Question

What was the cause of the increase in the numbers of black moths in England in the 1800s

Answer 1

Natural selection        is the answer

Answer 2

"Like many moths in forests, the peppered moth tends to rest (or "perch") on tree trunks during the day. They do most of their flying at night. So it would probably be a good thing if the moths look similar to the trees that they perch on, right? Then they can be camouflaged from birds that want to eat them.

Before the Industrial Revolution, the light peppered moth was common, while the dark form was very rare. The light moths blended in with the light-colored trees. However, the Industrial Revolution changed the tree colors.

Dark form of the peppered moth

After the pollution from the Industrial Revolution started affecting trees, most of the collected peppered moths were of the dark form. Click for more detail.

As the trees darkened with soot, the light-colored moths were easier to see. They were eaten by birds more and more, while the rare dark colored moths blended in better on the darker trees. This made the dark colored moths have a higher survival rate. They lived longer and passed their dark colored genes onto their offspring or young. " got this from https://askabiologist.asu.edu/peppered-moth use that link if you need more of a explanation