Physicists regularly invoke universal laws, such as those of motion and electromagnetism, to explain events. Biological and medical scientists have no such laws. How then do they acquire a reliable body of knowledge about biological organisms and human disease? One way is by repeatedly returning to, manipulating, observing, interpreting, and reinterpreting certain subjects--such as flies, mice, worms, or microbes--or, as they are known in biology, "model systems." Across the natural and social sciences, other disciplinary fields have developed canonical examples that have played a role...