Charles Darwin (1809-1882), British naturalist. Darwin studied medicine and theology, but was most interested in natural history. In 1831 he joined the HMS Beagle on its five-year-long voyage around the world. The data he collected during the trip helped him to formulate the evolutionary theories discussed in On the Origin of Species by Means of Natural Selection, which he published in 1859. In this book, Darwin proposed that natural variation within a species creates many individual traits, some more useful than others. He proposed that only those individuals with the most competitive traits (those that are best suited to their environment) survive to reproduce.