Before the industrial revolution, there wasn't much pollution/soot, and the moths were mostly light colored. When humans started using machines for everything, soot coated the trees and the light colored moths were easier for birds (predators) to see. The moths became mostly dark colored, because it was better camoflauge.
Now that we have environmental restrictions, there isn't as much soot everywhere. Now moths are mostly light colored again, because the dark moths are easier to see.
