gf.net Code // Was ist falsch?

CODE HIER FEHLER NACH DEM CODE

<!DOCTYPE HTML>
<html>
    <head>
        <title> Test </title>

    </head>
    <body>

        <script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
        <script type="text/javascript" src="http://underscore.org/underscore.js"> </script>
        <script type="text/javascript" src="http://backbonejs.org/backbone.js"> </script>

        <script type="text/javascript">
       var  Person = Backbone.Model.extend({
        initialize: function(){
            alert("Hallo");
        }
    });

    var Person = new Person({ name: "Name", age: 12, child: 'Vincenzo'});

    var age = person.get("age");
    var name = person.get("name");
    var child = person.get("child");

        </script>


    </body>
</html>


___________________ Fehler ______________

SyntaxError: syntax error
file:///Users/vincenzofloder/text-2.html
Line 14

TypeError: _ is undefined
http://backbonejs.org/backbone.js
Line 227

TypeError: Backbone.Model is undefined
file:///Users/vincenzofloder/text-2.html
Line 14

_________ Danke :)