Slavery started because people liked having other people work for free, so they enslaved others. They were brought by ships from Africa. They would usually work on farms and ranches. The jobs were different because of the climate. One type of climate made it possible to grow cotton, another to grow tobacco, another for crops, and so on. They were important because they were free. Since wealthy land owners didn't have to pay workers, they got extremely rich because they had almost no losses at all.
"The Doctrine of Chances" is a theory that was proposed by Alfred Russell Wallace that related to the theory of natura selection of Darwinists. This doctrine is what we now consider "statistics." Wallace's idea was to use this doctrine of chances and averages in order to study population genetics.
The German repeated attacks on the U.S.A