How to find radio button selection when a form is submitted?

<script type="text/javascript">

function findButton() {

var myForm = document.forms.animalForm;

var i;

for(i=0;i<myForm.marsupial.length; i++) {

if(myForm.marsupial[i].checked) {

break;

}

}

alert("You selected \""+myForm.marsupial[i].value+"\".");

}

</script>

<form name="animalForm" action="">

<input type="radio" name="marsupial" value="kangaroo" />Kangaroo

<br /><input type="radio" name="marsupial" value="Opossum" />Opossum

<br /><input type="radio" name="marsupial" value="Tasmanian Tiger" />Tasmanian Tiger

<input type="button" name="GO" value="GO" onclick="findButton()" />