In setting official standards for mutts, everyone has used the guiding principle that dogs belong to the same species, called the Canis familiaris. All dogs come equipped with the remarkable blend of high intelligence, fierce loyalty, human-like emotions, boundless love, and the ability to smell leftover meat two blocks away.
It is us humans that have established differences among dogs. Some dogs were selected over centuries for astonishing traits of size and temperament. These dogs were bred for high purposes, such as the Mastiff for defending the Roman Empire, the Newfoundland for rescuing sailors, the Chesapeake Bay Retriever for retrieving ducks, and the Saint Bernard for saving lost mountaineers. These are purebred dogs.
To create a purebred, all matters are arranged by a responsible breeder. On the other hand, other dogs, such as strays, neighborhood dogs or most dogs who have ever lived have met and mated as they saw fit, without the help of humans. This is evident by simply looking at them. These are called, simply, “dogs.”
All dogs are capable of the greatest genetic elasticity among mammals, providing a dazzling array of coat types, head types, tail configurations, and placements. Dogs astonish us with their rich diversity, the 250-pound Mastiff and the five-pound Miniature Poodle are members of the same species, a heart the size of a peanut and a heart as big as a fist both beating with love for us, their best friends. Breeders can’t perform this trick with other types of species.
In the last century or so, dog breeders, carefully molding remarkable lumps of canine clay, have turned out an amazing array of more than 450 distinct breed types, although only 146 breeds and varieties are formally recognized by the American Kennel Club. Dogs meeting dogs for entirely unsupervised dates have done it a little differently. For example, Yorkies serenading a song of seduction to strong, silent Shepherd types, and Terriers falling for Chihuahuas. The results of this dizzying merry-go-round of random matches is nothing less than the richest, maddest, crazy-quilt variety of mammals of a single species on Earth: mixed-breed dogs. Surely there are thousands of types of mixed-breed dogs, roughly familiar, and yet each one unique, like snowflakes.