It has been suggested that the demonstrated relationship between melanism and pollution can not be fully proven because the exact reason for increase in survivability can not be tracked and pin-pointed. However, as air quality has improved in industrial areas of America and Britain, through improved regulation, offering the conditions for a natural experiment, melanism has sharply declined in moths including "B. betularia" and "Odontopera bidentata". Cook and J. R. G. Turner have concluded that "natural selection is the only credible explanation for the overall decline", and other biologists working in the area concur with this judgement.