This is volume 99 issue 4 of The Quarterly Review of Biology. The Quarterly Review of Biology (QRB) has presented insightful historical, philosophical, and technical treatments of important biological topics since 1926. As the premier review journal in biology, the QRB publishes outstanding review articles of generous length that are guided by an expansive, inclusive, and often humanistic understanding of biology. Beyond the core biological sciences, the QRB is also an important review journal for scholars in related areas, including policy studies and the history and philosophy of science. A comprehensive section of reviews on new biological books provides educators and researchers with information on the latest publications in the life sciences.