Explain the increase in the numbers of melanic moths in the urban areas of post-industrialization period in England

Industrial melanism is an adaptation where the moths living in the industrial areas developed melanin pigments to match their body to suit the covered surroundings.
The moths exist in two strains :

  1. Light coloured or whitel
  2. Melanic black.
    After industrialization barks got covered by smokers, so white moths were selectively picked by birds. But black moths were escaped unnoticed so they manage to survive, resulting in more population of black.