Final answer:
Historical evidence refutes the claim that racial mixing leads to the downfall of civilizations. Ancient civilizations like the Greeks and Romans did not view racial differences as barriers, and modern thinkers like Frederick Douglass have praised the strength derived from societal diversity. Pseudo-scientific theories that promoted such views have since been discredited.
Step-by-step explanation:
The idea that "racial mixing" led to the downfall of all civilizations is not supported by a consensus of historical scholarship. The fear and belief that racial mixing could degrade societies has, however, been a recurring theme in certain ideologies and pseudo-scientific theories, especially during the era of Social Darwinism. This misguided belief was often coupled with a fear that 'inferior' genetic traits could proliferate within a population and has been used to justify racism and classism.
However, historians like Frank Snowden, Jr. have pointed out that ancient societies such as the Greeks and Romans did not hold color as a basis for judging a person, suggesting that the concept of 'biological racism' and the subsequent fear of racial mixing are largely modern constructions. In sharp contrast, figures like Frederick Douglass argued that America's diverse makeup was a source of strength, not weakness. Additionally, Polish-Austrian sociologist Ludwig Gumplowicz and others postulated that conflicts and power dynamics within civilizations did not necessarily correlate with racial mixing but rather with social and cultural hegemony of one group over others.
Anthropological research and theories, like the ones from Lewis Henry Morgan regarding a hierarchy of civilizations, have since been discredited, but the damage from perpetuated stereotypes has lingered. Further challenging the notion of racial purity as a prerequisite for success, the colonial history of the Western Hemisphere displays calamitous effects on indigenous populations caused not by racial mixing, but by factors including colonization, and diseases introduced by Europeans, as documented in works like "Guns, Germs, and Steel".