by John J. Miller

About the author: John J. Miller is national political reporter for the National Review, a biweekly journal of conservative opinion.

Senator Tom Harkin wanted to sound sophisticated at a hearing on the complicated subject of embryonic-cell research in December 1998. “It is my belief and my opinion, based upon a lot of study of this, that [stem cells] do not fall under the ban on human-embryo research,” declared the Iowa Democrat. Then he read the law aloud, including the part that defines embryos as organisms derived from human diploid cells. “I don’t even know what that is,” he announced.

A seventh-grade biology textbook might have come in handy. Diploid cells are those that contain complete sets of chromosome pairs. In other words, all cells are diploid cells except...