At the turn of the nineteenth century, the population expanded rapidly toward the West. Between 1800 and 1840, western New York State, western Pennsylvania, Ohio, Michigan, Indiana, Kentucky, and Illinois were settled.
The stream of immigration lasted for about five decades. The process of westward expansion of the population that had begun in the early 1800s was hastened by the arrival of these new immigrants.
“incipient decline.” That is, population size was still increasing but, at current rates, it would not continue to do so for long.