In his popular and widely praised Running to the Mountain, Jon Katz wrote of the strength and support he found in the massive forms of his two yellow Labrador retrievers, Julius and Stanley. When the Labs were six and seven, a breeder who'd read his book contacted Katz to say she had a dog that was meant for him -- a two-year-old border collie named Devon, well bred but high-strung and homeless. Katz already had a full canine complement, but in