Using existing functions as part of my hooked function?

I’d like to be able to use a hook, but include part of the original functionality in the hooked function. Here’s my problem.

I’m doing my own authentication if the user is passed from a different site. The HTTP_REFERRER is checked, and if it’s the right site, the username and password are parsed.

I’m overriding wp_authenticate using:
add_action(‘wp_authenticate’, array(&$this, ‘authenticate’), 10, 2);

Problem is, if it’s from the current wordpress site, I’d like to use the existing login function. The above loses this.