There are 153 letters in this collection, many addressed to Aubrey Lackington Moore, an English writer who tried to reconcile evolution and traditional Christianity. Some letters discuss scientific questions of the day, while others are formal social notes. Also includes 24 clipped signatures, inlcuding those of Sir Henry Wentworth Acland; Goerge Bentham; H. A. Bruce; S. H. Butcher; Matthewss Duncan; Percy Gardner; George Gritchett; G. Grote; Alfred Cort Haddon; Sir Prescott Gardner Hewitt; William Guyer Hunter; Thomas Henry Huxley; John Lingard; Thomas Babington Macauley; Miss K. Aubrey Moore; Louis Pasteur; Sir John Russell Reynolds, bart.; Thomas Smith; James Leigh Strachan-Davidson; Agnes Strickland; and Arthur Thomason.

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.

One of the most important natural historians in nineteenth century Britain, Charles Darwin provided the first compelling mechanism to account for organismal evolutionary change. Although lacking a coherent model of heredity, Darwin's natural selection has exerted an enormous influence over the biological sciences and since the introduction of Mendelian genetics, had remained the key unifying principle in the discipline. The APS Darwin Papers are a large a valuable assemblage of Darwin's correspondence with scientific colleagues, including Charles Lyell and George J. Romanes. They are included in the print version of the Correspondence of Charles Darwin (Cambridge Univ. Press).

There is correspondence concerning geology, botany, natural history, natural selection, evolution, British science and scientists, and other scientific topics. Most of the letters are written by Lyell, with a very few items by others.