An excerpt published by the Sunday Times reveals Hawking had concerns that at some point in this century, humans would advance genetic engineering to a point at which they’ll be able to create a race of superhumans. According to the excerpt, which was earlier reported on by the Washington Post, Hawking wrote that he believes people—and especially the wealthy—could alter human genes to create people who are extremely intelligent or even aggressive. He called it “self-designed evolution.”

“I am sure that during this century people will discover how to modify both intelligence and instincts such as aggression,” he wrote

Hawking, a Cambridge University professor, died in March. He was one of the world’s foremost physicists and considered one of the most respected scientists in the world. He had a motor neuron disease similar to Lou Gehrig’s disease.

Hawking’s new essays are entitled “Brief Answers to Big Questions” and will be published on Tuesday. His 1988 “A Brief History of Time: From the Big Bang to Black Holes” was one of his most successful works and sold more than 10 million copies.

You can pre-order Hawking’s new book now on Amazon.