A bit of history...
Jack Russell Terriers are a short stocky breed originating from Britain and it was originally known as Fox Terrier. It was renamed on the late 1800's after Reverend John Russell who bred with care, a predominantly white terrier that followed the hunt on foot and went to ground after the fox. Russell had a passion and understanding for the breed that is still highly admired today.
It is said that while he was in Oxford, Russell was intrigued by a female British White Terrier named "Trump" that was owned by a milkman. What made Trump unique for a white terrier was that she had dark tan spots over her eyes, ears and tail. When the young Russell saw the dog, he was immediately struck with an idea, and bought her from the milkman on the spot. Russell would soon use Trump as the foundation for his breeding program, breeding her with the standard Fox Terrier of the time in order to create a new, more distinctive coloring pattern, while still retaining the desirable features already present in the breed. After several generations and 12 years of careful breeding, Jack Russells became popular among British aristocrats.
Up to the late 1800's Jack Russells where mainly used as working dogs for the fox hunt (one of the most popular sports in England) and the first major change for the breed was during World War II when the breed made its transition from working in the field to the household pet.
The breed developed specific features between each branch of its family tree. Eventually these differences have begun to be referred as signs of two separate, but related breeds: the Jack Russell Terrier (pet breed) and the Parson Russell Terrier (this one being the one recognized as the "show breed"). Both of them are part of the same genetic stock, but due to selective breeding, Parsons have longer head, large chest, a slightly larger body size and more obedient, making it more apt for shows.
The breed developed specific features between each branch of its family tree. Eventually these differences have begun to be referred as signs of two separate, but related breeds: the Jack Russell Terrier (pet breed) and the Parson Russell Terrier (this one being the one recognized as the "show breed"). Both of them are part of the same genetic stock, but due to selective breeding, Parsons have longer head, large chest, a slightly larger body size and more obedient, making it more apt for shows.