During post-industrialisation period (in 1920) in England, there dark-winged moths became dominant in numbers because

Before industrialisation set in, thick growth of almost white-coloured lichen covered the trees. In that background, the white-winged moth survived but the dark-coloured moths were picked out by predators. During post-industrialisation period, lichens which were sensitive to pollution were killed. The tree trunks became dark due to industrial smoke and soot. Under these conditions the white-winged moth did not survive due to predators, dark-winged or melanised moths survived as they could camouflage themselves (hide in the background).