Why are there so many different kinds, or species, of living things on earth, each uniquely fitted to its environment? For Charles Darwin, this question represented the "mystery of mysteries." Darwin first began to formulate an answer during a youthful voyage around the world on the H.M.S. Beagle from 1831 to 1835. Darwin's answer, known as the Theory of Natural Selection, changed the way we think about life on earth. In a clever...