In the history of most breeds there was a period when crossbreeding was allowed, or at least ignored. There were also many instances when a deliberate cross was made to another breed.
In the late 18th century, a dog named Trojan was one of the top foxhound stud dogs. He was a reject from a Harrier pack.
In the 1890s, the Earl of Antrim is recorded to have shown two dogs at a show, winning Best of Breed with each. One was a Dandie Dinmont, and the other a Bedlington. The dogs were littermates.
At the turn of the 20th century, the defining difference between Cocker Spaniels and Field Spaniels was weight: A dog less than 25 pounds was a Cocker, while one more than 25 pounds was a Field. There was a saying: “Before breakfast a Cocker, and after breakfast a Field.”
These are not isolated instances. Scotties, Westies, and Cairns were all sorted out from the same source. Believe it or not, the Skye Terrier and the Yorkshire Terrier share a common ancestor in the extinct Clydesdale Terrier.
In general, it can be said that our breeds were developed from common sources; these dogs were manufactured by dedicated fanciers in the image and for the function that they wanted. An interesting example is the corgi breeds, which started from totally different origins. In the 19th century, when interbreeding was common, the two breeds became more and more alike. It was not till the 20th century that dedicated breeders recreated them as two distinct breeds.
As you can see, there is a very good reason for judges to study the history of a breed: in order to know and instantly recognize any sign that a dog is reverting to one of its earlier ancestors.
It is crucial that every judge place the utmost importance on recognizing and rewarding those characteristics peculiar and individual to that breed. And the closer the breed being judged is to another breed, the more important are the differences.
A judge should know and understand every breed he judges, and he should also know all related and ancestral breeds. All dogs have faults, but those faults that result in a loss of breed type or a reversion to the past are by far the most serious.
—D.M., “The Judge’s Eye,” AKC Gazette