Eaton Vance Corp.’s chief financial officer and treasurer is resigning, based on a “mutual decision’’ that it is time for him to leave, the Boston investment fund manager reported Tuesday. The duties now handled by Robert Whelan will be divided between two executives. A spokeswoman said Whelan was not fired. He was in the job almost five years. A statement from the company said Whelan, 50, “resigned to pursue other opportunities.’’ Laurie Hylton, chief accounting officer since 1997, will take the role of CFO. Daniel Cataldo will take the job of treasurer, reporting to Hylton. He has been director of financial planning and analysis since 2000.