Random samples of B. betularius males show a clinal variation in the degree of melanism: from high melanic frequencies in Denmark and south-west Sweden to almost no melanism in south Finland. Old samples from Denmark dating back to the end of the 19th century have strikingly lower melanic frequencies than recent samples showing an increase in melanism during the 20th century in NW Europe. Levels of air pollution and melanic frequency coincide fairly well indicating that the spread of melanism is a response to increased air pollution as also is demonstrated elsewhere. In Britain, in central Europe, and in USA a black morph has evolved whereas in NW Europe the grey morph (insularius) is the predominating melanic form indicating that evolution of melanism in B. betularius has followed its own course in NW Europe.