From a blog about the science of dogs, a comparison of photos of purebred dogs from 1915 to those of today. You can see how much the dogs have changed in just under 100 years, in some cases for the worse. For instance, the difference in the Bull Terrier (aka the Spuds MacKenzie dog) is marked and a bit disturbing:
Pure breeding has also introduced medical problems for some breeds.
The English bulldog has come to symbolize all that is wrong with the dog fancy and not without good reason; they suffer from almost every possible disease. A 2004 survey by the Kennel Club found that they die at the median age of 6.25 years (n=180). There really is no such thing as a healthy bulldog. The bulldog’s monstrous proportions makes them virtually incapable of mating or birthing without medical intervention.