Mara Hvistendahl is worried about girls. Not in any political, moral or cultural sense but as an existential matter. She is right to be. In China, India and numerous other countries (both developing and developed), there are many more men than women, the result of systematic campaigns against baby girls. In “Unnatural Selection,” Ms. Hvistendahl reports on this gender imbalance: what it is, how it came to be and what it means for the future.