  Kettlewell found that carbonaria was twice as likely to survive in soot-darkened environments (27.5 percent) as was typical (12.7 percent). He worried, however, that his results might be an artifact of his experimental procedures. Perhaps the traps used were more attractive to one type of moth, that one form of moth was more likely to migrate, or that one type of moth just lived longer. He eliminated the first alternative by showing that the recapture rates were the same for both types of trap. The use of natural boundaries and traps placed beyond those boundaries eliminated the second, and previous experiments had shown no differences in longevity. Further experiments in polluted environments confirmed that carbonaria was twice as likely to survive as typical. An experiment in an unpolluted environment showed that typical was three times as likely to survive as carbonaria. Kettlewell concluded that such selection was the cause of the prevalence of carbonaria in polluted environments.