Fifty per cent and all that: what Haldane actually said

Abstract In 1924, J. B. S. Haldane used the observation of increasing melanic frequencies in peppered moths (Biston betularia L.) to illustrate strong selection in a natural population. Since the phenomenon was first observed, there has been criticism and misinterpretation of work on industrial melanism in moths coming from a number of directions, increasingly on the Internet. Haldane’s calculation, its reception and his other interests in peppered moths are reviewed. An example of Internet comments attributing opinions to him, and their origin and background, are discussed.