Jack Russell terriers are yappy ill-mannered vicious little POS dogs that should have been bred out of existence before they actually DID exist.
WTF???
We'll see if you say that when you've got a rat or mice problem. They were bred for a purpose and a damned good one. If not for dogs (and cats), we'd never have been able to sustain an agrarian society. My dad's cow dogs often made the difference between profit and loss by the end of the year. They were well-mannered and quiet, but would have torn your hand off if you'd done anything to threaten my dad, his property, his livestock, or his kids. But those kinds of dogs deserve big homes; folks on small properties owe it to their pets to have small dogs.
If I was going to have a dog, there's a good chance it would be a JR -- aggressive, nasty, and as metmom says, small enough for anyone to drop-kick across the room if need be.