One of the larger flaws in his thesis:

"A priori, there is no firm reason to anticipate that the intellectual capacities of peoples geographically separated in their evolution should prove to have evolved identically." ... Our wanting to reserve equal powers of reason as some universal heritage of humanity will not be enough to make it so. Rather than face up to facts that will likely change the way we look at ourselves, many persons of goodwill may see only harm in our looking too closely at individual genetic essenses."
He also points to a search for genes that significantly affect a person's intelligence, and characterizes it as a "very hot potato."

...is that it has made a HUGE jump to a wrong conclusion. The leap is in the assertion that races have followed a different evolutionary track. They have not. In fact, there is no such thing as diverse "races" in homo sapiens at this point. We are all one race.

Thus, though the assertion that "...there is no firm reason to anticipate that the intellectual capacities of peoples geographically separated in their evolution should prove to have evolved identically" may be true...

...there has been no "geographically separated evolution" at this point. And now, with the world of communication and transportation such as it is, even if a few million years down the road it looks as though there COULD have been such "geographically separated evolution", it is unlikely that such a thing would ever occur.