Incoming and outgoing manuscript, typescript, and carbon letters generated and collected by Cain throughout most of his career. Correspondence is almost entirely professional, with the exception of several letters written by his wife, J. T. Cain, during his 1953 expedition to the British Solomon Islands. The series is arranged alphabetically by correspondent's name and then chronologically within each file. When a correspondence file includes letters to a third party (i.e. not Cain), the name is indented on the next line.

There are cross references for all third party correspondence.

These files consist primarily of correspondence between Cain and his colleagues from the University of Liverpool and other academic institutions in Great Britain and abroad, and with students, publishers, editors, officers of professional societies, and admirers of his work in evolution, systematics, and genetics. In his correspondence, Cain frequently discusses his research, expeditions, grant applications, reviews of colleagues' journal articles and books, his own scientific publications, references for current and former students, requests for reprints, and invitations to attend and present papers at biological conferences and meetings.

Cain's correspondence records his wide range of research interests in evolutionary biology, systematics, and genetics. His work on gastropods and other molluscs and on birds forms an important core of the Series. Among Cain's most frequent correspondents were Bryan C. Clarke, George Morgan Davis, Ernst Mayr, Philip M. Sheppard, Mario von Cranach, and David S. Woodruff. The correspondence file for Collins Publishers contains Cain's manuscript versions of chapters written for the Reader's Digest Book of British Birds, 1968. Other manuscripts of Cain's are lacking from the collection.

For the most part, enclosures within letters have been retained with the original covering letter. Examples of such materials include unpublished scientific papers and articles, research notes, charts, graphs, maps, and forms for grant applications. Reprints, offprints, and photocopies of publications have been moved to the library's printed collections and are referenced in the card catalog under Cain's name. Photocopies of the title page of all transferred printed items have been filed with the original covering letter.

Materials which highlight the major scientific activities and accomplishments of Cain's career. The series is arranged alphabetically by subject; each subject file contains a variety of documents, including material such as minutes, reports, programs, graphs, maps, and miscellaneous research data.

A small amount of correspondence which originally appeared in the subject files has been added to Series I under the correspondents' names. Researchers interested in the following subject files are advised of related material to be found in Series I:

Series II, Animal Breeding Research Organization -- see also Series I, Sheppard, Philip M.

Series II, Hutchinson University Library -- see also Series I, Douglas, Ann (editor)

Series II, Science Research Council grant -- see also Series I, Arnold, Richard W.

Cain and Sheppard (whose papers are also housed in the APS Library) were both members of the Agricultural Research Council of the Animal Breeding Research Organization; consequently, information on this group is available within a subject file as well as Sheppard's correspondence file. Hutchinson University Library, responsible for the publication of Cain's Animal Species and Their Evolution, also retained Cain as an editor of their Biological Series from 1967-1975; much of Cain's correspondence with their editor, Ann Douglas, involves his reviews. Finally, details regarding the Science Research Council (SRC) grant received by Cain are located in a subject file as well as the correspondence file of his Senior Research Officer/Associate, Richard W. Arnold.

Subject files often record Cain's membership and participation in professional societies and organizations such as the Animal Breeding Research Organization (1965-1968), Genetical Society (1979-1985), Institute of Biology (1964-1977), Population Genetics Group (1967-1988), Serengeti Research Institute (1965-1969), and Systematics Association (1963-1984). These files generally document business such as minutes, reports, and programs of meetings. Other subject files exist for biological conferences and meetings attended by Cain. In many cases, files such as these contain papers given by Cain as well as colleagues at symposia. These papers remain within the subject file they were originally found. Other material within subject files for scientific meetings includes programs, lists of attendees, and notes taken by Cain.

Other subject files of note include the British Guiana Expedition (1952-1959) and the Science Research Council grant (1961-968). The former, an Oxford expedition conducted in 1959, holds preparatory material in the form of notes and reports. The latter involves a grant to study functional systematics through natural selection in the wild (particularly involving Cepaea) and contains detailed documents of the grant application process and reports as the research progressed.

A small selection of unpublished papers received by Cain throughout his career. These works are primarily undated typescripts in carbon or photocopy and are filed in this series because they were not found with any accompanying correspondence. Most papers of this nature were originally transmitted to Cain by the authors with letters discussing their contents. In these cases, such unpublished papers were filed in Series I with the correspondence of the author. When no cover letter was found, unpublished papers of colleagues were arranged alphabetically by author's name in Series III.