Final answer:
The historian would focus on the availability of high wages in northern factories and the push and pull factors that influenced the African American population shift.
Step-by-step explanation:
The historian would focus on two themes as he researches his hypothesis about the African American population shift from southern rural areas to large urban centers in the North between 1910 and 1930. The first theme would be the availability of high wages in northern factories. African Americans were attracted to these urban areas because they offered better job opportunities and higher wages compared to the rural South. The second theme would be the push and pull factors that influenced the migration. Push factors such as racial segregation, limited educational opportunities, and economic hardships in the South pushed African Americans to leave, while pull factors such as better schools, voting rights, and the absence of legally enforced segregation in the North attracted them.