Jack Russell terrier, breed of dog developed in the 19th cent. by an English clergyman, the Reverend John (Parson Jack) Russell, 1795–1883, for hunting. The Jack Russell resembles the fox terrier and, like it, has two varieties, one with a short smooth coat, the other—the Parson Jack Russell terrier—wirehaired. It is a smaller dog than the fox terrier, however, weighing 9 to 18 lb (4 to 8 kg) and standing about 10 to 15 in. (25 to 38 cm) at the shoulder. The color is white with reddish brown, black, or tan markings. The Jack Russell has dark, almond-shaped eyes and V-shaped ears that are carried forward. The terrier is not a recognized American Kennel Club breed and less emphasis has been placed on its conforming to a breed standard than on its abilities as a hunting and companion dog. The breed can become aggressive and destructive if not given the attention and exercise required by a dog bred for hunting.