Darwin’s thinking about evolution was not only deep, but also very broad. He was interested in fossils, animal breeding, geographical distribution, anatomy and plants. “That very comprehensive view allowed him to see things that others perhaps didn’t,” says Robert J. Richards, a historian at the University of Chicago. “He was so sure of his central ideas — the transmutation of species and natural selection — that he had to find a way to make it all work together.”
Richards’s recent book The Tragic Sense of Life: Ernst Haeckel and the Struggle over Evolutionary Thought traces the life of Haeckel, a contemporary of Darwin who did much to popularize Darwin’s work.