Charles Darwin is often cited as the greatest biologist in history. His most famous work, On the Origin of Species, explains the theory of evolution by natural selection, providing numerous supporting examples. Darwin believed that all of life on earth had descended from a common ancestor, whose offspring could vary slightly from the original. Successive generations of life took part in a struggle for existence in which the best adapted variations survived to seed new generations. Less well adapted variations became extinct.
Darwin established that the South American content is gradually rising from the sea. Charles Lyell, whose geology book had influenced Darwin on the voyage, arranged that Darwin should present this work to the Geological Society of London at the beginning of 1837.
Darwin’s thoughts had increasingly turned to the question of how different species had formed. In July 1837 Darwin began keeping a notebook of his thoughts and theories about the variation of plants and animals. By this time, he had completely accepted that species could evolve