The disastrous results of intermixing

"The totally disastrous results that can be produced by a superior race
intermixing with inferior races had been fully perceived by the most ancient
civilized peoples. This was undoubtedly the origin of the caste system which
prevents any union between peoples of different races, and which we find still
in place in many old societies. Without mankind would perhaps have never
gone beyond the dawn of civilization. Thanks to this system powerfully
sanctioned by religious law, the ancient Aryans, when they penetrated into
India, at the time inhabited by savage hordes with dark skin, guarded
themselves against any crossbreeding and, consequently, degradation and the
final absorption that menaced them. Without the caste system, the brilliant
civilization that they founded on the banks of the Ganges would never have
taken root, and history would not have occupied itself with them. This system
therefore played, in fact, an immense role in the history of the early
civilizations; if, with our modern ideas, we find it unjust, the fact is that,
fortified by long-time traditions, it has outlived in many peoples the necessities
that called it into existence."

The Influence of Race in History by Doctor Gustave Le Bon