Domestic dogs come in a bewildering variety of shapes and sizes. They have
been selectively bred for millennia for various behaviors, sensory capabilities,
and physical attributes, including dogs bred for herding livestock (collies,
sheperds, etc.), different kinds of hunting (pointers, hounds, etc.), catching
rats (small terriers), guarding (mastiffs, chows), helping fishermen with nets (Newfoundlands,
poodles), pulling loads (huskies, St. Bernard's), guarding carriages and
horsemen (Dalmatians), and as companion dogs. Some kinds were even bred simply
as lap warmers (Pekingese). Their basic morphology though, no matter how
modified, is that of their wild ancestors, gray wolves.
Like many other predatory mammals, the dog has powerful muscles, fused wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing. Compared to the bone structure of the human foot, dogs technically walk on their toes.