In 1857, Charles Darwin, now 48 years old, had not yet published his theory of evolution. On this day, he sent a letter to Asa Gray, a Harvard botanist, discussing his theory. The encouragement which followed from Gray and others, and new knowledge that Alfred Russell Wallace had independently developed the same theory, prompted Darwin to end 20 years of indecision and publish his ideas. more From So Simple a Beginning: Darwin's Four Great Books, by Charles Darwin, Edward O. Wilson. - book suggestion.