This passage is about the speech of President Jackson in Congress, where he defended the process of remorse of the Indians.
The use of the word "progessive" refers to the continuation of a process begun earlier, where the Indians were massacred and expelled from their territories to the detriment of the white man's civilization. The former president used that term as a way to slow down an attitude that today would be understood as barbarism, but one that is part of US history.
The limitations would be how much evidence was available and the truthfulness of the witnesses. The Code dealt with violations of the law. The law specified punishments based on who you were...slave or master, etc..
