What are you talking about? Jacks are adorable (unless you're a cat or a fox).
To people, yes.
To dogs, jacks are a menace. If you are attacked by a pitt, the only thing better than another pitt for defense is probably a jack russel. He will lose, but he is fast enough to buy you enough time to get a weapon or get away. He is also just as ornery and bull headed in a fight as a pitt.
Or a person.
Jack Russell terriers are yappy ill-mannered vicious little POS dogs that should have been bred out of existence before they actually DID exist.