Industrial and non‐industrial melanism in the peppered moth, Bistort betularia (L.)

. 1. Estimates of the relative crypsis of the melanic and typical morphs of B.betularia have been made at fifty‐two sites in southern England and south Wales and these estimates were compared with melanic frequencies in samples from these sites. 2. The decrease from east to west in the frequency of the melanic form, carbonaria , and increase in the frequency of a second melanic form, insularia , across the southern part of England and Wales is largely independent of changes in smoke concentration, sulphur dioxide concentration or relative crypsis. 3. When all the available information on the distribution of the melanics of B.betularia is considered, sulphur dioxide concentration is correlated with the geographic variation in carbonaria frequency. This, together with evidence of the close relationship between carbonaria crypsis and sulphur dioxide levels, indicates that outside southern England and south Wales, either selective predation, or some direct selective effect of pollutants, is of major importance in determining the variation in carbonaria frequency. 4. Some features of the pattern of the spread of carbonaria in England and Wales during the last century give reasons for expecting non‐industrial selective factors to be of greater importance in determining the distribution of melanics of B.betularia in southern England and south Wales than in the rest of Britain.