INSERT INTO from a SELECT query

I am trying to INSERT multiple rows to a table using a stored procedure
something like this:

CREATE FUNCTION test(varchar) RETURNS int2 AS '
DECLARE
id_list ALIAS FOR $1;
BEGIN
INSERT INTO history (media_id, media_type) SELECT media.media_id,
media.media_type WHERE  media.media_id IN (id_list);
.
.

So I would call this function by passing desired media_id's to be put in the
history table like this:
SELECT test( '24,25,26,27' );
In the INSERT statement, I am taking values from a table called Media, and
adding a row to a table called History. In this example, the only rows
copied would be rows where the media_ID was 24,25,26 or 27.  This function
is working for me, but it only works if the varChar being passed has only
one value, like this:
SELECT test('24'); This works.
But when I try to pass more then one value in the list ( '24,25'), the
function runs with no errors but does not add the rows, it does nothing.

What am I doing wrong? Do I have the syntax wrong for using INSERT with a
SELECT-IN statement?