We rescued a French Bulldog/Pug/Jack Russell mix. I think he was a attempt to breed a more robust/less unhealthy pug - failed in that he's not very pug-like, but successful in that he's a delight and very healthy.
And that's because they've been bred for use, not for looks for tens of generations. As is proven by the high variance on dogs you can call "Jack Russell". You get tiny ones with stubby-ish legs, you can tall ones with long legs, you have completely smooth haired ones and the ones that look like a ball of wire wool. =)
Source: have owned JR's and still do.