GREY'S ANATOMY - "Invasion" - The residents of Seattle Grace and Mercy West vie for surgeries and their careers as day one of the hospital merger begins. Meanwhile, Arizona stands by an enraged Callie when her father, Mr. Torres, employs a priest to help convince her to date men, and Izzie experiences the brunt of her new surgical competition, on "Grey's Anatomy," THURSDAY, OCTOBER 15 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/ERIC MCCANDLESS) JUSTIN CHAMBERS, KATHERINE HEIGL