Darwins Theory

The nineteenth century witnessed a number of books asserting that living species had evolved from earlier ones. Before 1859, these works were often more geological than biological in content. Most successful among them was the anonymously published Vestiges of the Natural History of Creation (1844), written by Robert Chambers (1802-1871). Books of this genre sold well but contained many flaws. They proposed no mechanism to account for evolutionary change. They supported the outmoded concept of a scale of being, often as a single sequence of evolutionary "progress." In geology, they supported the outmoded theory of catastro-phism, an idea that the history of the earth had been characterized by great cataclysmic upheavals. From 1830 on, however, that theory was being replaced by the modern theory of uniformitarianism, championed by Charles Lyell (1797-1875). Charles Darwin read these books and knew their faults, especially their lack of a mechanism that was compatible with Lyell's geology. In his own work, Darwin carefully tried to avoid the shortcomings of these books.

Darwin brought about the greatest revolution in biological thought by proposing both a theory of branching evolution and a mechanism of natural selection to explain how it occurred. Much of Darwin's evidence was gathered during his voyage around the world aboard HMS Beagle between 1831 and 1836. Darwin's stop in the Galápagos Islands and his study of tortoises and finchlike birds on these islands is usually credited with convincing him that evolution was a branching process and that adaptation to local environments was an essential part of the evolutionary process. Adaptation, he later concluded, came about through natural selection, a process that killed the maladapted variations and allowed only the well-adapted ones to survive and pass on their hereditary traits. After returning to England from his voyage, Darwin raised pigeons, consulted with various animal breeders about changes in domestic breeds, and investigated other phenomena that later enabled him to demonstrate natural selection and its power to produce evolutionary change.

Darwin delayed the publication of his book for seventeen years after he wrote his first manuscript version. He might have waited even longer, except that his hand was forced. From the East Indies, another British scientist, Alfred Russel Wallace (18231913), had written a description of an identical the-

Charles Darwin and the Beagle

In 1831, a twenty-two-year-old Charles Darwin, who had been studying for the ministry at Cambridge, by luck was offered a position as naturalist on the ship HMS Beagle, which was about to embark on a round-the-world voyage of exploration. His domineering father was against the trip at first, but he finally relented. The expedition would turn the young man into a scientist. Over the next five years, Darwin recorded hundreds of details about plants and animals and began to notice some consistent patterns. His work led him to develop new ideas about what causes variations in different plant and animal species:

[The] preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest slight modifications, which in any way favoured the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved. . . .

—On the Origin of Species by Means of Natural Selection, 1859

Until Darwin and such colleagues as Alfred Russel Wallace, the "fixity" or unchangingness of species had been accepted as fact, and the appearance over time of new species remained a mystery. Darwin's lucky trip laid the foundation for today's understanding of life and its diversity.

Strait of __ Magellan

Cape Horn

Strait of __ Magellan

Cape Horn ory and submitted it to Darwin for his comments. Darwin showed Wallace's letter to Lyell, who urged that both Darwin's and Wallace's contributions be published, along with documented evidence showing that both had arrived at the same ideas independently. Darwin's great book, On the Origin of Species by Means of Natural Selection, was published in 1859, and it quickly won most of the scientific community to a support of the concept of branching evolution. In his later years, Darwin also published The Descent of Man and Selection in Relation to Sex (1871), in which he outlined his theory of sexual selection. According to this theory, the agent that determines the composition of the next generation may often be the opposite sex. An organism may be well adapted to live, but unless it can mate and leave offspring, it will not contribute to the next or to future generations.

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