Harold Garnet Callan, known to family, friends and colleagues as "Mick", was born in 1917 in Maidenhead, Berkshire. He was educated at King's College School, Wimbledon and graduated from St John's College, Oxford in June 1938. In September 1938 he was awarded a scholarship for postgraduate research in Naples where he became impressed by lampbrush chromosomes and excited by the potential they offered for studying the organisation of hereditary material and gene action. The outbreak of war interrupted his research and he left in June 1939. He then went to work at the John Innes Horticultural Institute at Merton, Surrey until May 1940.
From 1940 until September 1945 Callan was employed by the Ministry of Aircraft Production. He trained in radar electronics and then went to an anti-aircraft gun site at Wingham, Kent. In 1941 he was transferred to the installation and operational development of Mark VII Aircraft Interception gear in Beaufighters. In late 1941 he returned to Telecommunications Research Establishment at Worth Matravers, Dorset and trained on a blind bombing and navigational device. He was then sent to 35 Squadron of Bomber Command to work on the fitting and operational use of this instrument in Halifax bombers of the Pathfinder Force. In early 1943 he was seconded to the Royal Navy to join a flotilla of rocket-firing tank-landing craft and given an Honorary Commission in the R.A.F. as a Flight Lieutenant. Callan stayed in Gibraltar until shortly after the invasion of Sicily, in which the flotilla was engaged. He was recalled to the Telecommunications Research Establishment to work first on a night fighter tail warning system fitted in bombers and later on a blind bombing and navigational device for the bombing of Berlin. In January 1944 he was promoted to Squadron Leader and posted to 614 Bomber Squadron to help with the formation of a pathfinder force flying out of Foggia, Italy. In May 1944 he was appointed Scientific Adviser to the signals section of Mediterranean Allied Air Forces and spent most of his time at Caserta, Italy at which time he took the opportunity to go to Naples and marry Amaryllis Dohrn.
In October 1945 Callan returned to the John Innes Horticultural Institute for temporary employment which he left in April 1946 to work as a Research Assistant to Professor Sir AV Hill at the Biophysics Department, University College London. Callan was not happy there and accepted a position offered by Professor Conrad Hall Waddington at the Agricultural Research Council, first at Hendon and later at the Institute of Animal Genetics at Edinburgh University.
In September 1950 Callan was appointed to the Kennedy Chair of Natural History at the University of St Andrews, successor to D'Arcy Wentworth Thompson. This was his first teaching post, and he was to remain in it until his retirement in 1982. He found the first year of teaching difficult but he had a flare for teaching and was enthusiastic in his belief in the dual role of teaching and research. He built up the Department of Zoology from a few staff into a thriving modern department. He was responsible for the additional building work to the Bute Medical School and instigated the remodelling of the Bell Pettigrew Museum (which was carried out by David Burt in the 1960s). Callan ruled the Department as a "benevolent dictator" and was sorely missed when he retired. His early retirement at 65 was voluntary, to protect the jobs of younger scientists, a decision which was characteristic of his generosity. However he did not retire from science and was given laboratory and office space at the Gatty Marine Laboratory, St Andrews. He used the next four years to work on his monograph, Lampbrush Chromosomes (1986). Once the monograph was published he was able to re-immerse himself in research which he did on frequent visits to Baltimore to work in Joseph Gall's laboratory. He continued in this vein until 1992 when he was diagnosed with diabetes and he decided to retire from science. He was honoured by the University of St Andrews in 1984 with an honorary degree of Doctor of Science and again in 1988 when he was given the honorary title of Research Professor.
Callan's research interests centred upon the lampbrush chromosomes of amphibians. He specialised in cell physiology undertaking ground-breaking research into genetics and DNA. He was the first to demonstrate the linearity of DNA along the chromosome. His work attracted world renowned scientists to come to St Andrews to work with him including: Joseph Gall; Dennis Gould; Ronald Hill; James Kezer; Georgio Mancino; Oscar Miller; Ulrich Scheer; Herbert Taylor and Charles Thomas. He travelled around the world to collaborate with other scientists and to lecture and teach, in Italy, United States, Russia, China, Ecuador and Australia. In addition, Callan made frequent short visits to various places to attend conferences and give lectures and courses. He was a strong supporter of international exchange and collaboration.
In contrast to the trend of modern academic science where there is great pressure to keep up a furious publication rate, Callan published only 88 papers during his career from 1938 to 1992, and just one book. Callan was also keen to see quality papers published by other scientists and served as editor of the Quarterly Journal of Microscopical Science from 1960 to 1964, an experience which proved invaluable when he became one of the two founding editors of the Journal of Cell Science in 1966.
Callan was honoured nationally and internationally. In 1963 he was elected to the Royal Society of London in recognition of his research. He served as a member of the UK Advisory Council on Scientific Policy from 1963 till 1964 and as a trustee of the British Museum from 1963 to 1965. He was a member of the Science Research Council, 1972-1976. In 1974 he was elected as a foreign honorary member of the American Academy of Arts and Sciences and in 1982 as a foreign member of the Accademia Nazionale Dei Lincei.
Callan was an outstanding scientist, but science did not dominate his life. He worked a nine to six, five day week. He never worked evenings or weekends. He spent his leisure time fishing, hunting, shooting or with his wide circle of friends. He and Amaryllis were famous for their generous hospitality and for their happy home which consisted two daughters, one son, and later many grandchildren. Callan has been described as: "a man of immense and solid integrity' (Herbert Macgregor) and "a straightforward, down-to-earth man, who had the time of day for all with whom he came in contact' (St Andrews Citizen, 12 November 1993, p.11). He was widely respected and loved by colleagues, friends and family.