Your question intrigued me as I'd never heard of a Parson Russell, so I looked it up. It appears the Parson is taller and lighter in weight, having a slightly squarer face and coarser coat. The longer legs were bred so that they could accompany horses on hunts, presumably to run faster. I'm now trying to find out more! As a matter of trivia, the French are really keen on Jack Russells; many of them are called d'Jack! The d' is to overcome the soft French pronunciation of the letter j - weird, eh?!