This collection is primarily correspondence and also notes on experiments concerning the cytology research of Lewis and his wife, Dr. Margaret Reed Lewis, at Johns Hopkins University, Carnegie Institution of Washington, and finally at the Wistar Institute of Anatomy and Biology in Philadelphia. There is significant material relating to his professional society activity in the American Association of Anatomists (President, 1934-1936) and the International Society for Experimental Cytology.

This is a diverse collection, including correspondence, drafts of letters, notes and notebooks (on biometric methods; tables and formulae; science, nature and method; coefficient and correlation; vitalism; Japanese language); commonplace book (1924); autobiography; and over one hundred folders of unpublished writings. The correspondence and other material covers a variety of topics, including biology, eugenics, evolution and natural selection, human heredity, paramecia, protozoa genetics, U.S. immigration policy. There is much on the Seventh International Congress of Zoology (1907); letters to his father and wives, and from students and colleagues on his seventy-fifth birthday, 1943 (1 v.); diplomas and certificates of membership; and photographs.