Discover Charles Darwin's most important ideas... When the eminent naturalist Charles Darwin returned from South America on board the H.M.S Beagle in 1836, he brought notes and evidence that would form the basis of his landmark theory: that species evolve by a process of natural selection. This theory, published as The Origin of Species in 1859, is the basis of modern biology and the concept of biodiversity.It also sparked a fierce scientific, religious and philosophical debate that still rages today. WITH AN INTRODUCTION BY DARWIN'S GREAT-GREAT-GRANDDAUGHTER, RUTH PADEL