Recent work on the rise of science in the nineteenth century has encouraged historians to look again at the role of correspondence. Naturalists relied extensively on this form of contact and correspondence was a major element in generating a community of experts who agreed on what comprised valid knowledge. As a leading figure in the development of North American botany, Asa Gray found that letters with botanists and collectors all over the world greatly expanded his areas of influence. Lasting friendships were made and the collections at Harvard were materially advanced. Letters also brought Gray into contact with Charles Darwin, who became a close friend. After publication of Darwin's Origin of Species in 1859, Gray defended Darwinism in the United States and corresponded with him about evolution. This article sets Gray's correspondence with Darwin in the context of the reception of Darwinism in the United States.