These letters, from the Wellcome Historical Medical Society's collections, are on a variety of topics, primarily natural history and medicine.

Collection of 283 letters assembled and presented to the Academy of Natural Sciences of Philadelphia by John Torrey. Correspondents include: Louis Agassiz, Zaccheus Collins, Peter S. Du Ponceau, Elie M. Durand, Asa Gray, William Hembel, Alexander von Humboldt, William H. Keating, James Mease, Samuel G. Morton, George Ord, Charles Pickering, Constantine S. Rafinesque, Benjamin Silliman, Sr., John Torrey, Charles Waterton, Alexander Wilson.

This collection includes letters to and from Americans (including South America and the West Indies) and letters about America selected from the Society's manuscripts (Classified Papers, Letter Books, Letters and Papers, Royal Society Letters, Miscellaneous Correspondence, and other official groups) and from collections of private papers (Sir Charles Blagden, William Buckland, John Canton, Sir John F. W. Herschel, Sir Edward Sabine, and others). The documents range in time and character from John Winthrop, Jr., "A Description of the Artifice & Making of Tarr & Pitch in New England" (1662), to letters from Sir Thomas Edward Thorne to his wife describing the American West, where he was on a surveying party in the 1880s.