The Jack Russell Terrier is a small terrier that has its origins in fox hunting in England. Jack Russells are an energetic breed that rely on a high level of exercise and stimulation. They are relatively free from any serious health complaints. Originating from dogs bred and used by the Reverend John Russell in the early 19th century, from whom the breed takes its name, the Jack Russell has similar origins to the modern Fox terrier. As it is primarily a working terrier, its most important physical characteristic is its chest size, which must not be so large that it prevents the dog from entering and working in burrows. The red fox is the traditional quarry of the Jack Russell terrier, so the working Jack Russell must be small enough to pursue it. Their high energy and drive make these dogs ideally suited to a number of different dog sports such as flyball or agility. Obedience classes are also recommended to potential owners, as Jack Russells can be stubborn at times and aggressive towards other animals and humans if not properly socialized.