Ford had a long working relationship with R.A. Fisher. By the time Ford had developed his formal definition of genetic polymorphism, Fisher had got accustomed to high selection values in nature. He was most impressed by the fact that polymorphism concealed powerful selective forces (Ford gave human blood types as an example). Like Fisher, he continued the natural selection versus genetic drift debate with Sewall Wright, whom Ford believed put too much emphasis on genetic drift. It was as a result of Ford's work, as well as his own, that Dobzhansky changed the emphasis in the third edition of his famous text from drift to selection.