What's one of the main motivations for individuals in the economic system of capitalism?

Well most plainly that would be the accumulation of wealth, monetary gain or in other words self-interest. Roots of this philosophy can be traced to Adam Smith and his monumental work The Wealth of Nations which argues that an individual acting in his own best interest and guided by the principle of justice  benefits himself and  the society as a whole. This can also be looked at from the viewpoint of the rational choice theory on which the profit motive functions.

d. migration

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.