The breed history is wrapped in legend. The parson favored a dog that was bold enough to follow the fox into its earthen den but not so aggressive that it would kill the quarry. Breed fanciers describe their favorite as a strain of fox terrier, kept pure from the early days, much as fanciers of some hunting breeds have continued field strains of their breeds that today differ from the dogs seen in the show ring. The Jack Russell Terrier is first and foremost a working dog, so character and skill are more important than uniformity of size and coat type. The dog must appear balanced and be in fit condition.
Ears are moderately thick with the flap tipped forward towards the front of the skull.
The chest is relatively shallow and narrow, giving an athletic appearance. Movement should be free, lively, and well-coordinated. Faults in the breed include shyness, disinterest, overly aggressive, lack of muscle tone, and lack of stamina or lung reserve. The Jack Russell is a fearless, happy, alert, confident, intelligent and lively hunting dog. These qualities make him a sturdy, vigorous companion, ready to meet the world on a moment's notice, and, unless he is appropriately trained and exercised, can be subverted into wanton destructiveness.