"If I live till I am 80 years old," Charles Darwin wrote after fin- ishing his first book, "I shall not cease to marvel at finding my- self an author." That book was The Voyage of the Beagle (1837), in which Darwin recorded his experiences and observations as a naturalist on the Beagle's globe-circling journey, begun 150 years ago. From this trip came much of the raw material and in- spiration for Darwin's great work, On the Origin of Species (1859), in which he first propounded...