The collection is primarily professional letters written to Russell, but there are also a few letters from his wife and a few by him. There is also an autobiographical letter written by Russell. Most of the correspondence pertains to the Royal Meteorological Society, snow crystals, dew, frost, hail, fog, etc. Russell had wide-ranging interests, however, and some of his non-scientific concerns appear in the letters.

A prime exponent of evolutionary theory in the late nineteenth century, the British naturalist Alfred Russel Wallace arrived independently at the theory of natural selection nearly simultaneously with Charles Darwin. The numerous publications that emerged from his extended field excusions into the Amazon Basin and the Malay Archipelago (Indonesia) Wallace resulted in major contributions to evolutionary theory, biogeography, ecology, and ethnography, and made Wallace, by the end of his life, one of the best known naturalists in Britain. A Socialist, social progressive, and Spiritualist, Wallace's distinctive take on evolutionary change differed from the Darwinian mainstream in significant ways. The Wallace Collection is a miscellaneous assemblage of letters written by and to Alfred Russel Wallace, primarily during the last twenty five years of his life. Varied in content, the letters touch on Wallace's views on evolution, Spiritualism, and to a less degree, his progressive social commitments.

Thomas Henry Huxley (1825-1895) was an eminent Victorian-era biologist, best known as a passionate defender and popularizer of Darwin's theory of evolution. His partisanship earned him the nickname of "Darwin's Bulldog," although he did not accept the theory uncritically. In addition to his work in biology, he performed original research in zoology and paleontology. He is also remembered as the progenitor of a family of highly successful scientists and thinkers. This collection is made up of letters, primarily written by Huxley, on a variety of topics including the age of man, evolution, education, natural history, science, geology, spiritualism, and vivisection. The most voluminous correspondence is with Sir James Thomas Knowles (30 letters) and with Huxley's daughter, Ethel Huxley Collier, "Babs," (44 items). The 30 letters to Knowles appear in neither the Life and Letters, nor are they listed in Dawson's guide to the Huxley Papers at the Imperial College. The other major correspondents include Charles Edward C. Appleton, Matthew Arnold, William B. Carpenter, Sir Henry Cole, George Dixon, Sir John Donnelly, Thomas Campbell Eyton, Sir William Henry Flower, Sir Joseph Dalton Hooker, Leonard Huxley, Sir William Jenner, Sir Charles Lyell, Louis Compton Miall, Alphonse Milne-Edwards, Sir James Paget, Harry Quilter, Harry Govier Seeley, Alfred R. Wallace, and William Crawford Williamson.

The collection is a miscellaneous group of letters of mainly British scientists and physicians, purchased as an existing autograph collection. There are a few American signatures. The letters are primarily from the nineteenth century and focus on medical and geological topics, but also there are some earlier and later dates. In addition to the letters are anatomical drawings of surgery, sketches of bones, and one geological notebook.

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.