My roommate's two year jack russell terrier, Chance, has recently become very territorial towards my 10 week old aussie, Remi. When I first got Remi two weeks ago, it took about three days before Chance would play and run around with him. But for the past two/three days Chance has become very agressive towards him when in the house. If their outside they play together, and are fine most of the time. There are times when playing fetch that he will growl and avoid Remi. Chance will bare his teeth and let out a little growl but only a few times has he ever snapped at him. Now we keep them apart most of the time. We aren't sure what to do, as I have never had an agressive dog, and this is my roommates first dog. Chance has gone as far as to pee on my pillows, and just about anything else in my room. And he only does it when we're not around. Is there anything we can do to Chance to at least tolerate Remi, and anything we can do to teach Remi when to leave Chance alone?