Among the major influences in the 1940s were World War II, Morgan’s retirement, and the return of Beadle. 
The Division faculty and Divisional programs were well in place, and in full-scale operation by the 1940-1941 school year. The United States entered World War II in December, 1941, bringing normal operations to a halt. The Biology Division and its staff served in many ways during the war. The plant physiologists, including Bonner, spent the war looking for new plants that could be sources of rubber, to replace the Hevea brasiliensis plantations of Malaysia, which had been seized by the Japanese. Others joined the armed forces, Ed Lewis, for example, completing his Ph.D. in Biology under Sturtevant, then earning an M.S. in meteorology before shipping out to the Pacific as a navy meteorologist.
During the war years, no new members were added to the professorial faculty. Van Overbeek left (eventually becoming chair of the Biology Department at Texas A&M University), and Morgan retired (in 1942). The Division was without a chairman from 1942 until 1946, with Sturtevant serving in lieu of chair as Chairman of the Biology Council, a committee of the full professors (Borsook, Haagen-Smit, Sturtevant and Went). 
In 1946 George Beadle left his faculty position at Stanford to return to Caltech as the new Division Chair. He immediately began hiring new faculty, starting with Max Delbrück, who had spent the years between his departure from and return to Caltech on the faculty at Vanderbilt. 
Shortly afterward Norman Horowitz (former graduate student of Tyler at Caltech, then postdoc with Beadle at Stanford) and Ray Owen (from the University of Wisconsin) were hired as Associate Professors, and Ed Lewis as Instructor (1947) – he was soon Assistant (1948) and Associate (1949) Professor.
By the 1949-1950 school year Herschel Mitchell (from the Tatum lab at Stanford, then a Senior Research Fellow with Beadle at Caltech from 1946) joined as Associate Professor. Among the postdoctoral fellows and visiting researchers were Renato Dulbecco, Arthur Galston, Sam Wildman, Barry Commoner, Robert Bandurski, Seymour Benzer, William Drell, Melvin Green, Urs Leupold, S.E. Luria, Clement Markert, Roger Stanier, and others, who would become renowned biologists in the 1950s and 1960s.
The year 1949 also marked the opening of two new buildings for the Biology Division – the Biology Annex (the facility now under the Alles Patio) and the Earhart Plant Research Laboratory, torn down in the 1970s to make way for the Beckman Behavioral Biology building.
“The Division of Biology of the Institute has now been in operation twenty-one years. Its steady growth and development during this period is a tribute to the imagination and foresight of those who planned and established it. Its first Chairman, the late Professor Thomas Hunt Morgan, built its foundations well and guided its growth wisely for thirteen years…During the year the work of the Division was carried on by eleven full professors, eight research associates, seven associate professors, eight senior fellows in research, twenty-two research fellows, thirty-two graduate students and seventy-six research technical and laboratory assistants…” G.W. Beadle, Caltech Biology Annual Report, 1949.
