Selecting multiple <option> within a <select>

HTML

<select multiple id="foo">
    <option value="1">Cat</option>
    <option value="2">Dog</option>
    <option value="3">Fox</option>
    <option value="4">Hen</option>
    <option value="5">Albatross</option>
</select>
<span id="dog">Also select Dog and Hen</span>

CSS

select { height:200px; }
span { 
    text-decoration:underline;
    cursor:pointer;
    color:Blue;
}

JavaScript

$("#dog").click(function(){
    $("#foo").val($("#foo").val().concat(1, 2));
});