A long time ago, more White Moths were reproducing than Brown Moths. This was because the White Moths were able to camouflage into the white trees. Because of this, the Brown Moths were beginning to get eaten by the birds.
But once Power Plants were built, they created tons and tons of dark smoke. The smoke then eventually reached the trees which the moths were hiding on.
When the smoke had reached the trees, the colour of the tree had changed. The tree was now a dark green colour. This made the White Moths noticeable for the birds. Eventually, the Brown Moths started to reproduce again.