After many decades of human manipulation, English bulldogs are so far from their natural ancestors that they commonly suffer from abnormalities, such as small windpipes and closed nostrils. Our efforts to breed so many of these dogs have diluted the bloodline, thinning and weakening it further with every generation. Rather than creating a sustainable breed of dog, we have opted for an unsupportable dog breed that cannot be maintained for much longer. The word manipulation has a connotation in this passage.