Which group was at the bottom of aztec society?

If you're talking about their social classes, then slaves were at the bottom.

The transcontinental railroad provided a means of fast transportation. it cut a 6 month trip into 6 days, created jobs for many people and encouraged immigration. the homestead act played a huge role in western expansion; it allowed settlers to begin a fresh start, and gave them land in exchange for their residence for up to five years and a small fee.

Democracy and darwinism would be my answer.