Asa Gray’s extensive correspondence with naturalists shaped the early years of exploration and botanical research in North America. These letters brought Gray into contact with Charles Darwin, who became a close friend. With the publication of Darwin’s On the Origin of Species, Gray soon emerged as the key proponent and defender of natural selection in the U.S.  On Thursday, October 21, beginning at 6 pm, Janet Browne, Aramont Professor of the History of Science at Harvard, will discuss their letters , which reflect a warm personal relationship as well as the making of an intellectual revolution. Free and open to the public, Geological Lecture Hall, 24 Oxford Street. Part of the Asa Gray Bicentennial series.  For more information, log on to www.hmnh.harvard.edu.