I found this gem by Darwin of all people:

"Both sexes ought to refrain from marriage if they are in any marked degree inferior in body or mind"

>>2
There's still a problem with that though. Even if the partners are equal in intelligence, the IQ of the offspring will regress towards the racial average of inferior race.

For example, two white people both with an IQ of 100 will produce a kid with an IQ of 100 on average.

However a white and black couple both with an IQ of 100 will produce a kid with an IQ ~90 on average.