Born in Shrewsbury, Shropshire, England, on February 12, 1809. After graduating from the elite school at Shrewsbury in 1825, Darwin went to the University of Edinburgh to study medicine. In 1827 he dropped out of medical school and entered the University of Cambridge, in preparation for becoming a clergyman of the Church of England. There he met two figures: Adam Sedgwick, a geologist, and John Stevens Henslow, a naturalist. Henslow not only helped build Darwin s self-confidence but also taught his student to be a painstaking observer of natural phenomena and collector of specimens. After graduating from Cambridge in 1831, the 22-year-old Darwin was taken aboard the English survey ship HMS Beagle, largely on Henslow s recommendation, as an unpaid naturalist on a scientific expedition around the world.

Darwin s job as naturalist aboard the Beagle gave him the opportunity to observe the various geological formations found on different continents and islands along the way, as well as a huge variety of fossils and living organisms. In his geological observations, Darwin was most impressed with the effect that natural forces had on shaping the earth s surface.

At the time, most geologists adhered to the so-called catastrophist theory that the earth had experienced a succession of creations of animal and plant life, and that each creation had been destroyed by a sudden catastrophe, such as an upheaval or convulsion of the earth s surface. According to this theory, the most recent catastrophe, Noah s flood, wiped away all life except those forms taken into the ark. The rest were visible only in the form of fossils. In the view of the catastrophists, species were individually created and immutable, that is, unchangeable for all time.

Aboard the Beagle, Darwin found himself fitting many of his observations into Lyell s general uniformitarian view. Beyond that, however, he realized that some of his own observations of fossils and living plants and animals cast doubt on the Lyell-supported view that species were specially created. He noted, for example, that certain fossils of supposedly extinct species closely resembled living species in the same geographical area. In the Gal pagos Islands, off the coast of Ecuador, he also observed that each island supported its own form of tortoise, mockingbird, and finch; the various forms were closely related but differed in structure and eating habits from island to island. Both observations raised the question, for Darwin, of possible links between distinct but similar species.

After returning to England in 1836, Darwin began recording his ideas about changeability of species in his Notebooks on the Transmutation of Species. Darwin s explanation for how organisms evolved was brought into sharp focus after he read An Essay on the Principle of Population, by the British economist Thomas Robert Malthus, who explained how human populations remain in balance. Malthus argued that any increase in the availability of food for basic human survival could not match the geometrical rate of population growth. The latter, therefore, had to be checked by natural limitations such as famine and disease, or by social actions such as war.

Darwin immediately applied Malthus s argument to animals and plants, and by 1838 he had arrived at a sketch of a theory of evolution through natural selection. For the next two decades he worked on his theory and other natural history projects. In 1839 he married his first cousin, Emma Wedgwood, and soon after, moved to a small estate, Down House, outside London. There he and his wife had ten children, three of whom died in infancy.

Darwin s theory was first announced in 1858 in a paper presented at the same time as one by Alfred Russel Wallace, a young naturalist who had come independently to the theory of natural selection. Darwin s complete theory was published in 1859, in On the Origin of Species.Often referred to as the "book that shook the world," the Origin sold out on the first day of publication and subsequently went through six editions.

Darwin s theory of evolution by natural selection is essentially that, because of the food-supply problem described by Malthus, the young born to any species intensely compete for survival. Those young that survive to produce the next generation tend to embody favorable natural variations the process of natural selection and these variations are passed on by heredity. Therefore, each generation will improve adaptively over the preceding generations, and this gradual and continuous process is the source of the evolution of species. Natural selection is only part of Darwin s vast conceptual scheme; he also introduced the concept that all related organisms are descended from common ancestors. Moreover, he provided additional support for the older concept that the earth itself is not static but evolving.