Max Delbrück has often been called the founder of molecular biology. Although educated as a physicist, Delbrück quickly became interested in bacteriophages, a type of virus that infects bacterial cells. He perfected a method of culturing bacteriophages and found that they could infect a bacterial cell and, within twenty minutes, erupt out of the cell in a hundredfold their original number. Each of these offspring bacteriophages was then ready to infect another bacterial cell. Among his many contributions to the field, Delbrück and another researcher together discovered that bacterial cells could spontaneously mutate to become immune to the bacteriophages. He also found that two different types of bacteriophages could combine to create a new type of bacteriophage. Perhaps as much or more than his discoveries, he forged the field of molecular biology through his involvement in the work of so many other...