One demographic change that took place in the u. s. in the mid 19th century was an increase in

There was a lot of immigration because of the better employment opportunities in the country.

Explanation:

In the period of the nineteenth century, from 1880s to 1920, there was an immigration in large number mostly from Mexico. The people were migrating to the United States of America because there were jobs available in this country which helped the people to enjoy a better lifestyle. The improved employment opportunities acted as a source of attraction for people to migrate here from Mexico.

the exclusive possession or control of the supply or trade in a commodity or service.