"As it had been explained to David long ago, genetic diversity was very, very important. The more diverse the human gene pool was, the better were humanity’s chances of adapting to any new and unexpected conditions it might encounter, now that it was beginning to push outward into Space, to say nothing of surviving any unexpected natural disasters such as polar shifts or meteor strikes on Earth. Unfortunately, humanity had been both unlucky and foolish. Out of the dozens of races that had once lived in the world, only a handful had survived into modern times. Some ancient races had been rendered extinct by war. Some had been simply crowded out, retreating into remote regions and forced to breed amongst themselves, which killed them off with lethal recessives. That had been the bad luck. The foolishness had come when people began to form theories about the process of Evolution. They got it all wrong: most people interpreted the concept of “survival of the fittest” to mean they ought to narrow the gene pool, reducing it in size. So this was done, in genocidal wars and eugenics programs, and how surprised people were when lethal recessives began to occur more frequently! To say nothing of the populations who died in droves when diseases swept through them, because they were all so genetically similar there were none among them with natural immunities."