Most of the time in monarchies the rule is passed down through the bloodline, so my train of thought is feudalism.
Some of the peoples it conquered didn't want to be ruled by the Incas and were rebellious, mainly, and it was so big that it couldn't keep the conquered in line very well.
Answer:
Social Darwinism describes the various theories that emerged in Western Europe and North America in the 1870s which applied biological concepts of natural selection and survival of the fittest to sociology, economics, and politics. Social Darwinism posits that the strong see their wealth and power increase while the weak see their wealth and power decrease. Different social Darwinist groups have differing views about which groups of people are considered to be the strong and which groups of people are considered to be the weak, and they also hold different opinions about the precise mechanisms that should be used to reward strength and punish weakness. Many such views stress competition between individuals in laissez-faire capitalism, while others were used in support of authoritarianism, eugenics, racism, imperialism, fascism, Nazism, and struggle between national or racial groups.