primogeniture:

Generally, the law or custom that the first-born or oldest surviving son should be the presumed inheritor. That’s why there are a lot of younger sons who go into the clergy or military to seek their fortunes. Edward Ferrars is the eldest son in Sense and Sensibility, however his mother irrevocably grants his inheritance to his younger brother Robert after she learns of Edward’s engagement to Lucy Steele. So the inheritance can be changed.