阅读理解
Natural selection is the process by which one type of animal within a species thrives because of certain characteristics that make it more likely to live than others in its group. The history of the peppered moth（灰蛾）is an example of the natural selection process.
In 19th century England, certain types of peppered moths were able to better blend（融合）into their surroundings. During that time period, great changes were happening in Great Britain. The Industrial Revolution was part of this change, and with it came air pollution. Natural selection often takes hundred or even thousands of years to occur. For the peppered moth, this process occurred comparatively quickly.
At the beginning of the Industrial Age, most peppered moths in England were light-colored and covered with black markings, although a few moths had dark-colored wings. Because the light-colored moths blended into the light-colored bark on the trees, they could not be easily seen by birds that would eat them. As the air grew more polluted, however, tree trunks became covered with soot（烟煤）and became darker. The light-colored moths became easy for birds to see against the dark tree trunks. Since the dark-colored moths now had the advantage, their numbers grew. Within 50 years, the peppered moth went from being mostly light-colored to being mostly dark-colored.
In the 20th century, the air cleared up, and the peppered moth population changed again. As tree trunks lighted due to less soot in the air, light-colored moths once again had an advantage. Their numbers increased as soot levels declined. Depending on their environment, the coloration of the moths helped them to be “naturally selected” to survive.
