Sentence:  Close to the window, where he could get more than his fair share of fresh air, the other twin, James—the fat and the lean of it, old Jolyon called these brothers—like the bulky Swithin, over six feet in height, but very lean, as though destined from his birth to <WSD>strike a balance</WSD> and maintain an average, brooded over the scene with his permanent stoop; ... 
Meaning of the word marked with <WSD> tags:  To find a compromise between two conflicting things.