basic function question

hi guys

Im not too hot on js so this script does not do exactly what i want it to.

i have a drop down form and i want it to add a value to a form area field every time its clicked on

so drop down has say two values 'one' and 'two'. if user selects 'one' the word one is added to the form area field. if user then selects 'two' from the drop down the area field will then say onetwo

at the moment my script just replace one with two or vice versa it wont leave them both there

variable is the drop down field
copy is the area field

function variable_selector() {
window.document.compose.copy.value = window.document.compose.variable.value;
return false;
}

any idea? help would be great!

Japes