Every movement needs a meliorist. In Ronald Green, an ethicist at Dartmouth College, proponents of genetic engineering have found an eager one. “We are entering the era of directed human evolution,” Mr. Green writes in “Babies by Design” (Yale University Press, 288 pages, $26). “Having vastly expanded our control over the world around us, our species is now rapidly developing the ability to alter the world within. The question is not whether we will do this, but when and how.”

Mr. Green gives readers a sampling of the science that will make this world possible, but he devotes most of his attention to answering the ethical questions these techniques raise. He is particularly concerned with parsing — and overturning — the distinction between what is therapy and what is enhancement. After positing that “gene doping” in sports is in our immediate future, for example, he writes, “This raises the question of why we would ban gene doping or prenatal genetic enhancement when we celebrate nature’s unequal bestowal of athletic talent.” He evinces a similar enthusiasm for extending the human life span, noting that as we learn more about the genetic basis of aging, “an extended life span may be one of the items on a future menu of prenatal choices.”