Aghast (adjective) – shocked and upset
“James was aghast. He forgot to be embarrassed. “But Helga! Good heavens! Don’t you see that if we-I mean people like us-don’t have children, the others will still have. That’s one thing that’s the matter with us. The race is sterile at the top” (231, online version)
The word ‘aghast’ tells us how James facial reaction was towards Helga’s response. It shows that he was not expecting Helga, who wants black people to be appreciated, so he thought she would have kids with another black man like him. But was not the case so it took him by surprise and thought that she didn’t understand the importance of it.