Since the late 1970s, 163 million female babies have been aborted by parents seeking sons. BY JONATHAN V. LAST 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... (snip) In nature, 105 boys are born for every 100 girls. This ratio is biologically ironclad. (snip) Yet today in India...