The dog is the first animal domesticated by humans, and we have done a brilliant job creating a variety of dog breeds to meet our needs. Selective breeding, which began roughly 9,000 years ago and expanded dramatically during the Victorian era, has resulted in dogs specialized for herding, hunting, retrieving, ratting and companionship. We also created a single species ranging in size from two to 200 pounds. While veterinarians love the variety of dogs we care for, the challenge for us is to understand all the variations that make up normal dogs in order to provide excellent veterinary care. Here are a few of the issues in medical care for species with a 100x range in size.