Charles Robert Darwin (1809 - 1882) was a British naturalist, and co-discoverer of the principle of evolution by natural selection. He studied medicine at Edinburgh and theology at Cambridge. After graduating from Cambridge, Darwin travelled the world as a naturalist on board the Beagle (1832 - 1837).

After the Beagle voyage, Darwin worked over his collection of birds from the Galapagos Islands. He had initially supposed that the Galapagos finches were all one species, but it now became clear that each island had its own distinct species. How easy to imagine that they had evolved from a common ancestral finch! It was probably these observations of geographic variation that first led Darwin to accept that species can change.

Darwin sought to explain why species change. The notebooks Darwin kept at the time still survive. They reveal how he came upon the explanation while reading Malthus's Essay on the Principle of Population:

"Fifteen months after I had begun my systematic enquiry, I happened to read for amusement 'Malthus on population', and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved and unfavorable ones to be destroyed. The result of this would be the formation of a new species."

Because of the struggle for existence, forms that are better adapted to survive will leave more offspring and automatically increase in frequency from one generation to the next. Another British naturalist, Alfred Russel Wallace, had independently arrived at a very similar idea to Darwin's natural selection. Darwin and Wallace gave a simultaneous announcement of their idea at the Linnean Society in London in 1858; but by then Darwin was already writing an abstract of his full findings: that abstract is the scientific classic On the Origin of Species .

This portrait of Charles Darwin was painted in 1840.