The main way in which the <span>federal system helps prevent the centralization power is that it reserves a good deal of power for the individual states--meaning that this power "checks" the power of the central (federal) government. </span>
In this quote, Thurgood Marshall is talking about the time between the signing of the Emancipation Proclamation (which was supposed to free slaves in the Confederacy) to the Brown vs. Board case which ruled that "separate but equal" is unconstitutional.
The reason he brings up these two events is because it shows how little progress American society has changed in these 90 years. Even though African-Americans were supposed to be free citizens after the Emancipation Proclamation, they were still treated as second class citizens in the US. They were constantly targets of voter discrimination, violence, and prejudice.
This was a mixture of things.
The increase in population from soldiers and influx of immigrants caused the need for more food to be produced and more jobs to be demanded. So farmers started over producing which led to the Dust Bowl and, eventually, the Great Depression.
