These are primarily letters to Gray on various aspects of natural history, with many references to collections at the British Museum of Natural History. There are numerous letters from British scientists and from European colleagues. There are letters from Americans too, such as those from Agassiz which relate to his Florida expeditions and the creation of natural history collections.

A miscellaneous collection of letters, mainly of English scientists, written to various people. There are mostly one or two letters, except for those of John Peter Gassiot, William Huggins, Edwin R. Lankester, James Mackintosh, David Prain, and William Whewell.