The tenure of any office not herein provided for may be declared by law; when not so declared, such office shall be held during the pleasure of the authority making appointment, but the legislature shall not create any office the tenure of which shall be longer than four years, except that appointments under a merit system in civil service shall not be subject to such limitation. The legislature may make provisions for a merit system under which appointments and promotions in the civil service of this state and all civil divisions thereof, shall be made according to merit and fitness, to be determined, so far as practicable, by examination, which, so far as practicable, shall be competitive.