Charles Darwin was an English-born naturalist and biologist of the 19th century. His most notable work is his 1869 book, The Origin of Species, which describes his theory of evolution. Darwin studied variation in plants and animals during his five year worldwide voyage on HMS Beagle. Darwin concluded that evolution happens by natural selection, animals and plants adapt to their environment over time in order to have a higher change of survival and reproduction. Individuals with weaker genes die and those with stronger genes pass their successful traits on to their offspring.