The replacement of grey coloured moth by dark coloured melanic species due to industrial smoke is called industrial melanism. Initially on the lichen covered tree trunks, the mutant moths was more conspicuous due to its black colouration and was therefore more susceptible to predation by birds. The large scale burning of coal during industrial revolution resulted in the deposition of sooty particulate matter on tree trunks. Hence the grey coloured moths became more conspicuous to predatory birds. Thus the population of black moths increased considerably.