the second season
... that William Hartnell was the only original Doctor Who cast member who remained by the end of the second season?
... that <a href="./William_Hartnell" id="mwBtg" rel="mw:WikiLink" title="William Hartnell">William Hartnell</a> was the only original <i id="mwBtk"><a href="./Doctor_Who" id="mwBto" rel="mw:WikiLink" title="Doctor Who">Doctor Who</a></i> cast member who remained by the end of <b id="mwBts"><a href="./Doctor_Who_(season_2)" id="mwBtw" rel="mw:WikiLink" title="Doctor Who (season 2)">the second season</a></b>?