No I was just referring to the possibility of selection because of unusual features of human societies.
The story is about the trend amongst the so-called intelligencia (bearing in mind this story was written in the early 60's) to have few or no children. It put forward the idea that intelligence is inherited, and that if the trend continued the majority of individuals would have lower than average IQ in the future.
If you are from Europe (for example) how many Mauries (SP?) do you know ... or Bantu, or Inuit, or ....
We have geographic isolation and we also have cultural isolation in many cases. Some individuals refuse out-right to take partners outside of their own race, religion, or what-have-you.
Sexual selection is also a part of Darwin's original concept.
While we are able to travel, few do in order to meet diverse mates. The 'slow-down'/stop to evolution that we seem to have may be more to do with the apparent genetic bottleneck that humans have encountered sometime in recent (geological) time.