as a purposeful movement toward more complex and better-adapted life forms, in which each organism was driven by some inner force which acted to improve the species, not a random process.
Read this short description of Lamarck's life and theories at the Berkeley Paleogeology site.
As an example, he used the giraffe (which in his time had only been recently discovered). He thought its early form must have been similar to that of an antelope. During a drought, when grass disappeared and the lower leaves of trees had been eaten by shorter animals, the antelopes would stretch their necks to get leaves higher in the trees. They would pass this "longer neck" characteristic to their offspring. As each generation stretched its neck, the neck became longer and longer, resulting in the giraffe.
Not everyone shared this view of "evolution toward perfection". Lamarck's ideas were opposed in France by Georges Cuvier (whom we met in the last unit, as the father of catastrophism), and by a number of British philosophers, who thought Lamarck too materialistic. Cuvier used the classification system developed by Carl Linneas, adding phyla and kingdom groupings to locate his fossils in relation to living animals. In his position as professor of comparative anatomy at the Muséum National d'Histoire Naturelle, Cuvier had access to an enormous collection of fossils to study, supplemented periodically by deliveries from Napolean's conquering armies. As his reputation grew, many correspondents sent him samples or at least pictures of samples. By comparing a single bone (which was often all he had from a given specimen) to others in his collection, Cuvier believed that he could predict the appearance of the rest of an animal's skeleton, and many of his predictions have since been confirmed by the discovery of more complete skeletons.
One of the bones Cuvier examined, now in the Museum of Evolution in Paris.
© 2007 Christe Ann McMenomy
Like Linneaus, Cuvier believed in fixed species--that no new species developed after the original Creation, but that many species had become extinct, and that the world was very old. He used his classification system on fossils as well as on living species. This coupled with his idea of catastrophism in opposition to the more popular theories of gradualism (gradual change) or uniformitarianism (all changes through the same constant mechanisms) led many in the early twentieth century to discount his work. However, the re-emergence of a form of catastrophism in the recognition of mass extinctions has made many geologists think Cuvier's theories worth re-examining.
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