Answer: what do they have in common is a narrowly racist or (perhaps in a more scientific language) social Darwinian perspective that was supposed to justify 19th-century colonization. It means that both 19th-century colonialism and 20th-century genocide were justified as a Darwinian "law of history" according to which the strongest dominate or eliminate those who are weaker.
Explanation: Racism is a surprisingly modern term (from the 19th century) that was imported from natural sciences (in natural sciences appeared in 18th century). This import took place especially thanks to social Darwinism which spread across Europe during the 2nd half of the 19th century. Genocide was practiced always (not always with this name)....unfortunately it seems that it is only Homo sapiens sapiens whos is able to commit such a massive killing.
