A classic case demonstrating natural selection is referred to as industrial melanism. A change occurred in a number of moth species populations in the late 1800ís in industrialized areas of England and the United States. Prior to this period, moth populations were primarily composed of light colored mottled moths with occasional mutant dark colored (melanic) moths. During this period, over a number of generations, the populations changed such that the majority of moths were dark colored. This could be explained by a change in the bark of trees in the woods surrounding the industrial areas. The bark had gone from being whitish in color to a soot-covered black. Moths rest on bark during the day. Light colored moths were now conspicuous to birds whereas dark moths were not. Dark colored moths therefore had higher fitness and left more offspring for the next generation.