What is the change that occurred in peppered moths?

In the 1800's in the UK (England), lots of factories were burning coal and wood which put up a bunch of smoke and soot in the air. Well, most of this had to come down somewhere. The trees in this area were a light greyish brown color originally. There were these light colored moths called peppered moths - they were whitish color with some light grey spots here and there. They blended in perfectly with the trees. There were other moths of the same species with different colors, mostly dark grey with whitish speckles, those were often eaten off the trees because they were easy to see. As the soot fell over many many years, it stained the trees a dark brown - blackish color. NOW, those dark grey moths blended in perfectly, and the light colored moths acutally started to die off because the birds could see them easier, so they ate them. Over time, the dark colored moths became more in numbers. Check out this cool site I use with my students: http://www.biologycorner.com/worksheets/pepperedmoth.html