In the present, we see groups of people on Earth that could be classified by potential based on how much intelligence they manifest: 1. Brilliant and 2. Not Smart at all.
One professor in college explained it this way without any reference to race: In one culture, intelligent people were not permitted to marry and have children, but in another culture, intelligent people were encouraged to marry and have children. He explained that the result was that in the culture where smart people were encouraged to marry and have children, that culture had increased numbers of smart people. And the level of intellect grew higher as well.
This, coupled with negatives such as criminals passing on their negative tendencies as well as cruelty, was passed on in some cultures in a similar manner. The examples were not racial but vividly based on culture.
A friend told me once, "Some people should not be allowed to reproduce based on their behavior." Simplistic and dangerous, yes. Mainly because who gets to decide which people are worthy of reproduction and which are not? We know of one time during the twentieth century when this was extreme in Europe.
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